Rocks and Minerals_1405305940

RO C K SP O C K E T N A T U R E

AND MINERALS

MONICA PRICEKEVIN WALSH

DORLING KINDERSLEY

ROCKSAND MINERALS

P O C K E T N A T U R E

LONDON, NEW YORK, MUNICH, MELBOURNE, AND DELHI

DK LONDONSenior Art Editor Ina StradinsSenior Editor Angeles Gavira

Editors Georgina Garner, Bella PringleDTP Designer John Goldsmid

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First published in Great Britain in 2005 byDorling Kindersley Limited

80 Strand, London WC2R 0RL

A Penguin Company

Copyright © 2005 Dorling Kindersley Limited

All rights reserved. No part of this publicationmay be reproduced, stored in a retrieval system, or

transmitted in any form or by any means,electronic, mechanical, photocopying, recording or

otherwise, without the prior written permission of the copyright owners.A CIP catalogue record for this book is available from the British Library

ISBN 1-4053-0594-0

Reproduced by Colourscan, SingaporePrinted and bound by South China

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CONTENTS

How This Book Works 6

What are Minerals? 8

What are Rocks? 9

Rock Identification 10

Mineral Identification 14

Sedimentary Rocks 20

Igneous Rocks 43

Metamorphic Rocks 68

Ore Minerals 86

Rock-Forming Minerals 142

Glossary 217

Index 219

Acknowledgments 224

The bulk of minerals that constitute rocks are not ores,although some have important uses in industry. This chapterfeatures minerals that are found in a wide range of rock types(pp.143–67), and those found mainly or exclusively insedimentary rocks (pp.168–75), igneous rocks (pp.176–96),and metamorphic rocks (pp.197–216). Talc, for example, isfound exclusively in metamorphic rocks, such as the cliffs ofKynance Cove, England (below). Hydrothermal minerals thatare neither ores nor their secondary minerals are alsoincluded in this chapter.

MUSCOVITE SERPENTINEGYPSUMBERYL

Rock-forming Minerals

6 I N T R O D U C T I O N

How this book worksThis guide covers over 320 of the mostimportant rocks and minerals found all over the world. The book begins with a shortintroduction, which focuses on the process ofidentifying different rocks and minerals. Thefollowing pages divide rocks into three groups:sedimentary, igneous, and metamorphic; and minerals into two groups: ore minerals androck-forming minerals. Introductions to each ofthese sections define the groups and explain howentries are organized within them.

GROUP INTRODUCTIONSEach chapter opens with anintroductory page describing theparameters of the group, as well asany further sub-groups.

PHOTOGRAPHSThe main images illustratetypical examples of the rock ormineral. Secondary picturesshow named varieties.

LOCALITIES ANDASSOCIATIONSShows important rock or mineral localities, or shows therock or mineral in a typicalassociation.

ROCK OR MINERAL NAME

CHEMICALFORMULA OF THE MINERAL

CAPTIONDescribes the featured area orassociation in which the rock ormineral may be found.

NOTESDescribe unique features, orprovide interesting historical orcontextual background.

PICTURESPhotographs of representativespecimens show the diversitywithin the group.

CHAPTER HEADING

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R O C K - F O R M I N G M I N E R A L S 215

THE HIGH mountainsof Badakhshan inAfghanistan have beena rich source of lapislazuli for thousands of years.

Lazurite(Na,Ca)8Al6Si6O24[(SO4),S,Cl,(OH)]2

Since ancient times, lazurite has been highly prized for itsexquisite blue coloration as the principal mineral in therock known as lapis lazuli. Lazurite is a member of thefeldspathoid group. It is always deep or vibrant blue, and it was once the source of the artist’s pigment ultramarine.Most lazurite is massive or in disseminated grains, anddistinct crystals – which are usually dodecahedral – aremuch sought after. Lapis lazuli forms by contactmetamorphism of limestones. At its finest, this rockconsists of lazuritespeckled with goldenpyrite, but white calciteand other feldspathoidsare normally present too.

polished surface

whitecalcite

dodecahedralcrystals withdull lustre

COMPOSITION Silicate.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Imperfect/Uneven.LUSTRE/STREAK Vitreous to dull/Bright blue.HARDNESS/DENSITY 5–5.5 / 2.38–2.45.KEY PROPERTIES Bright blue streak; doesnot fizz in dilute HCl like azurite (p.113).Should not be confused with lazulite (p.149).

NOTE

The best lazurite crystals come fromBadakhshan Province in Afghanistan, which isalso the source of the many lapis lazulispecimens in old collections said to be fromPersia (now Iran). The stone was traded, butnot mined, in Persia. Other deposits of lapislazuli ar

e in the USA, Chile, and Russia.

LAPIS LAZULI CABOCHONS

POLISHED LAPIS LAZULI ROUGH LAPIS LAZULI

SECTION SHOWN

ULTRAMARINE

rich bluecolour

goldenpyritegrains

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208 R O C K -

THE FIRST descriptions ofkyanite were of crystalsfrom the schists of Zillertalin the Austrian Alps.

Stauro(Fe,Mg,Zn)3-4

Staurolite is redblack. It normahexagonal or dsurfaces. Cross-

STAUROLITE is oftenassociated with kyanite, as in this muscovite schistfrom St Gotthard,Switzerland.

KyanitAl2SiO5

Blue, white, anand these are gcrystal. The elohardness is maits length. Kyanof its polymorpmica schists, gnveins and pegm

shades of blue

bladedcrystals

triclinicprismaticcrystals

oss�

pseudo-orthorhombiccrystals

FULL-PAGEENTRIESRocks or minerals that exhibita more varied or complexrange, are of special interest,or are particularly important,are all given full-page entries.

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H O W T H I S B O O K W O R K S 7

ROCK ANDMINERAL ENTRIES The typical page featurestwo entries. Each has amain image, which issupported by one or more secondary pictures.Annotations, scale artworks,and a data box add keyinformation for each entry.

COLOUR BANDSBands are colour-coded, with a different colour for each ofthe five chapters.

ANNOTATIONCharacteristic features of the rock or mineral are picked outin the annotation.

DESCRIPTIONConveys the main features and the distinguishingcharacteristics of the rock or mineral.

DETAIL PICTURESThese tinted boxes show different aspects of the rock or mineral, including gem cuts, different habits, and colour variations.

OTHER KEY INFORMATION – MINERALSThese panels provide consistent information on the following points:COMPOSITION: the mineral class to which the entry belongs.Some silicate minerals also inlude an additional group classification.CRYSTAL SYSTEM: the crystal system to which it belongs.CLEAVAGE: the grading, from poor to perfect, of the way inwhich the mineral splits along flat planes.FRACTURE: the description of the typical appearance of a surfacewhere a specimen has broken.LUSTRE: the different ways the mineral typically reflects light,from dull to adamantine or metallic.STREAK: the colour of the mineral when it is in fine powderedform.HARDNESS: the hardness of the mineral when compared to thestandard minerals on Mohs’ scale.DENSITY: the typical weight of the mineral, measured in gramsper cubic centimetre.KEY PROPERTIES: the key identifying characters of the mineral,sometimes suggesting similar-looking minerals and distinguishingfeatures to help tell them apart.

MICROGRAPHSShows a section through the rock,seen through a microscope, to showthe constituent minerals.

SCALE MEASUREMENTSTwo small scale drawings are placed next to each other in everyentry, as a rough indication of the size of the featured specimen.The hand represents anaverage adult hand of18cm height.

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OTHER KEYINFORMATION – ROCKS These panels provide consistentinformation on the following points:GRAIN SIZE: the typical size or size range of the rock grains.ESSENTIAL COMPONENTS:lists the essential mineral constituents. ADDITIONALCOMPONENTS: lists mineralsthat may appear in the rock, but that are not essential. ORIGIN: describes the process bywhich the rock type is formed.SIMILAR ROCKS: lists rocks thatlook similar to the one featured, andoften provides distinguishing featuresto help tell them apart.

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48 I G N E O U S R O C K S

light beigematrix

BOTH diorite andgranodiorite occur inlarge intrusions, such as here in theAustrian Alps.

Felsite

Felsite is a general term for medium- to fine-grained, light-coloured, pink, beige, or grey igneous rocks fromsmall intrusions, such as sills and dykes. Blocky jointing iscommon, and occurs perpendicular and parallel to thewalls of the intrusion. Felsite may be slightly porphyritic,

with small phenocrysts, oftenof quartz, or it may

contain sphericalstructures.

COLUMNAR jointing in a thick sill of felsite isshown here on thenorthern coast of theIsle of Eigg, Scotland.

Diorite

An intermediate, coarse-grained igneous rock, dioriteconsists of white plagioclase and dark hornblende inroughly equal proportions, but other dark minerals mayinclude biotite and augite. With the addition of small

amounts of quartz and alkali feldspar itbecomes a granodiorite; with largeramounts, a granite. These three rock typesoften occur together in large intrusions.

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Plagioclase,hornblende.ADDITIONAL COMPONENTS None.ORIGIN Crystallization of an intermediatemagma in a major intrusion.SIMILAR ROCKS Syenite (top right), whichhas more alkali feldspar.

GRAIN SIZE Less than 2mm.ESSENTIAL COMPONENTS Quartz,feldspars.ADDITIONAL COMPONENTS None.ORIGIN Crystallization of an acid orintermediate magma in a minor intrusion.SIMILAR ROCKS Rhyolite (p.56), dacite(p.59), and porphyry (p.60).

MICROGRAPH

plagioclase

quartzphenocryst

darkerweatheredsurface

hornblende

minerals in equalproportion

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� SECTION SHOWN

F O R M I N G M I N E R A L S

olite(Al,Fe)18 (Si,Al)8O48H2-4

ddish brown, yellowish brown, or nearlyally occurs as prismatic crystals, which arediamond-shaped in section, often with rough-shaped penetration twins are common. It

forms by regional metamorphism ofargillaceous (or clay) rocks,

and is found in medium-grade schists and gneisses.

te

nd green are the usual colours of kyanite, generally mixed or zoned within a singleongate, flat, bladed crystals are often bent;rkedly greater across a crystal than along nite forms at temperatures between those phs andalusite and sillimanite. It occurs inneisses, and associated hydrothermal quartzmatites.

COMPOSITION SilicateCRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect and distinctcleavages at 90°/Splintery.LUSTRE/STREAK Vitreous to pearly/Colourless.HARDNESS/ DENSITY 5.5 along crystal, 7across crystal / 3.53–3.65.KEY PROPERTIES Bladed, blue crystals.

muscovite schist

cross-shapedtwin

vitreouslustre

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic, pseudo-orthorhombic.CLEAVAGE/FRACTURE Distinct/Nearlyconchoidal.LUSTRE/STREAK Vitreous to dull/Pale grey.HARDNESS/DENSITY 7–7.5 / 3.74–3.83.KEY PROPERTIES Brown; cr

ss-shaped twins.

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8 W H A T A R E M I N E R A L S ?

What are Minerals?Minerals are natural, inorganic substances, composed of the atoms ofeither one single chemical element or a number of different elements.There are over 4,000 different minerals, and each one is distinguishedby its chemical composition (the particular ratio of its chemicalelements) and its crystal structure. Nearly all minerals are crystalline:the atoms are arranged in a regularpattern; when allowed to growfreely, they form symmetriccrystals with flat faces.

Minerals are in all the rocks of the Earth. Theycan be found wherever rocks have been exposed,either naturally or by man. Some minerals arerich in those metals we use in our everyday lives,and we exploit these as ores.

Most minerals that make up the bulk of rocksand veins are neither metallic nor noticeablyheavy, and many are not particularly colourful.There are important exceptions, however, andsome of the most richly coloured are gemminerals that are beautiful, durable, and rare.

Ores and their secondary minerals frequentlyoccur in mineral veins, which are sheet-likestructures that result when minerals fill fractureswithin existing rocks. Many ore minerals look metallic, and some are noticeably heavy.Secondary minerals may be formed whenprimary ore minerals are altered by rain andgroundwater. They are often brightly coloured,and some may themselves be of economic value.

Rock-forming and Ore Minerals

LEAD OREGalena, the principal ore of lead,can be seen as a metallic greyband in this mineral vein.

MINERAL VEINS

Galena

ROCKS AND CAVESCalcite makes up the bulk of rockssuch as limestone and marble, aswell as forming stalagmites andstalactites in limestone caves.

ROCK FORMATIONS

Calcite

COMPOSITION Microcline is composed ofpotassium, aluminium, silicon,and oxygen atoms in the ratio 1:1:3:8, giving it the chemical formula KAlSi3O8. It is a silicate mineral, and a member of the feldspar group.

ROCK-FORMING MINERALS

ORE MINERALS

MICROCLINE

tabular crystalswith triclinicsymmetry

flat-facedcrystal

solid (likevirtually allminerals)

What are Rocks?Rocks are naturally-occurring consolidated substances, which may bemade up of minerals, other rock pieces, and fossil materials, such asshells or plants. Rocks are the result of various geological processesthat occur both at and beneath the Earth’s surface or, in the case ofmeteorites, in other parts of the Universe. Rocks can be studied and

differentiated between by grouping together those types that share a similar

appearance, similarcomposition, and the

same process offormation.

W H A T A R E R O C K S ? 9

Dynamic processes acting on the Earth'scrust allows rock material to be recycled. Atthe Earth’s surface, weathering and erosionbreak down pre-existing rocks intosediments, which form new rocks such as sandstone. These rocks may be buriedbeneath the Earth’s surface; heat andpressure of large-scale movements in turn cause fracture, deformation(alteration caused by stress), andeventually, melting. For example, sandstone is transformed into gneiss, and melted gneisssolidifies into granite.Uplift of deeper parts of the Earth’s crust bring thesenew rocks to the surface.

The Rock Cycle

quartz

biotite

sandstone

SEDIMENTARY

GRANITE

FELDSPARBIOTITE QUARTZ

COMPOSITIONGranite is alwaysmade up of threedifferent kinds of

minerals: white orbeige-coloured feldspar,

clear or grey quartz, andblack mica (biotite).

feldspar

METAMORPHIC

gneiss

ERO

SIO

N

BU

RIA

L

MELTING

light colourand squarecorners

thinsheetsand darkcolour

transparentand glassy

granite

IGNEOUS

Rock Identification1 0 I N T R O D U C T I O N

There are many features of rocks that can be used in identification; the size and shape of the grains, the colour, and determination of theconstituent minerals are all important. The processes that producerocks also give rise to characteristic textures and structures, forexample, lava can produce glassy rocks with flow structures.

TRAVERTINE TURBIDITE

OBSIDIANGABBRO

SCHISTMIGMATITEGARNETSCHIST

BASALT

Types of RockExamples of the three main types of rock – sedimentary, igneous, andmetamorphic – are shown below, but some other types of rock also feature inthis book: deformation rocks, which result from Earth movements; meteorites;and surface impact rocks, which are produced when meteorites strike the Earth.

SEDIMENTARY ROCKS

Sedimentary rocks result from the consolidation of sediments. One type ofsediment is deposited as grains by water or wind, in layers known as bedding;another is formed from biological material, producing rocks such as limestone.

IGNEOUS ROCKS

Intrusive igneous rocks form when magma (molten rock beneath the Earth’ssurface) solidifies, and are made up of crystals, which can be aligned or layered.Volcanic rocks are extrusive and form when lava solidifies; they may containglass, gas bubbles, or show flow structure.

METAMORPHIC ROCKS

These rocks are produced by alteration due to increased heat and pressure. Theyoften show features of deformation, such as flattening, streaking, or folding.Distinctive minerals, such as garnet, are good indicators of this type of rock.

VESICULARBASALT

lineationstreakinessfoliation

(parallel sheets)foldedlayer

alignedminerals

metamorphicgarnet

interlockingcrystals

volcanicglass

tourmaline(igneousmineral)

calcite (sedimentary mineral)

grainscross-bedding

bedding

gradedbedding

igneouslayering

gas bubbleholes

flowstructure

fossil

SANDSTONE FLAGSTONE CROSS-BEDDED SANDSTONE

CUMULATE ROCKPEGMATITE

TECTONITE METATUFF MYLONITE

CHALK

R O C K I D E N T I F I C A T I O N 1 1

Some minerals are restricted to particular types of rock, and determining mineralcontent can help identification. Obvious mineral grains can give a clue to arock’s identity, for example, garnet only appears in metamorphic rocks.

Mineral Content

In sedimentary rocks, grain size depends on how far the grains have travelled;for igneous rocks it is determined by how long the rock took to crystallise;metamorphic grain size depends on the pre-existing rock. Size limits for coarse,medium, and fine grains vary according to the group of rocks.

Grain Size

The shape of individual grains are more easily observed using a hand lens. Somefeatures to consider include the degree of roundness, whether all the grains aresimilar in shape, and how well developed any crystals are. Well-developed crystals have their flat faces intact and straight edges.

Grain Shape

PEGMATITE (IG.)

CONGLOMERATE BRECCIA GRANITE AUGEN GNEISS

TRAVERTINE (SED.)

calcitecrystals

garnet

feldsparquartz

diopside

roundedgrains

angulargrains

rectangularcrystal

CONGLOMERATE

GRANITE

PARAGNEISS

COARSE

SEDIMENTARYROCKS

METAMORPHICROCKS

IGNEOUSROCKS

MEDIUM FINE

IRONSTONE CLAY

RHYOLITE

SLATE

MICROGRANITE

QUARTZITE

eye-shapedgrains

ECLOGITE (META.)

TEKTITE

tear-dropshape

1 2 I N T R O D U C T I O N

Texture refers to the rock’s surface appearance or therelationship between the individual grains in a rock. If therock has no visible grains and is a single mass of mineral orglass, it is known as massive. Many igneous rocks are madeup of interlocking grains – a texture known as granular.

Texture

Structure refers to those featuresproduced by geological processes.Some structures, such as bedding,igneous layering, and gneissicbanding, are formed at the same timeas the rock itself, but others, such asfolding and shearing (stretching ofmineral grains), occur later. Manystructures exist over a vast range ofscale, for example, folding can beseen in microscopic crystal grains as well as across entire cliff faces.Geologists use structuralinformation to construct geologicalhistories and produce geological maps.

Colour is an obvious but useful property in rock identification. It can be used totell apart different varieties of a rock, and it can also provide information on thecomposition of the rock. In many cases, the colour of a rock will be affected byweathering, so a fresh surface should always be exposed.

Structure

Colour

PORPHYRY PISOLITE

GABBRO

FLINT GRANODIORITE

ROCK GYPSUMporphyritic (crystalsin fine matrix)

oolitic (small,rounded grains) glassy

green sandstone

pinksandstone

greysandstone

red sandstone

light intermediate mainly dark dark

massive

crystalline

shearing

folding

banding

IGNEOUS ROCKS In general, the higherthe silica content of an igneous rock, thelighter its colourshould be.

VARIETIESEach sandstone colouris determined by itsconstituent minerals;these depend on theoriginal sediment andthe geological history.

bedding

FOLDEDMYLONITE

GNEISS

SANDSTONE

TECTONITE

DESERTSANDSTONE GREENSAND ORTHOQUARTZITE GREYWACKE

GRANITE DIORITE PYROXENITE

layering

CUMULATEROCK

R O C K I D E N T I F I C A T I O N 1 3

Rocks are found everywhere, but the best examples are found where the effectsof weathering are least, often in settings created by human activity, such as road-cuts, mine dumps, and quarries. Good examples are also found where nature has

recently exposed fresh rock, such as on sea cliffs,beaches, mountains, and volcanic terrains. Manyof the harder, more resistant, and more attractiverocks are used for building; these can be seen assupport structures, as well as interior and exteriordecoration. Geological maps show the distributionof rocks in particular areas, and can be importantand useful tools for finding and recognising rocks.

Where to find Rocks

Geological Environments

RIVERS LAKES

CAVES

BUILDINGS

MOUNTAINS VOLCANIC REGIONS BEACHES

ROAD CUTS MINES QUARRIES

DESERT OCEANS AND SEAS

conglomerate siltstone

tuff

slatevolcanic bomb

dolomite

kimberlite

Rock types rarely occur in isolation. Normally, several different but related typesof rock form together in one geological environment. For example, basalt, scoria,spilite, and volcanic bombs are found together in the volcanoes of Hawaii.Knowing these associations can help to identify rocks that occur together and,also, to recognise their ancient environment – greensand and chalk of theCretaceous Period are evidence of warm, calm seas of 100 million years ago.

oolitic limestone

travertine

scoriaVOLCANOES

amber

loess greensand


Mineral IdentificationThe fact that a single mineral can occur in a variety of crystal shapesand in a range of colours can be confusing. However, every mineralcan be identified by investigating a combination of properties. Someproperties, such as colour, are visible to the eye; others, such ashardness, need to be measured using simple pieces of equipment.

Minerals are divided into thefollowing classes according to theirnegatively charged component, forexample, carbonates (CO3

2-). Manyproperties of a mineral result from its chemical make-up, for example,colour, streak, and magnetism.Chemical elements and their symbolsappear on p.19.

Mineral Composition

ELEMENTSNative elements occur uncombined with any otherchemical elements.

HALIDES These include fluorides(-F) and chlorides (-Cl); they often havecubic symmetry.

BORATES Borates are mainlywhite or grey; theircomplex structures link-BO3 units together.

borax

kyanite

baryte

halite

manganite

silver chalcocopyrite

cobaltite

Some minerals are grouped together because they have closely related chemicalcompositions and crystal structures. As a result of this, they share certain keyproperties and may be difficult to tell apart. Many silicate minerals belong togroups, which include the feldspars, garnets, micas, amphiboles, and pyroxenes.

Mineral Groups

SPESSARTINEGROSSULAR

THE GARNET GROUPGrossular, almandine,and spessartine allbelong to this group.The group name isuseful to use whenunsure what membermineral you have.ALMANDINE

calcite

CARBONATES The carbonates (-CO3)include rock-formingminerals such ascalcite and dolomite.

SULPHIDES ANDSULPHOSALTS Many sulphides (-S)and sulphosalts (-AsSor -SbS) are ores.

crocoite

CHROMATES,MOLYBDATES, ANDTUNGSTATES These minerals (-CrO4, -MoO4, and -WO4)include important ores.

PHOSPHATES,ARSENATES, ANDVANADATES -PO4, -AsO4, and -VO4 minerals areoften brightly coloured.

pyromorphite

SILICATES SiO4 units connect toform a range ofstructures. Most rock-forming minerals arefrom this huge class.

OXIDES ANDHYDROXIDES Oxides (-O) andhydroxides (-OH) varywidely in appearance,and may be metallic.

SULPHATES Most sulphates (-SO4)are secondary minerals,but some, like gypsumand baryte, make uprocks and veins.

M I N E R A L I D E N T I F I C A T I O N 1 5

A crystal’s form is its geometric shape. Some crystals are single forms, such asoctahedrons; others are more complex and combine two or more forms, forexample, the quartz crystal (below right) combines a prism with two pyramids.

Crystal Forms

The crystal forms of a mineral indicate which crystal system the mineral belongsto. For example, elongate crystals with a square cross-section, such as that of thevesuvianite below, are found in minerals that belong to the tetragonal system.There are seven crystal systems, and each has certain elements ofsymmetry. Each of a mineral’s forms will always have theelements of symmetry of the system to which it belongs.

Crystal Systems

ORTHORHOMBIC More flattened thantetragonal crystals;often tabular orprismatic with wedge-shaped terminations.

CUBIC Crystals are commonlyoctahedral, tetrahedral,dodecahedral, cubic, orcombinations of these.

MONOCLINIC Crystals may betabular or prismatic;rhombic in section.

HEXAGONAL Typically six-sided in section; usuallyprisms, pyramids, or a combination of both these forms.

TRICLINIC The least symmetric of all crystal classes;crystals are tabular or show no obvioussymmetry.

TRIGONAL Less symmetric thanhexagonal; may berhombohedral orscalenohedral.

pyrite

vesuvianite

Twinned crystals form when different parts of one crystal grow as mirror imagesof each other. These may join at a crystal face, edge, or internal plane. Simpletwins have two parts; multiple twins have three or more. Twinning gives amineral different crystal forms, for example, this aragonite appears hexagonal.

Twinned Crystals

multiple twinsimple contacttwin

interpenetranttwin

eighttriangularfaces

prismpyramidtwelvefaces

OCTAHEDRON RHOMBOHEDRON DODECAHEDRON MORE COMPLEX

CALCITE STAUROLITE

six rhombus-shaped faces

baryte

axinite

calcite

beryl

gypsum

ARAGONITE

TETRAGONAL Crystals are typicallysquare or octagonal in section, and tabular, prismatic, or bi-pyramidal.

EQUANTTABULAR BLADEDPLATY

LAMELLAR FOLIATED COLUMNAR

SPHERULAR BOTRYOIDALRENIFORM

DENDRITIC EARTHY MASSIVE

EPIDOTE GOLD SAPPHIRE

These terms describe the way a mineralbreaks. Cleavage refers to the flat planesalong which a crystal tends to split; it isgraded from poor to perfect dependingon how clean and easy the break.Fracture describes the surface where amineral has broken, but where it has not cleaved. A parting looks like acleavage, but has other causes.

Mineral Habits1 6 I N T R O D U C T I O N

The habit of a mineral describes the general appearanceof its crystals or clusters of crystals. A mineral can havemore than one habit. For example, actinolite can bebladed, acicular, fibrous, or massive, depending on theconditions in which it forms. A massive habit indicatesnot the size but the absence of any visible crystal shape.Consequently, a tiny sample of a mineral can be massive.

Cleavage and Fracture

CITRINE

PRISMATIC

uneven fracture

conchoidal (shell-like) fracture

parting(looks like acleavage)hackly

fracture

very thin and tabular flattened and

elongated

same sizein alldirections

flattened, but not veryelongated

thin flatsheets

parallelprismaticcrystals

no visiblecrystals

needle-like

bent massesof lamellarcrystals

irregulargrains

rounded,kidney-like

like a bunchof grapessphere-

shaped

powderymass

elongateparallel faces

tree-like

perfect micaceouscleavage

perfect cubic cleavage

splaying out from a central point

hair-like

FIBROUS AND RADIATING

GRANULAR

ACICULAR

GALENA MUSCOVITE


All minerals featured in this book are transparent or translucentunless stated otherwise.Sometimes transparency may onlybe seen when looking through thinslivers of the mineral.

Transparency

Some minerals are always of the same colour,as a consequence of their chemicalcomposition and structure. Their colour is auseful aid to identification. Other minerals,such as fluorite, can be different colours dueto tiny amounts of chemical impurities,structural flaws, or to their different habits.Certain minerals are fluorescent – they glowdifferent colours under ultraviolet (UV) light.

Colour

OPAL OLIVINEPERIDOT

MICROCLINE

CARNELIAN

CHALCOCITE

translucent

opaque

The colour of the finely powdered mineralis called the streak. Even if a mineral’scolour varies, the streak will always thesame for a mineral. For example, no matterthe variety, hematite has a reddish brownstreak. Streak is tested by drawing themineral across a streak plate, which is thewhite unglazed back of a ceramic tile.

Streak

Lustre describes the way a mineral reflects light.Adamantine is the mostbrilliant lustre shown bytransparent or translucentminerals. The glass-likevitreous lustre is less brilliant.Slightly inferior lustres areprefixed ‘sub’, for example, asubadamantine lustre is slightlyless bright than adamantine. A mineral’s lustre may bedifferent on certain faces orcleavages, and it should alwaysbe examined on a clean,unweathered part of the mineral.

USEFUL KITSome properties are easier to see with a handlens or magnifying glass.Other useful tools include a streak plate, a compass for detectingmagnetism, a soft brush for cleaning

minerals, and coins and a knifefor testing hardness. Schools,

clubs, and societies may beable to providechemicals foracidity testing andGeiger counters.

dull

resinouswaxy

softbrush

streakplate

transparent

FLUORITE

coins

HEMATITE

HAWK’S EYE

TOPAZ GALENA

Lustre

silkygreasy

adamantine

purple

green

yellow

vitreous

crystal

kidneyore

metallic

ROSE QUARTZ

DIAMOND

pearly

HEULANDITE

NEPHELINE

hand lens

DensityMost minerals have a density ofbetween 2 and 4, but some areconspicuously lighter or heavier. Forexample, the crystal of baryte, below,would feel distinctly heavy compared togypsum. Density is measured innumber of grams per cubic centimetre.

MagnetismA few iron-bearing minerals aremagnetic; when placed beside acompass, they deflect the needle fromthe north. Magnetite, a particularly

strong natural magnet,attracts iron or

steel objects.


How hard a mineral is depends on theway its atoms are arranged. Hardness is measured by reference to the tenstandard minerals on Mohs’ scale, thesoftest mineral being talc, and thehardest, diamond. On Mohs’ scale, a crystal that scratches calcite but isscratched by fluorite would be 3.5 onthe scale. It may be possible todistinguish similar looking minerals by measuring their hardness.

Hardness

GYPSUM

BARYTE

COPPER COIN: 3

FINGERNAIL: 2.5

STEEL BLADE: 5.5

MEASURING KIT

It is useful to havesmall samples of allthe minerals onMohs’ scale, butother objects – yourfingernails, coins,and knife blades –can also be used toestimate hardness. Ahardness test shouldalways be carried outon an inconspicuouspart of the mineral,as it will leave apermanent scratch.

MOHS’ SCALE

density: 2.32

density: 4.5

Acid TestsIf a tiny drop of dilute hydrochloricacid (dil. HCl) is placed on certaincarbonate minerals, it fizzes and givesoff bubbles of carbon dioxide gas. Certain other minerals react to spot acidtests in other distinctive ways, by

changing colour ordissolving. Pleasenote, all acids can bedangerous: they mustalways be handledwith great care.

CORUNDUM: 9

DIAMOND: 10

GYPSUM: 2

FLUORITE: 4

TALC: 1

CALCITE: 3

APATITE: 5

ORTHOCLASE: 6

QUARTZ: 7

TOPAZ: 8

HCl fizzes oncalcite

RadioactivityMinerals that contain uranium orthorium are radioactive. Radioactivity isdetected using a Geiger counter. Allradioactive minerals require specialhandling and storage.

ironfilings

MAGNETITE

attractsthepaperclip


Some of the most important clues tothe identity of a mineral come fromthe rock matrix in which it occurs andthe other minerals that occur with it.Some minerals only occur in particulartypes of rocks; others aretypically associated withcertain other minerals. Forexample, apophyllite oftenoccurs with stilbite.

staurolite

Ac Actinium Ag Silver Al Aluminium Am Americium Ar Argon As Arsenic At Astatine Au Gold B BoronBa Barium Be Beryllium Bi Bismuth Bk Berkelium Br Bromine C CarbonCa CalciumCd Cadmium Ce Cerium Cf Californium Cl Chlorine Cm Curium Co CobaltCr Chromium Cs Cesium Cu CopperDy Dysprosium Er Erbium Es Einsteinium F Fluorine Fe Iron Fm Fermium Fr Francium Ga Gallium Gd Gadolinium

Ge Germanium H Hydrogen He Helium Hf Hafnium Hg Mercury Ho HolmiumI Iodine In Indium Ir Iridium K Potassium Kr Krypton La Lanthanum Li Lithium Lu LutetiumLw Lawrencium Md MendeleviumMg MagnesiumMn ManganeseMo Molybdenum N NitrogenNa Sodium Nb NiobiumNd Neodymium Ne Neon Ni Nickel No Nobelium Np NeptuniumO Oxygen Os Osmium P Phosphorous Pa Protactinium Pb LeadPd Palladium Pm Promethium

Po Polonium Pr Praseodymium Pt Platinum Pu Plutonium Ra Radium Rb Rubidium Re Rhenium Rh Rhodium Rn Radon S Sulphur Sb Antimony Sc Scandium Se Selenium Si Silicon Sm Samarium Sn Tin Sr Strontium Ta Tantalum Tb Terbium Tc Technetium Te Tellurium Th Thorium Ti Titanium Tl Thallium Tm Thulium U Uranium V Vanadium W TungstenXe Xenon Y Yttrium Yb Ytterbium Zn Zinc Zr Zirconium

TABLE OF ELEMENTS

muscovitemica

Geological Association

PEGMATITE ASSOCIATION

MICA SCHIST ASSOCIATION

kyanite

albitefeldspar

rhodochrosite

pyritesphalerite

basalt

stilbite

HYDROTHERMAL VEINASSOCIATION

apophyllite

elbaitetourmaline

BASALT ASSOCIATION

quartz

Sedimentary rocks are produced by the movement of water or wind. There are two groups: detrital, which result from the settling of rock grains, and chemical/organic, such as thechalk that forms the cliffs of England’s south coast (below),which are produced by water-deposited chemicals or fromorganic remains. Here, the detrital sedimentary rocks arearranged in order of decreasing grain size. This may depend onhow far the grains have travelled before deposition, or on the

energy levels of their environment. Forexample, only large grains can

settle in fast-moving water.The chemical/organic

rocks are grouped bycomposition.

Sedimentary Rocks

LIMESTONESEPTARIAN NODULE

BANDED IRONSTONE

TUFA

CONGLOMERATE, inter-bedded with sandstones, was formed in a riverchannel and flood plain.

S E D I M E N T A R Y R O C K S 2 1

Conglomerate

This common rock type is easily recognized by its largerounded clasts. The clasts, which were pebbles, may be ofone type, such as all quartz or limestone, or more. Theseclasts are in a finer matrix made up of smaller grains(typically sand-sized) and cement.Conglomerates are found withsandstone in rock formed inlarge river systems.

sandy matrix

Breccia

Unlike the clasts in conglomerate, those in breccia areangular, which indicates that they have not travelled farfrom their source. Breccias may form in volcanic systemsand in faults. Sedimentary breccias, however, are formed

from screes in mountains and along coasts, or in deserts due

to flash floods.

WELL-DEVELOPED screes on cliffs in southernMorocco; such depositsmay be the source of the clasts in breccia.

finematrix

GRAIN SIZE 2mm to several cm, in finermatrix.ESSENTIAL COMPONENTS Commonly rockfragments.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from coarse sediment.SIMILAR ROCKS Fault breccia (matrix is amineral), volcanic breccia (occurs next to lava).

POLYGENETIC BRECCIA

angularclast

roundedclast

POLYGENETIC CONGLOMERATE

differentclast types

differentclast types

GRAIN SIZE 2mm to several cm, in finer matrix.ESSENTIAL COMPONENTS Commonlyquartz, chalcedony, and rock fragments.ADDITIONAL COMPONENTS Sometimesgold, uranium minerals.ORIGIN Detrital, from pebbles.SIMILAR ROCKS Breccia (below).

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2 2 S E D I M E N T A R Y R O C K S

COARSE bedding andblocky jointing istypical in gritstoneoutcrops.

Tillite

A large variation in grain size – from powder to clasts ofseveral metres – is typical in this rock and gives rise totillite’s other name: boulder clay. In the past, material wasground up and transported as a result of a glacier movingdown a valley; when the glacier melted, it deposited itsentire load of materials or till. The resultant rock, tillite, is common in geologically recent deposits of ice ages in

northern Europe and North America, but also found in older rocks.

MELTING glaciers andthe deposition of theirice-carried sedimentproduce tillite rock.

Gritstone

Gritstone is made up of the cemented deposits of roundedgrains. These are the size of coarse sand to gravel. In somegritstones the grains can be easily rubbed out, while inothers a strong cement makes the rock suitable for use as agrinding stone, such as Millstone Grit. Quartz is always thegreatest component, but it often contains iron oxidesgiving it a yellow, brown, or red colour.

coarse quartzgrains

FELDSPATHIC GRITSTONE

SECTION SHOWN

grey claymatrix

largeclast

GRAIN SIZE Less than 0.005mm to manymetres.ESSENTIAL COMPONENTS Commonly rockfragments.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from coarse angularsediment.SIMILAR ROCKS Breccia (p.21).

GRAIN SIZE 1–4mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS Mica andmany heavy and resistant minerals, such asgarnet, rutile, and titanite.ORIGIN Detrital, from grit.SIMILAR ROCKS Sandstone (p.23) is finer;feldspathic gritstone contains more feldspar.

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NOTE

Sandstone is an attractive building stone and,since ancient times, it has been carved intomonuments, such as the Great Sphinx at Al-Jizah in Egypt. Its resistance to weatheringdepends on the nature and strength of themineral cement that holds together the grainsof sand.

Sandstone

One of the most common sedimentary rocks, sandstone isusually quartz-dominated with visible sandy grains. Thesegrains may be of various shapes when viewed with a handlens. Well-rounded grains are typical ofdesert sandstone, while river sandsare usually angular, and beachsands somewhere in between. Thecolour is also an indication of howthe sandstone formed and can beshades of white, red, grey, or green.Bedding is often seen insandstones as a series of layersrepresenting successivedeposits of grains. Beddingsurfaces may show otherdepositional features,such as ripples.

RED sandstones withwell-developed beddingare found in the desertof Arizona, USA.

roundedquartzgrains

finesandstone

coarse upperlayer

SLUMP-BEDDED SANDSTONE MICACEOUS SANDSTONE

folded beddingplanes

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS Mica andmany others.ORIGIN Detrital, from sand.SIMILAR ROCKS Orthoquartzite (p.25), arkose (p.25), greensand (p.26), flagstone (p.28).

IRON-RICH SANDSTONE

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LOESS is formed fromthe lightest fragmentsof sediment carried bydesert winds.

CROSS-BEDDED units in a series of yellowsandstone are parallelat the base of eachunit and truncated at the top.

Loess

This is a yellow or brown, soft and crumbly, low-densityrock. It comprises very fine rock and mineral material. Itcontains few clay minerals, so feels smooth but not stickywhen wet. It is another rock formed by the deposition of

material carried by the wind. The largestdeposits are found next to the Gobi

desert in China. European examplesare often accumulations of wind-blown debris from glacial deposits.

GRAIN SIZE Less than 0.005mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from wind-blown sedimentin a desert.SIMILAR ROCKS Clay (p.29), which is sticky when wet, has also a far greaterdensity.

yellow colour

spongytexture

SECTION SHOWN

mud-richlayer

CROSS-BEDDED MUDDY SANDSTONE

Cross-bedded Sandstone

When one set of beds is inclined to another it is known ascross-bedding. It forms in deserts where wind depositioncreates beds at different angles along dune surfaces, and itforms in water-lain sediments as competing currents drop

sediments at different angles. Desert sandstone is usually red due to the deposition of a thin

hematite layer over the quartz grains.

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS Hematite, mica.ORIGIN Detrital, from desert wind or water.SIMILAR ROCKS In other sandstones (p.23),the layers are parallel and not truncated.Cross bedding can occur in quartzites (p.76)and gritstones (p.22).

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truncatedbedding

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GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Quartz.ADDITIONAL COMPONENTS Heavyminerals, such as zircon, rutile and titanite.ORIGIN Detrital, from a quartz-rich sand.SIMILAR ROCKS Other sandstones(pp.23–27) have a lower proportion of quartz(p.143), and are darker coloured.

Orthoquartzite

White or pinkish sandstones composed of more than 95 per cent quartz are known as orthoquartzites todifferentiate them from similar metamorphic rocks. Theyare hard rocks and often form ridges. The quartz grains havebeen naturally sorted andhave travelled far.

quartzgrains

GREY ORTHOQUARTZITE


ORTHOQUARTZITE

forms prominent whiteridges in thislandscape.

grey colour ofmany ancientarkoses

smallresistantveinlet

Arkose

A pink sandstone, the colour of which is due to anabundance of feldspar, especially pink alkali feldspars. Theflat cleavage faces and angular grain shape of the feldsparsreflect light under a hand lens. Arkose forms by the fastdeposition of sandweathered fromgranites andgneisses.

TORRIDONIAN arkosefrom northwestScotland covers largeareas of flat-toppedmountainland.

pink colourof feldspar

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Feldspar, quartz.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from a feldspar-rich sand.SIMILAR ROCKS Other sandstones (pp.23-27) have more quartz (p.143) andless feldspar. Unlike other sandstones, mostarkoses occur in ancient rock sequences.

GREY ARKOSE

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GUANO deposits onNavassa Island, in theCaribbean, haveproduced a brownphosphorite.

GRAIN SIZE Some are not granular; othershave grains up to and greater than 1cm.ESSENTIAL COMPONENTS Phosphateminerals (apatite, variscite, vivianite, etc.).ADDITIONAL COMPONENTS Bones, guano.ORIGIN Organic or chemical precipitate.SIMILAR ROCKS Dark phosphorites mayresemble ironstone (p.35).

Phosphorite

Sedimentary rocks rich in phosphate minerals are knownas phosphorites. Several of these consist of phosphatesderived from animals, such as bone beds and guano, wherethe phosphorites are dark brown. Others are made of blackphosphates: deep sea nodules, pebble beds, andsandstones. Another common type is pale-

coloured and rich in apatite.

NODULARPHOSPHORITE

Greensand

Green-coloured sandstones are usually the result of thepresence of sand-sized and smaller grains of the iron mineralglauconite. They typically weather to a brown colour onsurface exposures. They are formed in shallow marineconditions and may contain shell fragments and largerfossils. Cretaceous and younger rocks of southern Englandand North America have many greensand beds.

green colourfrom glauconite

GREENSAND forms in quiet, shallowmarine conditions. The green colour is due to ferrous iron.

SECTION SHOWN

brown calcareousmudstone

MICROGRAPH

glauconite

quartz

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Quartz, feldspar, glauconite.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from glauconite-rich marine sand.SIMILAR ROCKS Rounded green grains ofglauconite (p.170) are unique to greensand. paler grains

mainly quartz

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phosphatic grain(fish bone)


GREYWACKE appears asa dirty-grey colouredseries of beds in a coastal outcrop.

Turbidite

A turbidite is a unit of rock formed from a turbiditycurrent: a fast-moving turbulent mass of water andsediment that travels over the continental shelf intodeeper ocean. Turbidites have a repeated series of different types of bed in a particular order. It starts with a thick greywacke, followed by thin graded greywackes,then siltstone, then mudstone. The lasttwo rocks may show ripples onbedding surfaces.

TURBIDITE units, eachtens of centimetresthick, representsuccessive influxes of sediment.

GRAIN SIZE 0.005–2mm.ESSENTIAL COMPONENTS Rock andmineral fragments.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from a turbidity current.SIMILAR ROCKS Turbidites are a mixture ofgreywacke (above), siltstone (p.28), andmudstone (p.29) present in repeating layers.

dirty greycolour

mixed angularfragments

dark, coarsegrains

SECTION SHOWN

fine grains

MICROGRAPH

grains of many differentsizes and types

GRAIN SIZE 0.005–2mm.ESSENTIAL COMPONENTS Rock andmineral fragments.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from muddy sand.SIMILAR ROCKS Other sandstones (pp.23-27) have less angular grains; turbidites (below)contain both greywackes and finer rocks.

Greywacke

Greywacke has formed from a mixture of sand and,proportionately less, mud. It is a dark-coloured rock: grey,dark green, or black. Sand-sized grains of rock fragmentsand minerals are visible, and may be randomly distributedor in graded layers with coarser fragments at the base,fining upwards. It is especially abundant in LowerPalaeozoic rocks worldwide.

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graded bedding


POLYGONAL dessicationcracks, seen here in silt,are often preserved insiltstones.

CAITHNESS flagstonefrom Achanarras innorthern Scotland, is a highly fissile, red,micaceous sandstone.

Siltstone

Siltstone is another detrital sedimentary rock, lying ingrain size between sandstone and mudstone. Likesandstone, it can form in different environments and havedifferent colours and textures, but reds and greys, andplanar bedding are typical. Plant fossils and other

carbonaceous matter are common indarker-coloured siltstones.

The presence of mica mayproduce a flaggy siltstone.

GRAIN SIZE 0.005–0.1mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS Plant fossils,coal, mica.ORIGIN Detrital, from silt.SIMILAR ROCKS Sandstone (p.23) iscoarser, mudstone (p.29) is finer, butsiltstone can grade into either.

dark colourfrom carbon

silt-sizedgrains

Flagstone

This is a type of sandstone that easily splits into flat sheetsalong certain bedding planes. These surfaces are rich insmall, flat-lying grains of mica, chiefly muscovite, whichmay sparkle as they reflect light. Mica grains areconcentrated because they settle through water moreslowly than quartz and other minerals. Flagstones are oftenused as paving stones and for building walls. flat surface

bed a fewcentimetresthick

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Quartz, feldspar,mica.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from a mica-rich sand.SIMILAR ROCKS Flagstones are differentiatedfrom other sandstones (pp.23-27) by a highercontent of mica, and the ability to be split.

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SECTION SHOWN

GRAIN SIZE Less than 0.005mm.ESSENTIAL COMPONENTS Clay minerals, such as kaolinite, illite, andmontmorillonite.ADDITIONAL COMPONENTS None.ORIGIN Detrital, from products of chemicalweathering.SIMILAR ROCKS Loess (p.24).


A PILLAR of more-resistant sandstone isperched on greymudstones, inNamibia.

Mudstone

A grey or black rock formed from mud, mudstones containboth detrital minerals, such as quartz and feldspar, andclay minerals and carbonaceous matter. Individual grains,however, are too small to be seen without a hand lens.Some mudstones are fossiliferous,others are calcareous and reactwith acid.

Clay

Clay is a rock made of minerals such as kaolinite, illite, andmontmorillonite. When damp, clay has a sticky feel. Addingwater allows it to be moulded by hand, but more water willcause it to disintegrate. Clay is typically dark to lightgrey but a pure white variety called chinaclay can occur. Individual grains may beseen only under powerfulmicroscopes.

CALCAREOUS MUDSTONE

KAOLINITE

SHRINKAGE cracks canbe seen in this quarry

face of grey clay.

limonitegivesyellowishcolour

mud-sizedgrains

palercolour

curvedfracture

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS Fossils arecommonly well-preserved.ORIGIN Detrital, from mud.SIMILAR ROCKS Greywacke (p.27), whichhas a greater range of grain size.

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extremelyfine grains

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calcite infillof shrinkagecrack

browncalcareousmudstone

IRON sulphide nodulescan form around orreplace a fossil, as seenin this ammonite.

Septarian Nodule

Septarian nodules are structures that form withincalcareous mudstones and marls. They are boulder-shapedareas within the rock, typically 20–50cm in diameter, andcomprise of harder rock than the surrounding matrix. The

nodule is often a dark brown colour due tothe presence of iron oxides. Within

each nodule, concentric andradial shrinkage cracks arelined with light-colouredminerals, such as white or

yellow calcite.

SECTION through aseptarian nodule inbrown clay on a beach cliff.

rusty andknobbled

GRAIN SIZE Nodules 1–10cm.ESSENTIAL COMPONENTS Pyrite ormarcasite.ADDITIONAL COMPONENTS Fossils,such as ammonites. ORIGIN Chemical reactions in host rock.SIMILAR ROCKS Meteorites (p.84) but thesehave a furrowed surface.

Iron Sulphide Nodule

Both pyrite and marcasite form similar nodules in clays,mudstones, shales, and chalk. These are often sphericaland usually about 5cm in diameter. The sulphide mineralforms a radiating structure and, on first exposure to air, hasan attractive yellow colour and metallic lustre.This lustre is soonlost as bothminerals areunstable.

EXTERNAL SURFACE

radiatingcrystals

outeroxidizededge

metalic lustreof pyrite

GRAIN SIZE Nodules 10cm–1m.ESSENTIAL COMPONENTS Calcite,

celestine.ADDITIONAL COMPONENTS None. ORIGIN Dehydration effects during rockformation.SIMILAR ROCKS Nodules are common inmany rocks; “septaria” are the infilled cracks.

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SHALE forms irregular,crumbling cliffs ofloose weatheredfragments, due to itsfissile nature.

NOTE

The Burgess Shales of British Columbia,Canada, are 505 million years old and containfinely-preserved fossils. These are among theearliest examples of complex life recorded onEarth and include a large range of organisms,some related to modern species and otherstrange animals long since extinct.

fissilesheets

Shale

Shale is a highly fissile rock, meaning that it breaks upreadily into thin sheets. Otherwise it resembles mudstonein grain size and colour. Shales are frequently fossiliferousand may contain bivalves, other molluscs, and plantfossils. Oil shales are rich in hydrocarbons and are ofeconomic importance. Black shales, also rich in carbon andits compounds, form in muddy water. They often haveconcretions of pyrite or gypsum that have formed aftersedimentation. Alum shales, common innorthern Europe, contain aluminium salts.These salts were once used in the tanningand dyeing industries.

brachiopod

fine, greyrock

aluminium-rich mineral

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Quartz, clays,mica.ADDITIONAL COMPONENTS Carbon(graphite), alum, fossils common. ORIGIN Detrital, from mud, clay, and organicmaterial.SIMILAR ROCKS Mudstone (p.29).

OIL SHALE

ALUM SHALE

hydrocarbondrops

FOSSILIFEROUS SHALE

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KARST topography istypical of limestoneoutcrops, here the lightgrey stone isCarboniferousLimestone.

greycolour

shell fragment

CRINOIDAL LIMESTONE

SECTION SHOWN

Limestone

Limestones are yellow, white, or grey rocks composedprimarily of the mineral calcite. They can easily beidentified by their reaction with dilute hydrochloric acid,which results in a rapid release of carbon dioxide gas and a fizzing sound. Calcite may be in the form of lime mud(micrite), detrital grains (calcarenite), tiny spheres (oolites),

calcite crystals (sparite), and/or fossils.The combination of these, leads to

a large number of textures, whichgive important clues to theenvironment in which thelimestone was formed.

FRESHWATER LIMESTONE

SHELLY LIMESTONE

very finegrains

brachiopodfossil

NOTE

Limestones are an excellent source of fossilsin rocks of all ages. Reef-forming animals,such as corals and algae, and shelledanimals such as bivalves, gastropods,brachiopods, ammonites, and echinoids are all common and sometimes form theentire rock.

GRAIN SIZE 0.005–2mm.ESSENTIAL COMPONENTS Calcite.ADDITIONAL COMPONENTS Dolomite,aragonite, quartz, and fossils common.ORIGIN Chemical and organic precipitation.SIMILAR ROCKS Dolomite (p.34), which does not react vigorously with dilute acid.

crinoidstem

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gastropod(snail)

Oolite

This is a type of limestone made of spherical grains ofcalcium carbonate. These are usually about 1–2mm indiameter and white or yellow in colour. The ooliths usuallyhave a concentric structure and have been formed by anucleus grain rolling around in lime-rich shallow seawater. Sometimes the ooliths are larger, and therock is then known as pisolite.


Chalk

A pure white fine-grained limestone, which formed in cleanshallow-water marine conditions by the accumulation ofthe hard calcium carbonate-rich skeletons of very tinyorganisms. Typical of the Cretaceous period, thick chalksare found in southern England. They contain layers of flint

nodules. Common fossils include echinoids,bivalves, and ammonites.

WHITE cliffs of chalk(here in East Sussex),are found along theEnglish Channel coast.

GRAIN SIZE Less than 0.005mm.ESSENTIAL COMPONENTS Calcite.ADDITIONAL COMPONENTS Fossilscommon.ORIGIN Accumulation of minute fossils inquiet marine conditions.SIMILAR ROCKS Chalk is a type of limestone(p.32), recognized by its pure white colour.very fine

white grains

WELL-BEDDED yellowoolitic limestone is seen in these quarry walls.

oolith

GRAIN SIZE 0.5–2mm.ESSENTIAL COMPONENTS Calcite.ADDITIONAL COMPONENTS Fossils,including bivalves.ORIGIN Chemical precipitation in shallow water.SIMILAR ROCKS Oolite is a type of limestone (p.32), containing ooliths.

SECTION SHOWN

large oolith lime mudmatrix

CHALK WITH FOSSIL

echinoid

PISOLITE

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WEATHERED outcropsof nodular grey marlare found in theBadlands, central USA.

THE DOLOMITES, amountain range insouth central Europe,give their name to the rock type.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Mud, calcite.ADDITIONAL COMPONENTS Fossils,gypsum, iron oxide minerals.ORIGIN Detrital, from lime mud.SIMILAR ROCKS Limestone (p.32) is softer;mudstone (p.29) has less calcite.

Dolomite

This rock is also known as dolostone to distinguish it fromthe mineral dolomite, which is a major constituent. It lookslike limestone but contains more crystalline material, anddoes not react with dilute acid unless finely powdered.Dolomite is often yellow-grey or brown. It has formed bythe reaction of magnesium-rich fluids passingthrough limestone before it hashardened into a rock.

fine, even-sized grains

browncolour

fine grain size

grey colour

RED MARLred ironoxides

MICROGRAPH

dolomitecrystal

Marl

A mudstone or clay that reacts with dilute acid because ofits high calcium carbonate content, marls are usually lightcoloured and may be grey, green, red, or variegated. Theyare often nodular with the nodules being better cementedthan the surrounding rock. Marls are commonly found in

lakes and other shallow water settings, such as lagoons.

GRAIN SIZE 0.005–2mm.ESSENTIAL COMPONENTS Dolomite.ADDITIONAL COMPONENTS Calcite, quartz.ORIGIN Chemical change of limestone, by magnesium-rich solutions.SIMILAR ROCKS Limestone (p.32), but unlikedolomite, it reacts vigorously with dilute acid.

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FORMED by theoxidation of a sulphidedeposit to iron oxidesminerals, ironstone isfound in Colorado, USA.

Ironstone

Sandstones and limestones that are very rich in hematite,goethite, siderite, chamosite, or other iron minerals, are known as ironstones. These minerals give the rock a dark red, brown, green, or yellow colour. They usuallyform by space-filling or replacement ofprevious minerals.

Bog Iron Ore

Marshes and shallow lakes in Scandinavia and Canada arethe source of a rich ironstone forming at the present day. It is typically a brown-yellow mudstone with yellow, red,brown, or black concretions of iron oxides and hydroxides,and contains up to 70 per cent Fe2O3. It was formerly usedas an ore, hence the name, but is rarely of suitable qualityfor modern iron production. Bog iron oreoften contains carbonaceousplant material, sometimespreserved by ironminerals.

SWAMPS in temperateclimates can be thesource of bog iron ore,where iron isconcentrated bychemical and

biological processes.

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Hematite,goethite, siderite.ADDITIONAL COMPONENTS Quartz orcalcite of host rock.ORIGIN Chemical change of limestone orsandstone, by iron-rich solutions.SIMILAR ROCKS Limestone (p.32).

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Mud, ironminerals especially goethite.ADDITIONAL COMPONENTS Plants.ORIGIN Chemical and bacterialconcentration of iron in swamps.SIMILAR ROCKS Mudstone (p.29), but thishas little iron-staining.

greymudstone

oxides andhydroxidesof iron

redhematite

ooliticgrains

MICROGRAPH

porespace

iron onoolith

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sandgrains

LATERITE is formed intropical and desertareas where heat playsa major role in theweathering process.

Laterite

A hardened iron and aluminium-rich nodular soil, laterite isformed in tropical and desert climates. The nodules oflaterite are red-brown or yellow, and contain grains of sandor hardened clay. In desert climates, it may have a shiny

surface, polished by the wind.It is formed by evaporationand leaching of minerals

from loose sediments andsoil, leaving behind

insoluble salts.

Banded Ironstone

Also known as banded ironformation (BIF), this rock is madeup of thin layers of alternatingred, brown, or black iron oxides(hematite or magnetite), andgrey or off-white chert. It is a very fine-grained rock thatbreaks into smooth splinterypieces. It is particularly abundant

in Precambrian rocks (more than500 million years old) and canform very thick sequences, such asin the Hammersley Range ofAustralia. It is as economicallyimportant as iron ore.

GRAIN SIZE Not granular or very fine.ESSENTIAL COMPONENTS Hematite,magnetite, chalcedony.ADDITIONAL COMPONENTS None.ORIGIN Chemical precipitation of ironminerals and silica.SIMILAR ROCKS Chert (right) does not haveas much red-coloured banding.

BANDED ironstones are often interbeddedwith chert and othersedimentary rocks, asin this example fromZimbabwe.

magnetite and chert layer

hematite-rich layer

GRAIN SIZE Not granular.ESSENTIAL COMPONENTS Hematite,magnetite, chalcedony.ADDITIONAL COMPONENTS Sand; clay.ORIGIN Chemical precipitation of ironminerals and silica.SIMILAR ROCKS Chert (above right) does nothave as much red-coloured banding.DESERT-VARNISHED

LATERITE

iron oxideminerals


polishedsurface

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Flint

Flint is a variety of chert that occurs in chalk as nodules,often in bands parallel to the bedding. These nodules

are irregular but rounded. Flint breaks with a conchoidal fracture and is

grey in colour and sometimesbanded. It is more resistant

to weathering thanchalk, so can form

thick beachpebble beds.

BLACK flint nodules,coated with white chalk, stand out against a background ofcream-coloured chalk.

conchoidalfracture

surfaceflaking

chalk

Chert

Chert is a rock composed of the mineral chalcedony. It is most commonly grey, white, brown, or black. It breaksalong flat to rounded, smooth surfaces and has a glassyappearance. It may occur as beds, or as nodules. Chertforms by precipitation from silica-richfluids and colloids and may fillfractures in lavas.

BEDS of chert invarious shades of grey;the orange streak isiron-staining fromsurface water.

vitreouslustre

flat fracturesurface

SECTION SHOWN

MICROGRAPH

chalcedony microcrystals

GRAIN SIZE Not granular.ESSENTIAL COMPONENTS Chalcedony.ADDITIONAL COMPONENTS None.ORIGIN Chemical precipitation of silica.SIMILAR ROCKS Flint (below), which has aconchoidal fracture.

GRAIN SIZE Not granular.ESSENTIAL COMPONENTS Chalcedony.ADDITIONAL COMPONENTS Sometimesfossils, such as sponges.ORIGIN Chemical precipitation of silica.SIMILAR ROCKS Chert (above), which has a flat fracture. Chalk coating is a key feature.

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grey-browncolour


Diatomite

A low-density rock made out of opal, Diatomite is derived from the remains of the hard parts of minuteanimals called diatoms. It can form in quiet marine andnon-marine conditions by the accumulation of millions of

tiny spiky exoskeletons, usually too small tobe seen even with a hand lens. The

hardness, size, and shape of theparticles make diatomite

a useful abrasive. It isalso used as a filter in

water purification.

DEPOSITS of diatomitefound at Lough Neagh,Northern Ireland wereformed when the lakecovered a larger area.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Opal.ADDITIONAL COMPONENTS None.ORIGIN Accumulation of silica-richorganisms.SIMILAR ROCKS Pumice (p.66), which isusually grey in colour, and associated withglassy volcanic rocks.

loose porousstructure

rough texture

Geyserite

Geyserite is a hard white, cream, or pink rock that forms inactive volcanic districts. It is also known as siliceous sinterand results from the evaporation of water from a silica-rich

solution, leaving a precipitate of opal. It iscommon around geysers, hence its

name, but also forms whereverwater escapes from or comes intocontact with hot lava.

SECTION SHOWN

GRAIN SIZE Not granular.ESSENTIAL COMPONENTS Opal.ADDITIONAL COMPONENTS Metal oxideminerals, such as hematite.ORIGIN Precipitation from hot water involcanic areas.SIMILAR ROCKS Travertine (p.39), but thisreacts with acid.

GEYSERITE depositsaround Clepsydra, ageyser in YellowstoneNational Park, USA.

multicoloured layers

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bandedstructure

Tufa

Tufa is poorly-cemented, soft, porous, white calciumcarbonate (usually calcite) that precipitates out of calcium-rich water by evaporation. It is not bedded andmay be stained yellow or red by iron oxides. It occurs inlimestone areas in temperate climates, but is ubiquitous indry river beds and desert surfaces in hotter climes.Where it is associated with loosesediments, it is known as calcrete.

Travertine

A compact and crystalline variety of calcium carbonate,travertine is normally creamy-white but may be stained bymetal salts. It often occurs in rounded, banded, or botryoidalstructures. It can be a product of hot springs,but is more usuallyseen underground incaves, where itcovers walls andforms stalagmitesand stalactites, avariety known asflowstone.

FLOWSTONE deposits in caves are made oftravertine, as seen inthis spectacularexample.

TUFA forms spires ofrock due to a reactionbetween underwater,spring water, and lakewater in Mono Lake,

California, USA.

petrifiedplant stem

GRAIN SIZE Less than 1mm.ESSENTIAL COMPONENTS Calcite.ADDITIONAL COMPONENTS None.ORIGIN Precipitation from calcium-richsolutions.SIMILAR ROCKS Travertine (below) is moresolid and often banded.

YELLOW TUFA

looselycemented

soft,powderytexture

GRAIN SIZE Wide range.ESSENTIAL COMPONENTS Calcite.ADDITIONAL COMPONENTS None.ORIGIN Precipitation from calcium-richsolutions.SIMILAR ROCKS Tufa (above), which has alighter density and is more porous; alabaster(gypsum rock, p.42), which is softer.

banding of iron-stained calcite

flowstoneSTALACTITE

calcitecrystals


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GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Carbon.ADDITIONAL COMPONENTS Fossils arecommon.ORIGIN Accumulation of organic debris.SIMILAR ROCKS Black shale (p.31) islaminated; lignite (below) and anthracite(right) do not break into rectangular lumps.

Coal

This low-density black rock is formed by the compression of thick layers of plant material. Peat is an intermediatestage between vegetation and coal, in which the plantmatter is still visible. In coal, the plant matter has been

reduced to carbon, although traces of plant fossilsmay remain. Depending on the grade, coal

may be sooty and powdery, orcompact and shiny.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Carbon, organicmaterial.ADDITIONAL COMPONENTS None.ORIGIN Accumulation of organic debris.SIMILAR ROCKS Other coals (above) areblack, rather than brown, and contain lessplant materials.

MINING of ligniteoccurs on a vast scale,as seen here inGermany.

Lignite

A low grade brown-coloured coal, lignite is a product of the stage between peat and black coal. Some plantmaterial is still visible and, although consolidated, it easilycrumbles. It has a lower density than other coals and awoody appearance. Most coal deposits are found inCarboniferous Age rocks, but lignites are usually muchyounger, though still economically important. This is

partly because gas and liquid petroleum productsare easily extracted from lignites.

SMALL coal mines, likethis, are rarer now thatlarger and moremechanized mines produce cheaper coal.

black colour

woodytexture

saltdeposits

SECTION SHOWN

PEAT

plantmaterial

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rectangular-shapedpiece

Anthracite

A high grade black and shiny coal, anthracite does not dirtythe fingers and breaks with a conchoidal fracture. It has ahigher carbon content than other coals, and so lights andburns at a higher temperature. Jet is a similar rock, hardand black, and with a conchoidal fracture, but with abrown streak. Jet occurs in layers in bituminous shales,notably at Whitby, England.

Amber

Another organic ‘rock’, amber is the fossilized resin of pinetrees. It is a soft, transparent to translucent yellow-orangesubstance that has a density similar to water and may infact float. Sometimes fossil insects, and other animals, andplants, are preserved in amber. It is foundworldwide, but much ambercomes from the Baltic Sea.

conchoidalfracture

THE BALTIC SEA coast,is a source of theworld's largest andhighest quality amber deposits.

BIG PIT, Blaenafon,part of the South Walescoalfield, is famous forits high gradeanthracite deposits.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Carbon.ADDITIONAL COMPONENTS None.ORIGIN Accumulation of organic debris.SIMILAR ROCKS Coal (left), but this ispowdery.

JET LAYER IN SHALE

shiny, blackcolour

GRAIN SIZE Not granular.ESSENTIAL COMPONENTS Amber.ADDITIONAL COMPONENTS Trappedanimals. ORIGIN Solidification of fossil resin.SIMILAR ROCKS Amber, unlike similarsubstances, softens and then burns with apine smell when heated.

trappedspider

resinouslustre


hardsurface

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orange colour

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EVAPORITES, includingrock salt, potash, andgypsum rock, areforming along thecoasts of the Dead Sea – the world’ssaltiest sea.

cubic crystals

orange colourfrom iron-richclays

sylvitecrystals

blue andwhite halite

NOTE

The minerals found in evaporite rocks can beseen on pages 171–75. Many of these occurtogether. Some continue to form in aridclimates, such as the Dead Sea area today,but others are only found in ancient evaporites,such as the 270 million year old deposits atStassfurt in Germany.

Evaporites

The commonest of the evaporite rocks is rock salt, whichlike the other evaporites, results from the evaporation ofsea water. Rock salt (formed of the mineral halite) iscolourless to orange, or rarely blue, and is soluble in water.The evaporite rock gypsum occurs as massive white layers,often in marl, or banded in light colours and is sometimesknown as alabaster. Potash rock is formed of solublepotassium salts, such as sylvite and carnallite often mixed with halite. It is granular, pink to brown in colour and has a greasy feel.

GRAIN SIZE Not granular to more than 1cm.ESSENTIAL COMPONENTS Halite, gypsum,or potassium minerals.ADDITIONAL COMPONENTS Clay. ORIGIN Evaporation of sea water.SIMILAR ROCKS Travertine (p.39), marble(p.77) but these are harder, have no taste,and are insoluble.

GYPSUM ROCKRARE BLUE ROCK SALT POTASH ROCK

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In the pages that follow, igneous rocks are arranged in theorder: intrusive, extrusive, and pyroclastic. Intrusive igneousrocks are those that have crystallized beneath the Earth’ssurface and are coarse-grained. Extrusive rocks, such as thoseof the Giant’s Causeway, Northern Ireland (below), are solidifiedlava and have a fine grain size. Each rock type is listed bydecreasing silicon dioxide content: high (acid), intermediate,and low (basic), followed by more unusual compositions.Pyroclastic rocks are formed from volcanic ejecta.

Igneous Rocks

PORPHYRITIC GRANITE

PUMICEPERIDOTITE SNOWFLAKEOBSIDIAN

4 4 I G N E O U S R O C K S

feldspar

quartz

granulartexture

phenocryst

GRAIN SIZE 2–5mm, phenocrysts to 10cm.ESSENTIAL COMPONENTS Quartz, alkalifeldspar, plagioclase.ADDITIONAL COMPONENTS Biotite,muscovite, hornblende, apatite.ORIGIN Crystallization of an acid magma ina major intrusion.SIMILAR ROCKS Diorite (p.48).

NOTE

Granite is a hard-wearing rock and is used asa building stone, both in blocks and inpolished slabs. Some especially attractiveexamples are porphyritic ones from Shap andfrom Dartmoor, in England, or the ones withred feldspars from Bushveld, South Africa, andAberdeen, Scotland.

SECTION SHOWN

PORPHYRITIC GRANITE

HORNBLENDE GRANITE

Granite

Granite is a common and familiar rock. Like other coarse-grained igneous rocks, its essential minerals can beidentified with the naked eye or a hand lens. For granite,these are quartz – usually in rounded glassy grains – andfeldspars. Feldspars are more or less rectangular, plagioclase,usually white, and alkali feldspars pink. Black grains ofhornblende and/or biotite are the most common dark-coloured minerals. In porphyritic granite some of thefeldspars, known as phenocrysts, are much larger than

other grains. Granite forms by the cooling of magmabelow the earth’ssurface and is thecommonest rocktype of thecontinents.

THESE TORS wereformed by theweathering of a graniteoutcrop along verticaland horizontal joints.

hornblende

WHITE GRANITE

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I G N E O U S R O C K S 4 5

ORBICULES are usuallyrestricted to a smallarea within a largergranite mass.

Orbicular Granite

This is an unusual but spectacular rock, characterized byspheres of concentric layers of granitic minerals withingranite. The orbicules are typically about 5–10cm indiameter and often richer than granite in darkerminerals. Excellent examples are foundin Finland. An example from Australiahas been quarried to provide polishedstone. Orbicular diorites are also found.

ORBICULAR DIORITE

GRAIN SIZE 2–5mm, orbicules 2–20cm.ESSENTIAL COMPONENTS Quartz,plagioclase, alkali feldspar, biotite.ADDITIONAL COMPONENTS Hornblende.ORIGIN Concentric growth within a granite magma.SIMILAR ROCKS This is a type of granite(left); orbicular diorite has no quartz (p.143).

granite

orbicule

biotite

GRAIN SIZE 2–10mm, or larger.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS None.ORIGIN Simultaneous crystallization ofquartz and alkali feldspar.SIMILAR ROCKS Granophyre (seemicrogranite, p.47).

feldspar andhornblende

Graphic Granite

In some granites, and also pegmatites, the minerals areintergrown in such a way that straight-sided quartzcrystals, which look like hieroglyphic characters, are set ina background of feldspar. The composition is roughly 30 per cent quartz and 70 per centfeldspar, with fewother minerals.The texture formswhen both mainminerals crystallizefrom the magma atthe same time.

feldspar

PEGMATITE veins withgraphic granite andgraphic amazonite havebeen found at PikesPeak, Colorado, USA.

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quartz


micafeldsparquartz

DARK gray amphibolitegneiss is cut by a seriesof minor intrusions of pegmatite. Theirregular shapes aretypical.

GRAIN SIZE More than 5mm to many metres.ESSENTIAL COMPONENTS Quartz, alkalifeldspar, plagioclase.ADDITIONAL COMPONENTS Mica, apatite,fluorite, beryl, tourmaline, cassiterite, etc.ORIGIN Fluid-rich crystallization in the finalstages of the formation of a granite.SIMILAR ROCKS Granite (p.44).

NOTE

Pegmatites are sometimes associated with aneven, medium- to fine-grained sugary-texturedrock called aplite. It has a similar compositionto granite (p.44), but little or no biotite (p.161),hornblende (p.163), or other dark-colouredminerals. Like pegmatite, it may contain topaz(p.178) or tourmaline (pp.180–81).

Pegmatite

Pegmatites are very coarse-grained rocks, mostly of agranitic composition, with individual crystals that rangefrom smaller than one centimetre to several metres in size.They are light-coloured rocks and occur in small igneousbodies, such as veins and dykes, or as patches in largermasses of granite. Quartz and feldspar dominate but many other minerals can form large beautiful crystals.Muscovite and other micas are commonly seen and occurin large flat sheets known as books. Some gemstones aremined from pegmatites, such as emerald, aquamarine,tourmaline group minerals, and topaz. Other pegmatitescontain important ore minerals that are the source ofindustrial metals, such as lithium, tin, tantalum, and tungsten.

GRANITE PEGMATITE

muscovitefeldspar(microcline) tourmaline

TOURMALINE PEGMATITE MICA PEGMATITE

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WELL-JOINTED

microgranite in aquarry at Threlkeldmining museum,Cumbria, England.

Microgranite

Microgranite is identical to granite in all but grain size andits occurrence in minor intrusions, such as sills and dykes.As with granites, porphyritic varieties are common. Inmicrogranite, granophyre, quartz, and feldspar areintergrown in the same way as in graphic granite, but this is only visible under a hand lens.

pinkfeldspar

mediumgrained

fine graphictexture

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Quartz, alkalifeldspar, plagioclase.ADDITIONAL COMPONENTS Biotite,muscovite, hornblende, apatite.ORIGIN Crystallization of an acid magma ina minor intrusion.SIMILAR ROCKS Granite (p.44).

GRANOPHYRE

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Muscovite,quartz.ADDITIONAL COMPONENTS Fluorite,tourmaline, topaz.ORIGIN Alteration of granite by hot fluids.SIMILAR ROCKS Mica schist (p.78), occurson a regional scale; aplite (see note, left).

Greisen

This rock is produced by fluids altering a granite in thefinal stages of crystallization. It is a coarse- to medium-grained mixture of muscovite and quartz, often withfluorite and minerals found in pegmatites. Similar rockscan be mixtures of quartz and tourmaline. All of thesetypes are common in dykes, veins, and marginal areas ofthe granites of south-west England, and elsewhere.

pegmatite

IRREGULAR quartz veinsoften cut throughgreisen rocks and thesemay carry ore minerals.

greisen

aplite

muscovite

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light beigematrix

BOTH diorite andgranodiorite occur inlarge intrusions, such as here in theAustrian Alps.

Felsite

Felsite is a general term for medium- to fine-grained, light-coloured, pink, beige, or grey igneous rocks fromsmall intrusions, such as sills and dykes. Blocky jointing iscommon, and occurs perpendicular and parallel to thewalls of the intrusion. Felsite may be slightly porphyritic,

with small phenocrysts, oftenof quartz, or it may

contain sphericalstructures.

COLUMNAR jointing in a thick sill of felsite isshown here on thenorthern coast of theIsle of Eigg, Scotland.

Diorite

An intermediate, coarse-grained igneous rock, dioriteconsists of white plagioclase and dark hornblende inroughly equal proportions, but other dark minerals mayinclude biotite and augite. With the addition of small

amounts of quartz and alkali feldspar itbecomes a granodiorite; with largeramounts, a granite. These three rock typesoften occur together in large intrusions.

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Plagioclase,hornblende.ADDITIONAL COMPONENTS None.ORIGIN Crystallization of an intermediatemagma in a major intrusion.SIMILAR ROCKS Syenite (top right), whichhas more alkali feldspar.

GRAIN SIZE Less than 2mm.ESSENTIAL COMPONENTS Quartz,feldspars.ADDITIONAL COMPONENTS None.ORIGIN Crystallization of an acid orintermediate magma in a minor intrusion.SIMILAR ROCKS Rhyolite (p.56), dacite(p.59), and porphyry (p.60).

MICROGRAPH

plagioclase

quartzphenocryst


hornblende

minerals in equalproportion

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Syenite

Syenites are attractive, multi-coloured rocks, which may bepolished and used as a decorative stone, as in the case ofNorwegian larvikite. Alkali feldspar is the main component.Many other minerals, such as plagioclase, biotite, pyroxene,and amphibole group minerals, minorquartz, or nepheline, canaccompany it.

attractive polishedsurface

hornblende

alkalifeldspar

DIFFERENCES in grainsize and compositionare typical across anoutcrop of syenite.

Nepheline Syenite

Both nepheline and alkali feldspar are essential minerals innepheline syenite, but this intermediate rock can containmany others, including unusual and attractive ones, such as eudialyte. Nepheline is typically brownishwhite with square crystals; alkali feldspar is white and rectangular. If the nephelinesyenite includes a pyroxene, itis usually aegirine and if anamphibole, it is commonlyarfvedsonite; both of whichare rich in sodium.

alkalifeldspar

IN ADDITION tonepheline, this examplecontains alkali feldspar,pyroxene, sodalite, and

andradite.

SECTION SHOWN

LARVIKITE

aegirine

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Alkali feldspar,pyroxene/amphibole.ADDITIONAL COMPONENTS Plagioclase,biotite.ORIGIN Crystallization of an alkalineintermediate magma in a major intrusion.SIMILAR ROCKS Diorite (below left).

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Alkali feldspar, nepheline.ADDITIONAL COMPONENTSAegirine, biotite.ORIGIN Crystallization of a low silicaintermediate magma in a major intrusion.SIMILAR ROCKS Syenite (above).

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nepheline


GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Plagioclase,augite, magnetite.ADDITIONAL COMPONENTS Olivine.ORIGIN Crystallization of a basic magma in amajor intrusion.SIMILAR ROCKS Diorite (p.48), which containshornblende (p.163), its main mineral.

Dolerite

A medium-grained rock with the same composition asgabbro. It is found in minor intrusions, sills, and dykes.Plagioclase crystals are usually tiny rectangles within largerpyroxene grains. Olivine is a common constituent, inrounded grains often weathered to an orange-browncolour. It is a hard and heavy rock, and used polished as a

decorative stone, roughfor paving, and crushedfor roadstone.

ALTERED dolerite, ordiabase, can be seen inthis irregular dyke withdarker-coloured chilledmargins.

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Plagioclase,augite, magnetite.ADDITIONAL COMPONENTS Olivine.ORIGIN Crystallization of a basic magma ina minor intrusion.SIMILAR ROCKS Basalt (p.61), occurs inlava flows and contains gas bubble holes.

plagioclase

brown weatherededge

Gabbro

A dark-coloured rock composed of coarse grains of darkgreen pyroxene (augite and lesser amounts oforthopyroxene), plus white- or green-coloured plagioclaseand black millimetre-sized grains of magnetite and/or

ilmenite. Gabbros occur in thick sills, in upliftedsections of oceanic crust called ophiolites,

and with cumulate rocks in layered igneous intrusions.

GABBRO is found in the Cuillins layeredintrusion, Skye,Scotland, in rugged

topography.

pyroxene

LEUCOGABBRO

plagioclase

SECTION SHOWN

plagioclase

pyroxene

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Cumulate Rocks

This is the name given to a number of different rock typesthat form in layered igneous intrusions, which are usuallybasic in composition. Each rock type consists of one or twoor more coarse-grained minerals, which occur in layers.Each layer can range in size from millimetres to hundreds of metres thick. The usual minerals are olivine, augite,orthopyroxenes, plagioclase, chromite, and magnetite, butmany others, such as apatite and alkali feldspar, may bepresent. To identify the combinations of mineralspresent, rock names, such as norite andtroctolite, are given.

LAYERED magnetiteand anorthosite formcumulate rock in theBushveld Complex,South Africa.

NORITE

Lora amitdolor sit

Lora amitdolor sit amit

CHROMITE-SERPENTINITE

TROCTOLITE

mottledanorthosite

serpentine

orthopyroxene

olivine

chromite

SECTION SHOWN

NOTE

Other cumulates include: troctolite (plagioclase,pp.166–67, and olivine, p.159), norite(plagioclase and orthopyroxene, p.162),chromitite (chromite, p.141) and magnetite(magnetite, p.126). Other rocks that formcumulates include gabbro (above left), dunite(p.52), anorthosite and pyroxenite (p.53).

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Any of olivine,plagioclase, orthopyroxene, clinopyroxene,magnetite.ADDITIONAL COMPONENTS Chromite,sulphide minerals, platinum group minerals.ORIGIN Crystallization in a layered intrusion.SIMILAR ROCKS Gabbro (left) is not layered.

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DARK BROWN hills ofperidotite stand outabove the surroundingland in Oman.

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Olivine, pyroxene.ADDITIONAL COMPONENTS Chromite,serpentine, spinel, amphibole, garnet.ORIGIN Crystallization of an ultrabasic magmain a major intrusion, or the Earth’s mantle.SIMILAR ROCKS Pyroxenite (right) but thismajor mineral has a cleavage.

Peridotite

Peridotite contains 50–90 per cent olivine. It is a coarse-grained light to dark green rock, which usually containspyroxene. Unlike olivine, pyroxene grains have a visiblecleavage when viewed under a hand lens. It forms much of the Earth’s mantle, and can occur as nodules brought

up from the mantle by basalt or kimberlitemagmas. Peridotite also occurs as

a cumulate rock.

Dunite

A green or brown coarse-grained rock consisting almostentirely of the mineral olivine, dunite also contains smallblack chromite or magnetite grains. In some countries,

if it contains magnetite, it is known as olivinite.Dunites are found as cumulate rocks in

layered intrusions and also as pipes andirregular bodies. Alteration of a dunite

by the addition of water producesa serpentinite.

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Olivine.ADDITIONAL COMPONENTS Serpentine.ORIGIN Crystallization of an ultrabasicmagma in a major intrusion.SIMILAR ROCKS Peridotite (below), containsless olivine (p.159).

THIS OUTCROP inCyprus, showschrysotile veining andthe contrast betweenfresh green and alteredbrown dunite.

brownweatheredolivine

SECTION SHOWN

GARNET PERIDOTITE

red pyrope

darkolivineandpyroxenecrystals

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green olivine

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Pyroxene.ADDITIONAL COMPONENTS Plagioclase,sulphides, and platinum minerals.ORIGIN Crystallization of a basic magma in amajor intrusion.SIMILAR ROCKS Serpentinite (p.72), is softerand fine-grained.


Anorthosite

A coarse-grained rock composed of more than 90 per centfeldspar, while pyroxene often forms most of the rest ofthe rock. Anorthosite is white or grey but it can be greencoloured and has a granular texture. It occurs in layeredigneous intrusions as a cumulate rock, but also in largemetamorphic gabbro-anorthosite complexes of thePrecambrian Era.

Pyroxenite

This is a coarse-grained, granular rock that contains at least90 per cent orthopyroxene, clinopyroxene or both. It is ahard and heavy rock, which is light green, dark green, orblack, and the surface often weathers to rusty brown.Pyroxenites may also contain olivine and oxide mineralswhen they occur as cumulaterocks in layered intrusions,or nepheline inalkaline intrusions.

RUM intrusion inScotland, is an exampleof where pyroxenitesform prominentresistant layers.

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Plagioclase.ADDITIONAL COMPONENTS Pyroxene,magnetite.ORIGIN Crystallization of a basic magma in amajor intrusion.SIMILAR ROCKS Marble (p.77), is softer andreacts with dilute acid.

pyroxene

sulphide

A LAYER of whiteanorthosite sits abovea weathered gabbrofrom the BushveldComplex, South Africa.

plagioclase

mottledorthopyroxene

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plagioclasefeldspar


GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Hornblende/Biotite.ADDITIONAL COMPONENTS Titanite.ORIGIN Crystallization of an alkaline magmain a minor intrusion.SIMILAR ROCKS Dolerite (p.50), does notcontain biotite (p.161) or hornblende (p.163).

BIG HOLE, Kimberley,South Africa, is anabandoned diamond minein a large kimberlite pipe.

Lamprophyre

Usually black when fresh, lamprophyre is hydrothermallyaltered and then weathers to a brown, green, or yellowcolour. It is a porphyritic rock with black or dark brownphenocrysts of mica and/or amphibole, but never withfeldspar phenocrysts like most other porphyries. It occursin minor intrusions, especially dykes. Common varieties

are alnöite (phlogopite, augite, andolivine phenocrysts) and minette

(phlogopite only).

BLOCKY jointing makesthis lamprophyre dykestand out from thesurrounding rock.

micaflake

brown colour

Kimberlite

Kimberlite is a rare rock, best known for being the majorsource of diamonds. It is found in dykes and carrot-shapedpipes called diatremes, up to a kilometre in diameter.When fresh, it is a blue-green colour but it weathers toyellow. It is coarse to fine-grained and contains clasts, or is

brecciated. Common minerals as phenocrysts includepurple pyrope, bright green chromium-rich

diopside, chromite, calcite, and phlogopite.

SECTION SHOWN

megacryst infiner matrix

yellowcolour

WEATHERED ROCK

brown weatheredsurface

WEATHERED KIMBERLITE

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GRAIN SIZE Wide range.ESSENTIAL COMPONENTS Serpentine,phlogopite, calcite, chromite.ADDITIONAL COMPONENTS Chrome-richdiopside, almandine, diamond.ORIGIN Extrusion of a fluid part of theEarth’s mantle.SIMILAR ROCKS Tuff (p.65).

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GRAIN SIZE Any size.ESSENTIAL COMPONENTS Country rock.ADDITIONAL COMPONENTS Thermalmetamorphic minerals.ORIGIN Incorporation of country rocks into amagma without melting.SIMILAR ROCKS Volcanic breccia (p.62),clasts are angular.

magnetite

granite

GRAIN SIZE Wide range.ESSENTIAL COMPONENTS Calcite and/ordolomite.ADDITIONAL COMPONENTS Magnetite,apatite, phlogopite, rare earth minerals.ORIGIN Crystallization of a carbonatemagma derived from the Earth’s mantle.SIMILAR ROCKS Limestone (p.32).

Carbonatite

Carbonatite is an unusual rock type – an igneous rockcomprising over 50 per cent carbonate minerals – usuallycalcite, dolomite, or siderite. It looks like marble, and maybe coarse-grained, if intrusive, or fine-grained if volcanic.Typically cream-coloured, yellow or brown, it may containphlogopite, magnetite (often as octahedral crystals) andrare-earth minerals, such as pyrochlore. It is usually foundin areas of continental rifting.

Xenoliths

Magma often incorporates fragments of the country rockthrough which it passes during intrusion. These often remainwithin the igneous rock that forms when the magma coolsand solidifies, and are known asxenoliths. Xenoliths are oftenrounded, may be any size,and may have beenmetamorphosed.

ROUNDED dark-colouredxenoliths are seen here in an outcrop of light-

coloured diorite.

CARBONATITES are ofteneconomically important,such as here atPhalaborwa copper mine, South Africa.

carbonatemineral

PERIDOTITE XENOLITH

xenolith

basalt

peridotite

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SNOWDON, the highestmountain in Wales, is built of ancientrhyolite flows, each of limited extent.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Quartz, alkalifeldspar, plagioclase.ADDITIONAL COMPONENTS None.ORIGIN Extrusion of an acid magma.SIMILAR ROCKS Andesite (p.58) and dacite(p.59), are darker coloured.

Rhyolite

Rhyolite is a fine-grained, light-coloured rock. It is oftencomposed largely of volcanic glass. Individual grains ofquartz, feldspar, and mica may be present, but are toosmall to be visible. Sometimes rhyolites are porphyriticwith millimetre-scale phenocrysts of quartz and/or feldspar.The granitic magma from which rhyolite crystallizes is veryviscous, and so, flow-banding is often preserved and can beseen on weathered surfaces. Devitrification of glassy

rhyolite produces tiny crystals arranged in radiatingspheres, usually of a centimetre or so in size: a

variety known as spherulitic rhyolite.

BANDED RHYOLITE

NOTE

The high viscosity of rhyolite magma hastwo other effects: the lava does not flow farfrom its source and forms domes; and gaspressure can build up in the magmaleading to explosive volcanicity, so rhyolitesare often associated with pyroclastic rocks,such as tuff (p.65).

dark glassclast

flowbandingquartz

phenocryst

phenocryst

pale-colouredrhyolite

MICROGRAPH

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banding


AN IRREGULAR dyke ofpitchstone, darker alongits edges, is shown herecutting a brown-

weathered basalt.

Pitchstone

This acid volcanic glass, is usually green-coloured but yellow,red, brown, and black varieties are also seen. It may breakwith either a flat or a conchoidal fracture. Small phenocrystsof feldspar or pyroxene may be present. It occurs both as avolcanic rock, as small intrusions, and as thebasal parts of rhyolite anddacite lava flows.

sharpedgeSNOWFLAKE OBSIDIAN

GRAIN SIZE Not granular.ESSENTIAL COMPONENTS Volcanic glass.ADDITIONAL COMPONENTS Quartzphenocrysts.ORIGIN Extrusion and rapid cooling of anacid magma.SIMILAR ROCKS Obsidian (below).

tinycrystals

glass

feldsparphenocryst

Obsidian

A black, acid volcanic glass with a well-developedconchoidal fracture, obsidian forms by the rapid cooling of an acid lava and is often found with grey pumice. The fragments of the rock are sharp and have been used as cutting implements. A variety withcentimetre-scale spheres of tiny whiteradiating crystals is known as snowflake obsidian.

smoothpitch-likesurface

GRAIN SIZE Not granular.ESSENTIAL COMPONENTS Volcanic glass.ADDITIONAL COMPONENTS None.ORIGIN Extrusion and rapid cooling of an acid magma. SIMILAR ROCKS Industrial glass, which is softer.

�

� conchoidalfracture

bottle-greencolour

PORPHYRITIC PITCHSTONE

THIS OBSIDIAN outcropis in Iceland, an islandwhere acid igneousrocks are rare.


DOME-SHAPED outcropsof trachyte are seen inthis volcano on theisland of Tenerife.

Trachyte

The volcanic equivalent of syenite, trachyte’s compositionis dominated by alkali feldspar. This is a major componentboth of the fine groundmass and of abundant phenocrysts,which are usually flow-aligned. It is often a vesicular lavaand may also occur as a pyroclastic rock. It is usually greycoloured and frequently banded or streaky.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Alkali feldspar,plagioclase.ADDITIONAL COMPONENTS Pyroxene,amphibole, biotite.ORIGIN Extrusion of an alkaline intermediatemagma.SIMILAR ROCKS Other porphyritic lavas.

feldsparphenocryst light-grey

groundmass

PORPHYRITIC ANDESITE

Andesite

An intermediate volcanic rock named after the Andes,andesite is usually grey coloured and porphyritic. Plagioclase,pyroxene, hornblende, and biotite may be present asphenocrysts, as well as, forming the finer groundmass. Itoccurs in the form of blocky or columnar jointed lava flows.

dark greycolour

smallphenocrysts

finegroundmass

alignmentof feldsparcrystals

MICROGRAPH

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Plagioclase.ADDITIONAL COMPONENTS Pyroxene,amphibole.ORIGIN Extrusion of an intermediatemagma.SIMILAR ROCKS Difficult to differentiate fromdacite (right).

BLOCKY-JOINTED

andesites can be seenhere in a lava field onthe island of Lanzarote.

feldsparphenocryst

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�


EXTRUSIVE andpyroclastic daciteforms much of thevolcano at Crater Lake,Oregon, USA.

Dacite

Dacite lies between rhyolite and andesite in composition.It is usually pink or a shade of grey and often flow-banded.Porphyritic varieties are common, with phenocrystsusually of plagioclasefeldspar and/or quartz.It occurs with rhyolitein continentalvolcanic districts,and with andesiteon continentalmargins.

Phonolite

Phonolite gets its name from a distinctive clinking soundthat it makes when hit with a hammer. It is the volcanicequivalent of nepheline syenite, and, like that rock, it maycontain unusual minerals. Porphyritic examples with well-developed crystals of sodalite, haüyne, and leucite areknown. It occurs in lava flows, minor intrusions,and as a pyroclastic rock. It is largely restrictedto areas of the Earth’s crust that have riftedapart, or to oceanic islands.

DEVIL'S TOWER,

Wyoming, USA is asmall intrusion or‘plug’ of phonolite that

has been exposed byerosion.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Alkali feldspar,nepheline.ADDITIONAL COMPONENTS Leucite.ORIGIN Extrusion of a low-silica intermediatemagma.SIMILAR ROCKS Similar to intermediate andbasic lavas, but rarer.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Plagioclase,quartz.ADDITIONAL COMPONENTS Pyroxene,amphibole, biotite.ORIGIN Extrusion of an intermediate magma.SIMILAR ROCKS Difficult to differentiate fromandesite (left).

hornblende

alkalifeldspar

feldsparphenocryst

pink-greycolour

quartzphenocryst

greygroundmass

SECTION SHOWN

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�


THESE PHENOCRYSTS

of feldspar in a granite porphyry havebeen aligned bymagmatic flow.

rhombicfeldspar

Porphyry

Porphyry is a general name for igneous rocks that containphenocrysts, especially if the phenocryst mineral is alsofound in the smaller crystals of the groundmass. It is mostoften used for rocks with a fine- to medium-grainedgroundmass, that have formed in minor intrusions or

lava flows. The term porphyry is often prefixed.This can be a reference to the minerals it

contains, such as quartz-feldspar-porphyry, which contains those two

minerals as phenocrysts.Alternatively, it can refer

to the composition,such as rhyoliteporphyry, or texture,such as rhombporphyry. The lattercontains feldsparswith rhombic cross-sections.

fine darkgroundmass

whitephenocryst

GREEN PORPHYRY

GRAIN SIZE Less than 0.1mm, phenocryststo 2cm.ESSENTIAL COMPONENTS Phenocrysts.ADDITIONAL COMPONENTS None.ORIGIN Two-stage crystallization of anigneous rock.SIMILAR ROCKS Any lava or minor intrusive rock.

RHOMB PORPHYRY

NOTE

Porphyries have been used as decorativerocks for several millennia because of theirhardness and beauty. A purple example fromEgypt, Imperial Porphyry, was much prized byRoman Emperors, to whom the colour signifiedauthority. The green porphyry (above) was alsoused in Ancient Rome, sourced from Greece.

SECTION SHOWN

rectangularfeldspar

�


fine grainsize

EXCEPTIONAL columnar-jointed basalt can beseen at Fingal’s cave,Staffa, Scotland.

VESICULAR BASALT

PORPHYRITIC BASALT

AMYGDALOIDAL BASALT

Basalt

Basalt is the most common lava in the Earth’s crust,forming almost all of the ocean floor and also large areason land. It is a fine-grained rock and, when fresh it is blackin colour, weathering to dark green, or brown. Somebasalts are porphyritic with feldspar, augite, and olivinephenocrysts being common. Gas bubble holes give avesicular texture, and these may be filled with laterminerals, known as amygdales. The surface of basalt lava

may be ropy and smooth,or blocky and rough.

columnar-jointedmassive basalt

amygdale

SECTION SHOWN

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Plagioclase,augite, magnetite.ADDITIONAL COMPONENTS Olivine,zeolites.ORIGIN Extrusion of a basic magma.SIMILAR ROCKS Dolerite (p.50) is coarserand occurs in minor intrusions.

phenocryst

Basalt lava flows can cover huge areas andform great thicknesses of rock. Amongst thelargest masses of basalt seen on land is theDeccan Traps of India, today covering morethan half a million square kilometres. Thesevolcanic rocks contain many of the beautifulzeolite group and related minerals.

NOTE

vesicle �


DARK red vesicular andnon-vesicular purplelava with flow lines,make up this volcanicbreccia.

Scoria

The top of a lava flow is made up of a highly vesicular,rubbly material called scoria. It has the appearance ofvesicular lava but is usually weathered to a brown colour,and forms piles of loose rubble with small pieces. It iscommon in areas of recent volcanism, such as the Canary

Islands and the Italian volcanoes, and is usually basalticor intermediate in composition. High drainage means

that areas of scoria often lack vegetation.

CINDER cones canproduce largequantities ofscoriaceous lava.

Volcanic Breccia

A brecciated rock formed by the interaction of lava andscoria, or by the mixture of cooled lava and flowing lava. Ittakes the form of centimetre-scale angular clasts of oftenvesicular lava in a more compact matrix. It commonlyforms at the top of a lava flow, and is then known as flow-top breccia. It is especially commonbetween basaltic lava flows.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Basalt.ADDITIONAL COMPONENTS None.ORIGIN Rubbly top of an extruded basic magma.SIMILAR ROCKS This rock is a type of basalt (p.61), but differs in beingunconsolidated.

GRAIN SIZE Less than 0.1 mm, clasts0.5–20cm.ESSENTIAL COMPONENTS Basalt.ADDITIONAL COMPONENTS Volcanic glass, spilite.ORIGIN Mixing of liquid and solid materialduring crystallization of a basic magma.SIMILAR ROCKS Agglomerate (p.64).

SECTION SHOWN

vesicular lava

oxidisedlava

many loose pieces

SECTION SHOWN

�

vesicle

�


POINTED bottoms androunded tops of this

pillow lava, show ithas the same

orientation aswhen the lavawas extruded.

Pillow Lava

When basalt is extruded under water it forms a ball of lavawith a thin solid skin. The ‘balls’ are typically 0.2–1m indiameter, and settle on top of one another,becoming pillow-shaped. They have basaltinteriors with chilled edges of finer grainedbasalt, often altered. Sometimes theyhave radially-oriented vesicles. Rubblymaterial or quartz fills the small spacesbetween pillows.

Spilite

A spilite is an altered basalt, usually by reaction with seawater. Calcium-rich plagioclase is converted to albite. Theedges of pillow lavas are often spilite, or it may result fromhydrothermal alteration by seawater circulatingthrough hot volcanic rocks. Spilite is lighter incolour than basalt, veined by calcite orchalcedony, and vesicles and cavities arefilled with secondary minerals.

A REACTION betweenmolten lava and sea

water, seen herein Hawaii,

producesspilite.

filledvesiclesand vein

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Plagioclase(albite), augite, magnetite.ADDITIONAL COMPONENTS None.ORIGIN Alteration of basalt by sea water.SIMILAR ROCKS Basalt (p.61) but is darkerand harder.

IRON-RICH SPILITE

alteredbasalt

veining

flowstructure

alterededge

GRAIN SIZE Less than 0.1mm, pillows0.1–1m.ESSENTIAL COMPONENTS Basalt.ADDITIONAL COMPONENTS Spilite.ORIGIN Underwater extrusion of a basic magma.SIMILAR ROCKS This rock is a mixture ofbasalt (p.61) and spilite (below).

quartzveining

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�

WHOLE PILLOW


BOTH the main andparasitic side vents canbe filled with ventagglomerate.

Agglomerate

An agglomerate is a pyroclastic rock in which coarse,centimetre-scale, rounded to sub-rounded clasts lie in amatrix of lava or ash. The clasts themselves may be lava orpyroclastic rocks, or they may be the country rocks, whichsurround and lie beneath the volcano. Rounding of the

clasts may have occurred in the magma, duringeruption, or by later sedimentary re-working.

COARSE angularfragments from severalsources can make upan agglomerate.

Vent Agglomerate

This is the rock which plugs the main or a satellite vent of a volcano. The outcrop of this rock is circular on ageological map and of limited extent. The rock itself, likeother agglomerates, contains a variety of clasts of differentsizes, shapes, and compositions from the lava, othervolcanic rocks, and country rocks. The matrix will be a fine-grained igneous rock.

GRAIN SIZE Less than 0.1mm, clasts0.5–20cm.ESSENTIAL COMPONENTS Igneous rockfragments.ADDITIONAL COMPONENTS Country rock.ORIGIN Pyroclastic accumulation of coarse material.SIMILAR ROCKS Vent agglomerate (below).

GRAIN SIZE Less than 0.1mm, clasts5mm–several metres.ESSENTIAL COMPONENTS Igneous andcountry rock fragments in lava.ADDITIONAL COMPONENTS None.ORIGIN Accumulation of material in avolcanic vent.SIMILAR ROCKS Type of agglomerate (above).

SECTION SHOWN

mixed country rockand igneous clasts

small igneousclasts

red dolomiteclast

carbonatitelava

fine-grainedash

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�lava matrix

CARBONATITE AGGLOMERATE


Tuff

Acid and intermediate magmas are more viscous than basicones, and this means that the volcanicity associated withthese magmas is more explosive, as greater pressures canbuild up. The major product of explosive volcanism isvolcanic ash which, when settled, forms a rock called tuff.The material comprises a mixture of crystal fragments, bitsof lava and pyroclastic rock, and volcanic glass. If any ofthese are dominant, the rock can be called crystal tuff,lithic tuff, or vitric tuff. Tuffs are usually light grey rocksand often exhibit graded bedding, either as a primaryfeature from settling ash, or as a secondary feature fromdeposition of ash in water or reworking of sedimented ashin the water.

NOTE

Some tuffs contain fossils of animals and plantstrapped in the volcanic ash. Tuffs from MountVesuvius, Italy, contain casts of humans killedduring the eruption of 79CE. At Pompeii the tufflayer is up to 3m thick, but at nearbyHerculaneum where the ash built up inpyroclastic flows and mud slides, it is 20m thick.

STEEP cliffs develop inlayered tuffs because itis an easily eroded rock.

CRYSTAL TUFF

LITHIC TUFF

small rockfragments

gradedbedding

GRAIN SIZE 0.0625–2mm.ESSENTIAL COMPONENTS Igneous rock,volcanic glass, and crystal fragments.ADDITIONAL COMPONENTS None.ORIGIN Pyroclastic accumulation of finematerial. SIMILAR ROCKS Turbidite (p.27), where theclasts are sedimentary grains.

SECTION SHOWN

mineral fragments

fragmentsof rockand glass

�

GRAIN SIZE 0.0625–2mm.ESSENTIAL COMPONENTS Igneous rockand crystal fragments, welded volcanic glass.ADDITIONAL COMPONENTS None.ORIGIN Pyroclastic flow of lava and hot gas.SIMILAR ROCKS Ignimbrite differs from other tuffs (p.65) in having deformed glass shards.


THE WHITE pumicelandscape of Sarakinikoin Greece has heavilydissected valleys.

Ignimbrite

Ignimbrite is a particular type of tuff formed when the ashwas so hot that any volcanic glass was re-melted, and sofused or welded the rock. The glass appears as deformed,curved shards surrounded by crystal and rock fragments.Ignimbrite is deposited by an ash flow – a mixture of hot

gases and pyroclastic material that travels at fastspeeds. Ash flows are responsible for

many casualties duringvolcanic eruptions.

NOVARUPTA, anAlaskan volcano,produced the largesthistorical quantity ofignimbrite in June 1912.

SECTION SHOWN

deformedglass shard

pale tuffmatrix

Pumice

Pumice is a light grey or white, glassy volcanic rock, full ofgas bubble holes, which may be spherical or compressed.These holes make it an extremely light rock which canusually float on water. It results from frothy acid lava andmay be associated with rhyolite and obsidian. Pumice canbe carried long distances from its source volcano by wateror even wind.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Vesicular acid lava.ADDITIONAL COMPONENTS None.ORIGIN Solidification of acid lava withtrapped gas bubbles.SIMILAR ROCKS Vesicular rhyolite, dacite,and andesite can have the same chemicalcomposition but are much denser rocks.

gas bubblehole

spiky rock

glassfragments

SECTION SHOWN

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�


Pélé’s hair

Named after the Hawaiian goddess of fire, Pélé’s hair isvery long and thin basaltic glass. It is formed byfountaining and rapid mid-air cooling of a very liquidmagma. It is pale yellow-brown to black and brittle. Similar tear-drop shaped glassesare known as Pélé’s tears.Other rock shapes aregenerally known asachneliths and includesmall spheres anddumb-bells.

Volcanic Bomb

A pyroclastic rock that has formed by the cooling of abatch of lava while flying through the air after eruption.During transport it becomes rounded and may be twistedand pointed, then called a spindle bomb. It is usuallybrown or red, weathering to yellow-brown andmay have a cracked, fine-grained, orglassy surface, then calleda breadcrust bomb.

VOLCANIC BOMBS areseen in a layer of ashbeneath a brown lavaflow in this road-cut.

GRAIN SIZE Bombs 0.02–1m.ESSENTIAL COMPONENTS Basalt.ADDITIONAL COMPONENTS None.ORIGIN Mid-air cooling of batches of basicmagma.SIMILAR ROCKS This type of basalt (p.61)can be recognised by its round shape, and by its size.

TEAR DROP

GRAIN SIZE Thickness 1mm.ESSENTIAL COMPONENTS Basaltic glass.ADDITIONAL COMPONENTS None.ORIGIN Rapid cooling of basalt in a lava fountain.SIMILAR ROCKS This rock has the samechemical composition as basalt (p.61) and isrecognized by its extreme fineness.

loafshape

fine, wispytexture

weatheredsurface

FOUNTAINS of lava onKilauea, Hawaii;which can produce

Pélé's hair and other achneliths.

basalticglass

blobs ofglass

twistedshape

ROUNDED SPINDLE BOMB

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�

pale browncolour

Metamorphism is the transformation of rocks by heat andpressure. High-grade metamorphic rocks form deep in theEarth’s crust, where temperatures and pressures are high; low-grade rocks, such as slate (below), form at shallower depths.Metamorphic rocks are grouped here by their precursor rock:firstly those that were basic igneous rocks, then acid igneousand sedimentary. Next are contact metamorphic rocks, formedby close proximity to igneous intrusions; deformation rocks,which show features of stretching, squashing, or fracture; andmeteorites and rocks formed bytheir impacts on Earth.

Metamorphic Rocks

BASALT PEGMATITE GRANITE OBSIDIAN

M E T A M O R P H I C R O C K S 6 9

GREEN metabasaltwith white quartzveins in a typicalfractured andweathered outcrop.

Metabasalt

This is a metamorphosed basalt which retains some of itsoriginal igneous features, such as gas bubble holes, or flowtop breccia. It differs from basalt in being dark green coloureddue to the presence of the mineral chlorite, and is sometimesknown as “greenstone”. Greenstones are common in long,narrow outcrops of the Precambrian age, between granitesor orthogneisses. Like the rock it is derived from,metabasalt has a very fine grain size andindividual crystals are too small to beidentified without a microscope.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Chlorite, albite,actinolite.ADDITIONAL COMPONENTS None.ORIGIN Low grade metamorphism of basalt.SIMILAR ROCKS Basalt (p.61) is black when fresh, and harder and less altered thanmetabasalt.

NOTE

One type of metabasalt is called komatiiticmetabasalt. It is rich in magnesium andrecognized by spiky crystals, called spinifextexture. These crystals were formed by rapidgrowth of olivine or pyroxene. If a metabasalt isdeformed it can form greenschist, a chlorite-rich schist (p.78).

KOMATIITIC METABASALTGREENSCHIST

green colour

fold

fine grainsize

spinifextexture

schistosity

brownweatheredsurface

�SECTION SHOWN

7 0 M E T A M O R P H I C R O C K S

COARSE pyroxene-richaggregates form thebrown spots in thisgranulite from Scotland.

Amphibolite

These are dark-coloured, coarse-grained rocks dominated byblack or dark green hornblende, or alternatively, by greentremolite-actinolite. They usually contain plagioclase, andmay contain epidote or garnet. The mineral grains, exceptfor garnet, are aligned and sometimes the rock isbanded. Amphibolites form deep in theEarth’s crust by high grademetamorphism of basicigneous rocks.

BODIES of dark-coloured amphibolitecontrast with greyschists in these Alpine hills.

Granulite

Granulite is a coarse-grained metamorphic rock formed athigh temperatures and pressures deep in the Earth’s crust.Its name derives from its even-grained granular texture.Some mineral groups, such as amphiboles and micas,cannot survive at the high metamorphic grade under

which granulites form, and they are converted intopyroxenes and garnets, by losing water.

Most granulites are of thePrecambrian age.

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Plagioclase,hornblende.ADDITIONAL COMPONENTS Epidote,garnet.ORIGIN High grade metamorphism of basicigneous rocks.SIMILAR ROCKS Gabbro (p.50).

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Quartz, feldspar,pyroxene.ADDITIONAL COMPONENTS Garnet.ORIGIN Very high grade metamorphism ofbasic igneous rocks.SIMILAR ROCKS Amphibolite (p.70) haselongated grains.

SECTION SHOWN

red garnet

greenhornblende

even grainsize

plagioclase

SECTION SHOWN

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BLUESCHIST can befound on theArgentario coast ofTuscany, Italy.

Blueschist

Glaucophane is the mineral responsible for the blue colourof this rock, but it is often so fine grained that individualgrains cannot be seen. The colour is a dark purple-blue, butthis colouring may not be particularly strong and, at firstglance, the rock may appear grey orblack. Outcrops of blueschist aretypically small areas withinstrongly folded andfaulted rocks.

Eclogite

A beautiful coarse-grained, dense rock of bright red garnet andcontrasting green omphacite, eclogite grains may be evenlydistributed or banded. It may occur as nodules in basalts andkimberlites, or as larger bodies of rock. It forms at very hightemperatures and pressures frombasic igneous rocks, deep inthe Earth’s crust andin the mantle.

ECLOGITE forms thedark rock below lightbrown schists in thiscliff at As Sifah, Oman.

GRAIN SIZE 2mm.ESSENTIAL COMPONENTS Glaucophane,chlorite, epidote.ADDITIONAL COMPONENTS Lawsonite,jadeite.ORIGIN Low temperature, high pressuremetamorphism of basalt.SIMILAR ROCKS Greenschist (p.69).

GRAIN SIZE 2-5mm.ESSENTIAL COMPONENTS Garnet,omphacite.ADDITIONAL COMPONENTS Kyanite, quartz.ORIGIN Very high pressure metamorphismof basic igneous rocks.SIMILAR ROCKS Garnet-bearing peridotite(p.52).

redpyropegarnet

greenomphacite

wavysurface

alignedglaucophanecrystals

DARKER EXAMPLE

banding

�

�


SOAPSTONE quarries of the Great Dyke,Zimbabwe providematerial for sculpture.

Serpentinite

An attractive rock with flowing bands of various colours,especially greens and yellow, largely made of serpentine. It is a soft rock, which is easily cut andpolished to make a decorative stone.It forms from the metamorphismof olivine and pyroxene-richultrabasic igneous rocks.Chrysotile asbestosdevelops in shearedserpentinites.

ROUGH columnarjointing is visible inthis serpentinite. Thewrinkled brown-greysurface is known as“elephant skin” texture.

Soapstone

A massive fine-grained rock easily recognized by its softness– it has a greasy feel and can be scratched with a fingernail.This is because talc, one of the softest minerals, is itsprincipal component. It may be green, brown, or black,especially when polished but on scratching becomes

white. It is associated with othermetamorphosed ultrabasic

igneous rocks likeserpentinite.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Talc.ADDITIONAL COMPONENTS Chlorite,magnesite.ORIGIN Water-rich metamorphism ofserpentine.SIMILAR ROCKS Serpentinite (above) isharder; clay (p.29) does not have a greasy feel.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Serpentine.ADDITIONAL COMPONENTS Chromite,magnetite, talc.ORIGIN Water-rich low grade metamorphismof olivine-rich rocks.SIMILAR ROCKS Marble (p.77), which reactswith dilute acid.

SECTION SHOWN

greencolour

mottled,patchytexture

soft, flakysurface

easilyscratched

SECTION SHOWN

�

�

GRAIN SIZE Less than 2mm.ESSENTIAL COMPONENTS Quartz, feldspar,chlorite.ADDITIONAL COMPONENTS None.ORIGIN Low grade metamorphism of tuff.SIMILAR ROCKS Tuff (p.65), which is looselycompacted; greywacke and turbidite (p.27),which have sedimentary grains.


METATUFF slates from the Honister quarries in Cumbria, England,frequently show gradedbedding.

Metatuff

An interesting rock in that it has igneous, sedimentary, andmetamorphic features. Metatuff is a low grademetamorphosed volcanic ash that preserves itsoriginal features, such as graded bedding. Thefine grain size means that metatuffsoften form slates. Sometimes, adetailed section of the preservedsedimentary features can beseen on cleaved surfaces.

Gneiss

This is a general term used for rocks that show “gneissictexture”. This refers to a medium- to coarse-grained rockwith oriented minerals. It is made of layers of contrastingmineral composition, grain size, or texture. Most gneissescontain quartz and feldspar as the main constituents butneither mineral is necessary for a rock to be called gneiss.

THIS QUARRY face inthe Sand River, SouthAfrica, shows gneissthat was formed overthree billion years ago.

FOLDED GNEISS

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS Mica, garnet.ORIGIN High grade metamorphism of rockscontaining quartz and feldspar.SIMILAR ROCKS In granite (p.44), the grainsare evenly distributed. See also orthogneiss,augen gneiss (p.74), and paragneiss (p.76).

BANDED METATUFF

slatycleavage

gradedbedding

beds ofvaryingthickness

pale quartzand feldspar

gneissicbanding

biotite

stretchedcrystal

�

� SECTION SHOWN


AUGEN GNEISS isaccompanied byquartzite, eclogite, andother high grade rocksat Sognefjord, Norway.

Orthogneiss

This is a type of gneiss, which before metamorphism was an igneous rock, commonly a granite. This fact givesorthogneiss its other name, granitic gneiss. It can also becalled quartzo-feldspathic gneiss because its principalconstituents are quartz and feldspar, usually accompaniedby biotite, and/or hornblende. It is an overalllight grey rock in outcrop and is typical of the Precambrian era.

DOME-SHAPED outcropsof granitic gneiss formbare hillsides in ashape similar to thatof a whale’s back.

Augen Gneiss

Augen are single-mineral, eye-shaped grains of a larger sizethan other grains in a rock. They are usually porphyroblasts:large crystals that have grown during metamorphism. Mostcommon are pink alkali feldspars (microcline or perthite),1–5cm in length, in a quartz, feldspar, and mica matrix.

They are typical of high gradegneiss terrains of the

Precambrian age.

GRAIN SIZE 2–5mm, augen 1–2cm.ESSENTIAL COMPONENTS Alkali feldspar,quartz, plagioclase.ADDITIONAL COMPONENTS Mica.ORIGIN Unknown.SIMILAR ROCKS Tectonite (p.83), whenaugen are formed by deformation, but theyalso form by other processes.

augenoffeldspar

hornblende,garnet, andquartz

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS Mica.ORIGIN High grade metamorphism of acidigneous rocks.SIMILAR ROCKS Paragneiss (p.76), whichcontains a greater variety of minerals andtextures.

rich in quartzand feldspar fold

rich in hornblendeand biotite

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MIGMATITICORTHOGNEISS

SECTION SHOWN


LIGHT-COLOURED

granite veins, greygneiss, and dark greyamphibolite can be seen

in this South Africanamphibolite.

Migmatite

A migmatite is a mixed rock: a mix of igneous granite andmetamorphic gneiss. It represents the highest grade ofgneiss, which has partially melted to form pockets ofgranite. The granitic parts consist of granular patches ofquartz and feldspar; the gneissic parts ofquartz, feldspar, and dark-colouredminerals. It is a commonPrecambrian-agedrock.

Charnockite

A high grade metamorphic rock, charnockite issimilar to granulite with which it occurs. It hasa granular texture and an overall compositionand appearance of granite, except it isdarker coloured. It does not containamphiboles as these have beenconverted to pyroxenes underextreme metamorphism.Mostly occurring inPrecambrianterrains, the best knownexamples arefrom India.

KABBAL DURGA, a hill near Bangalore, India, is

made of amphibolegneiss with

charnockite.

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Quartz, feldspar.ADDITIONAL COMPONENTS None.ORIGIN Partial melting of rocks containingquartz and feldspar.SIMILAR ROCKS Gneiss (p.73), which hasno granitic parts, and granite (p.44), whichhas no foliated parts.

granite

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Quartz, feldspar,pyroxene (enstatite).ADDITIONAL COMPONENTS Garnet.ORIGIN High grade metamorphism ofgranite or gneiss.SIMILAR ROCKS Granite (p.44), is paler anddoes not contain pyroxene.

granite-liketexture

foldedgranitelayer

dark gneiss

garnetgneiss

lightercolour

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greenishcolour


QUARTZITES, beingresistant, often formwhite vegetation-freehilltops and ridges.

Paragneiss

A paragneiss is a gneissic rock, which was a sedimentbefore metamorphism. Paragneiss can contain manydifferent minerals, depending on the sedimentaryprecursors. Changes in mineralogy and texture occuracross an outcrop or on the scale of a few hundred metres.Typical minerals in paragneiss are quartz and feldspar, pluskyanite, staurolite, garnet, and muscovite.

ALMANDINE andkyanite are theknobbly-weatheringminerals in this rock.

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Quartz, feldspar,garnet.ADDITIONAL COMPONENTS Kyanite, biotite.ORIGIN High grade metamorphism ofquartz-rich sediments.SIMILAR ROCKS Other gneisses (pp.73–74)have a more restricted range of minerals.

gneissic banding

almandinegarnet

Quartzite

Quartzite, or metaquartzite, is a hard, sugary-textured rock,usually white, cream, or pink. It has a medium grain size,and under a hand lens the grains appear to be fusedtogether. It can form by contact metamorphism ofsandstones a few metres around the edge of an igneous

intrusion, but is usually found as alaterally extensive regional

metamorphic rock.

roundedgrains

hard andresistant

very quartz-rich

KYANITE PARAGNEISS

kyanite

quartz andbiotite

GRAIN SIZE 2–5mm.ESSENTIAL COMPONENTS Quartz.ADDITIONAL COMPONENTS Heavyminerals, such as zircon, rutile, and titanite.ORIGIN Regional metamorphism oforthoquartzite.SIMILAR ROCKS Orthoquartzite (p.25),which has more pore spaces.

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CALC-SILICATE mineralsform the green andgrey parts of thismarble, which is being worked as adecorative stone.

Marble

Marble is metamorphosed limestone and, when pure, is awhite granular crystalline, fine- to medium-grained rock.Marble can easily be identified by its softness and vigorousreaction to dilute acid. The best examples are valuabledecorative rocks that can be carved or worked into tilingand facing stone. Many impure marbles are desirablebecause of their colour variation or texture of folding andbanding. Marble can be formed by contact metamorphismof limestone next to igneous intrusions, in which case, it isoften associated with skarn. Mostly, it is a regionalmetamorphic rock that occurs over large areas.

finecrystals ofdiopside

forsterite,tremolite, orserpentine

WHITE MARBLE CALC-SILICATE ROCK BLUE-GREY MARBLE

fine-grainedmetamorphicrock

brecciatedtexture

GRAIN SIZE Less than 0.01–2cm.ESSENTIAL COMPONENTS Calcite.ADDITIONAL COMPONENTS Dolomite,tremolite, diopside, serpentine.ORIGIN Contact or regional metamorphismof limestone.SIMILAR ROCKS Limestone (p.32) anddolomite (p.34), which are not crystalline.

NOTE

If silicate minerals form a considerable portionof the marble, then it is known as a calc-silicaterock and can contain metamorphic calcium-rich silicate minerals, such as green-colouredtremolite (p.198) and diopside (p.213), inpatches or bands. Others include dolomite(p.152), and serpentine (p.202).

iron-stainedveinlet

cream-whitemarble

granulartexture

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Schist

Schist is a metamorphic rock with a schistose texture,that is, sheets that are wrinkled, wavy, or irregular on a small scale. Many different mineral compositions canoccur, but it usually includes a mica. The rock colour will depend on the constituent minerals and the grainsize is usually medium. Larger crystals of garnet,staurolite, kyanite, or other metamorphic minerals may

be present. Schist is a medium-grade rockresulting from the metamorphism

of fine-grained sediments,and often occurs on a

regional scale inmountain belts.

TIGHTLY-FOLDED silver-grey schist, can be seenin this outcrop in theEuropean Alps.

crenulationsor wavy folds

kyanite

GARNET SCHIST

sheen onmica-richsurface

darkmineral

GRAIN SIZE 0.1–2mm.ESSENTIAL COMPONENTS Mica.ADDITIONAL COMPONENTS Quartz,feldspar, garnet, staurolite, cordierite, kyanite,sillimanite.ORIGIN Regional metamorphism of fine-grained sediments.SIMILAR ROCKS Phyllite (p.80).

MUSCOVITE SCHIST

KYANITE SCHIST

garnet muscovite

SECTION SHOWN

NOTE

To provide a more detailed rock name, themajor minerals present in schist arehyphenated and used as a prefix, for example,talc-tremolite-magnesite schist or quartz-sericite schist. Garnet-mica schist is aparticularly common variety, and it occursacross wide areas.

NOTE

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NORTH WALES hasmany large scalequarries of slate withvast waste tips.

Slate

Slate is a familiar rock type as it is used as a roofingmaterial. It is characterized by a strong and flat cleavage,enabling the rock to be easily split into sheets. The colouris usually dark: possibly grey, black, green, purple, or red. Itis a very fine-grained rock, but may contain well-developedcoarser crystals, such as pyrite. Slates are frequentlyquarried and being regional metamorphic rocks, theseworkings may extend over large areas, such as the slatequarries in north Wales. Welsh slate, like other examplesworldwide, is a Lower Palaeozoic rock.

BLACK SLATE

flatsurface

darkpurplecolour

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Quartz, mica.ADDITIONAL COMPONENTS Pyrite.ORIGIN Low-grade regional metamorphismof fine-grained sediments.SIMILAR ROCKS Shale (p.31), which iscrumbly; phyllite (p.80), which is rougher andwill not cleave into very thin sheets.

raised notches

NOTE

Being low-grademetamorphic rocks,some slates retainsedimentary origneous features,such as gradedbedding or fossils.

NOTE

SLATE WITH DISTORTED FOSSIL SLATE WITH PYRITE

cubic crystalof pyrite

brachiopodshell

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LIGHT reflected off thecleavage surfaces givesa characteristic sheenin this folded phyllite.

Phyllite

Phyllite is a dark coloured metamorphic rock, usually greyor dark green, with a fine grain size. It has a strongfoliation of aligned mica, which is often visible as a sheen

on cleavage surfaces. Thesesurfaces are more irregular

than slate but less thanschist, so it splits into

slabs. Phyllite is oftendeformed into folds

a few centimetresacross, and

veined withquartz.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Quartz, muscovite.ADDITIONAL COMPONENTS None.ORIGIN Regional metamorphism of fine-grained sediments.SIMILAR ROCKS Slate (p.79), which has flat,smooth cleavage surfaces; and schist (p.78),which is more crumpled.

SECTION SHOWN

Spotted Slate

The spots in a spotted slate are coarser-grained mineralsthat are scattered throughout the finer groundmass of theslate. Typical minerals that form the spots are cordieriteand andalusite, the former as darker diffuse areas and thelatter as distinct square-ended long prismatic crystals, often

of the variety chiastolite. Spotted slates areproduced by close proximity to an igneous

intrusion, in a narrow zone known as anaureole.

PRISMATIC andalusitecrystals can be seen inthis purple spotted slatefrom the Skiddawgranite aureole, England.

rough, slatycleavage

dark spot of cordierite

micasheen

darkcolour

GRAIN SIZE Less than 0.1mm, spots 1–5mm.ESSENTIAL COMPONENTS Quartz, mica.ADDITIONAL COMPONENTS Cordierite,andalusite, staurolite.ORIGIN Low-grade regional

metamorphism followed by contactmetamorphism.SIMILAR ROCKS Slate (p.79).

fairly flatsurface

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finegrain


MARBLE, in the top leftcorner, has beenconverted to darkerstriped calc-hornfels.

Skarn

This is a product of contact metamorphism of a limestoneor dolomite by an intermediate or acid igneous intrusion.Skarn is rich in carbonate minerals, as well as, in calcium,iron, and magnesium silicates, which may be fine tomedium-grained, but often occur as coarse radiatingcrystals or bands. Some skarns are rich in metallic ores and can form valuable deposits of metals such as gold,copper, iron, tin, tungsten,lead, and zinc.

MONTICELLITE formsthe darker layers inthis South Africanskarn. Calcite formsthe lighter layers.

Hornfels

Hornfels is a hard-to-break, fine-grained or glassy rock witha splintery fracture. It is formed by the metamorphism offine sediments in contact with an igneous intrusion. The

colour is usually even throughout, but variesdepending on the composition of the

original rock. An outcrop of hornfelsrarely extends more than a

few metres from thecontact and may pass outwards intospotted slate.

GRAIN SIZE Less than 0.1mm.ESSENTIAL COMPONENTS Many differentcompositions.ADDITIONAL COMPONENTS Calcite,cordierite, pyroxene.ORIGIN Contact metamorphism of fine-grained sediment.SIMILAR ROCKS Lavas (are of greater extent).

GRAIN SIZE 0.01–2cm.ESSENTIAL COMPONENTS Calcite.ADDITIONAL COMPONENTS Garnet,serpentine, forsterite, vesuvianite.ORIGIN Contact metamorphism oflimestone.SIMILAR ROCKS Calc-silicate rocks (p.77),which occur over larger areas.

black, hacklymatrix

dark and fine-grained

diopside andtremolite

garnet crystals

browncalcite

CORDIERITE HORNFELS�

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LINEATIONS, produced bystretching, in a myloniticgneiss with a much-reducedgrain size.

Fault Breccia

In some faults, pieces of rock can be broken up by crushingwithin the fault zone. When cemented, usually by a mineral,such as quartz or calcite, this crushed material becomes afault breccia. Fragments within the breccia are of the rocksfound on either side of the fault plane and may be ofany dimension, depending on the size andnature of the fault itself.

FAULT breccia withvariably sized clasts ina matrix of groundrock fragments.

Mylonite

A mylonite is a fine-grained rock with streaks or rod-likestructures that are evidence of the ductile deformation(stretching) of mineral grains. It is typically produced in a zone of thrusts, or low-angle faults. The fine grain size

may have been produced byrecrystallization under

pressure. In some cases,the rock is so fine it has

the appearance ofstreaky flint.

GRAIN SIZE Clasts of any size in a fine orcrystalline matrix.ESSENTIAL COMPONENTS Surrounding rocks.ADDITIONAL COMPONENTS Quartz andcalcite are common cements.ORIGIN Crushing of rock in a fault.SIMILAR ROCKS Sedimentary breccia (p.21),volcanic breccia (p.62).

GRAIN SIZE Less than 2mm.ESSENTIAL COMPONENTS Surroundingrocks, but usually the mylonite is a finer grain size.ADDITIONAL COMPONENTS None.ORIGIN Stretching of a rock in a large fault.SIMILAR ROCKS Hornfels (p.81), hasrandomly oriented grains.

orange quartzmatrix

fine grainsize

stretchedmineralgrains

angular slateclasts

SECTION SHOWN

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TECTONITES can befound in shear zones,such as this one within gneiss.

Pseudotachylite

Pseudotachylite is a dark-coloured glassy substance found in rocks that have experienced very severe stress or friction. This happens within large faults and thrusts, and beneath meteorite impacts. It is usually seen in gneiss,as a series of discontinuous veins, up to several centimetreswide, or as the matrix of a breccia.

VEINS of blackpseudotachylite withlight grey gneissfragments, found on theIsle of Barra, Scotland.

Tectonite

This rock has a well-developed deformation texture. It haselongated or flattened mineral grains or other components,such as pebbles. If flattening (foliation) is dominant, it iscalled an S-tectonite; if stretching (lineation), an L-tectonite,if neither, an L-S tectonite. Tectonites are found in shearzones, where a body of rock has been highly strainedwithin a narrow planar area. Many examples are coarse-grained rocks, such as gneiss.

GRAIN SIZE 0.2–2cm.ESSENTIAL COMPONENTS Any rock type,which shows evidence of deformation.ADDITIONAL COMPONENTS None.ORIGIN Stretching or squashing of a rock.SIMILAR ROCKS Mylonite (bottom left),which is finer grained. The term can beadded to, such as “gneissic tectonite”.

GRAIN SIZE Not granular.ESSENTIAL COMPONENTS Metamorphicglass.ADDITIONAL COMPONENTS Often containsgneiss fragments.ORIGIN Frictional melting in a fault zone.SIMILAR ROCKS Volcanic glass, such asobsidian (p.57), which is associated with lava.

gneiss

filling extensionvein

black pseudotachylite

PSEUDOTACHYLITE IN GNEISS

banded layers

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Meteorites

Meteorites are rocks from space, and can be divided intothree groups: irons, stony irons, and stony meteorites. Ironsare made of nickel-iron and readily rust in the Earth’satmosphere. If sliced, the inner surface may reveal metalcrystals. Stony irons consist of roughly equal amounts ofnickel-iron and silicate material; either igneous rock or

single minerals, such as olivine. Stonymeteorites may have the appearance

of igneous rocks, such as gabbro or anorthosite, but frequently

contain small metallicpatches too. One type,

known as chondritescontain spheroidalstructures and

graphite crystals.

METEOR crater, inArizona, USA, wasformed by the impactof the Canyon Diablometeorite.

IRON METEORITE

black fusioncrust

hollow

stonyinterior

NOTE

All meteorites may show thumb-likedepressions on the surface, known asregmaglypts, or they may have a layer of black,burnt material on the outside, known as a fusion crust. Constant weathering, overthousands of years, may again affect thesurface appearance.

GRAIN SIZE Typically 2mm–1cm.ESSENTIAL COMPONENTS Iron, nickel.ADDITIONAL COMPONENTS Pyroxene,plagioclase, olivine, graphite.ORIGIN As asteroids in space.SIMILAR ROCKS Basic and ultrabasicigneous rocks; man-made iron objects.

STONY IRON METEORITE

IRON METEORITE SLICE

thumb-shapeddepression

nickel-ironcrystals

olivinecrystal

nickel-iron

pointed metal

fusioncrust

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Impact Rocks

When a large meteorite hits the Earth’s surface, the rocksbeneath suffer great heat and pressure. This producesimpact breccia from shattering, and impact glass frommelting. If the molten material is thrown into the air, it canform interestingly-shaped glass objects, known as tektites.Impact glass mixed with impact breccia is known assuevite. Shatter cones are rocks with radiating cone-shapedfractures produced on impact. Meteorite impact sites canbe recognized by their circular shape, and rock and mineral products of impact. Meteorites themselves arerarely found at sites, because somevapourize on impact, and othersweather away quickly.

TEKTITE

SUEVITE, a type ofimpact breccia withsome glass clasts, isseen here at the Riesimpact site, southernGermany.

MOLDAVITE

radiatingcracks

GRAIN SIZE Breccia has clasts of about 1–10cm.ESSENTIAL COMPONENTS Usually glass, orshatter effects.ADDITIONAL COMPONENTS None.ORIGIN Impacts of meteorites on Earth.SIMILAR ROCKS Other breccias (pp.21, 62,82) and glasses (pp.57, 83).

shattercone

NOTE

Bottle-green tektites,from a large area ofthe southern CzechRepublic, are knownas moldavites.These tektitesprobably representejecta from the 215million year old Ries meteoriteimpact site inBavaria, Germany.flow

effects

pittedsurface

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greenglass

black glass

Ore minerals are those which we mine to obtain the metals weuse in our everyday lives. Chalcopyrite, for example, is an oreof copper; it was extracted at the Welsh mine pictured below.Many ores are hydrothermal in origin, crystallizing out of hotchemical-rich water; others form in igneous rocks; others stillin metamorphic skarn deposits. When exposed to water,primary ores may break down to form secondary minerals. In this chapter, ores and their secondary minerals are groupedby their principal metal. Those commonly found inhydrothermal veins precede those found in igneous rocks.

Ore Minerals

GALENA WULFENITE CYANOTRICHITEHEMATITE

GoldAu

Gold is normally found in nature in its elemental statebecause it does not bond with most chemical elements. It is opaque and metallic golden-yellow in colour, but it ispaler when alloyed with silver in the variety electrum. Goldcrystals are octahedral, rarely cubic or dodecahedral, and theyusually occur as dendritic aggregates. Most gold isfound as tiny grains, platy masses, and nuggets. It is very dense, soft, sectile, and easily bent, and it does not develop a tarnish. Gold is widespread in occurrence but it is usually found in minutequantities. It is most abundant in hydrothermalveins and placer deposits.

O R E M I N E R A L S 8 7

THE MINES at BanskáS̆tiavnica, Slovakia, were once one of theworld’s richest sources of gold and silver.

metallicgolden-yellow

grains of placergold

NOTE

Gold deposits are cemented into sandstones,siltstones, and conglomerates, and may besubsequently metamorphosed. Much of the world’s gold ore comes from an ancientplacer deposit in South Africa. Recent placers are sands and gravels where gold can be extracted by panning.

fine-grainedquartz

mass ofsoft puregold

flakes of goldcompressed intoa single nugget

SECTIONSHOWN

octahedralcrystals

COMPOSITION Element.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Hackly.LUSTRE/STREAK Metallic/Metallic golden-yellow.HARDNESS/DENSITY 2.5–3 / 19.30.KEY PROPERTIES Golden-yellow, dense, soft,sectile, and easily bent.

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8 8 O R E M I N E R A L S

DEPOSITS of sulphuroccur in fumaroles thatformed in 1971 in theKilauea Caldera ofHawaii, USA.

CinnabarHgS

Cinnabar is bright scarlet to deep greyish red. It usuallyoccurs as massive or granular aggregates and powderycoatings, but is also found as rhombohedral, tabular, or prismatic crystals. The principal ore ofmercury, cinnabar forms in low-temperature hydrothermal veins.Bright red powdered cinnabarwas used in the past as the artist’s pigmentvermillion.

CINNABAR is often seen as a powdery red coating on the rock matrix. The bestknown locality for the mineral isAlmadén, Spain.

SulphurS

Sulphur is normally bright yellow or orangish yellow, andforms pyramidal or tabular crystals, encrustations,powdery coatings, and granular or massive aggregates.Most sulphur forms in volcanic fumaroles, but it can alsoresult from the breakdown of sulphide ore deposits and

occur in some sedimentary rocks. Its lowthermal conductivity means sulphur

crystals may shatter in the warmth of a hand and should never beimmersed in water.

COMPOSITION Element.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Three,imperfect/Conchoidal to uneven.LUSTRE/STREAK Resinous or greasy/White.HARDNESS/DENSITY 1.5–2.5 / 2.07.KEY PROPERTIES A soft, low-density mineralwith poor cleavage.

SECTION SHOWN

tabularcrystalsin cavity

pisoliticaragonitematrix

thick tabularcrystals

rich redcrystallinemass

fine-grained rockmatrix containingcrystals of pyrite

COMPOSITION Sulphide.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect/Subconchoidal.LUSTRE/STREAK Adamantine to sub-metallic/Scarlet.HARDNESS/DENSITY 2–2.5 / 8.18.KEY PROPERTIES More dense than realgar(below right).

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yellow withresinouslustre


ORPIMENT formsaround hot springs and vents in volcaniclandscapes, such asthis one in Japan.

OrpimentAs2S3

Orpiment is a soft yellow or orange mineral, once used as anartist’s pigment. Typical orpiment is powdery or massive, orin foliated cleavable masses. It occurs in low-temperaturehydrothermal veins, hot spring deposits, and volcanicfumeroles. It also results from the alteration of arsenic-bearing minerals like realgar.

RealgarAs4S4

Bright red or orange-red, realgar is found as prismatic crystalsstriated along the length, or, as massive or granularaggregates and coatings. Realgar forms withother arsenic and antimony minerals in low-temperature hydrothermal veins. It is alsodeposited in volcanic fumeroles and hotspring deposits. When exposedto light, it disintegrates to formyellow powdery orpiment or pararealgar.

REALGAR is found withother sulphides andsulphosalts in dolomiterock at the Lengenbachquarry, Switzerland.

COMPOSITION Sulphide.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect, into flexiblesheets/None.LUSTRE/STREAK Pearly on cleavagefaces/Pale yellow.HARDNESS/DENSITY 1.5–2 / 3.49.KEY PROPERTIES Colour and cleavage.

COMPOSITION Sulphide.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good/Conchoidal.LUSTRE/STREAK Resinous or greasy/Orange-red to red.HARDNESS/DENSITY 1.5–2 / 3.56.KEY PROPERTIES A soft, sectile, red mineraloften found partially altered to yellow orpiment.

perfectcleavage intoflexibleyellow sheets

conchoidalfracture

rockmatrix�

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rare, stubbyprismaticcrystals

typicalfoliated habit

SECTION SHOWN

greasylustre

realgar altering toyellow orpiment


WHEAL coates andother mines inCornwall, England,were once a sourceof bismuth as wellas other metals.

BismuthiniteBi2S3

Bismuthinite usually occurs as delicate acicular crystals andfoliated or fibrous masses, but can form larger striated,prismatic crystals. It is opaque steel grey, often developing

an iridescent or silvery yellow tarnish. It is found inhydrothermal veins, granite pegmatites, and involcanic fumaroles. Bismuthinite occurs as large

crystals at Tazna in Potosí, Bolivia.

delicate fibrouscrystals

COMPOSITION Sulphide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Brittle; slightlysectile.LUSTRE/STREAK Metallic/Lead grey.HARDNESS/ DENSITY 2–2.5 / 6.78.KEY PROPERTIES More dense and lesscommon than stibnite (above).

CAVITIES in thishydrothermal veincontain radiatingsprays of stibnitecrystals on white calcite.

radiatingprismaticcrystals

dull andtarnishedwhere exposedto light

calcite

SECTION SHOWN

metallic grey

StibniteSb2S3

Stibnite is opaque silvery grey, but becomes tarnished anddull on exposure to light. It normally occurs as elongate,prismatic crystals with the curious property that they maybe bent or twisted. Coarse irregular masses or radiatingsprays of acicular crystals are typical, but it can also be

granular or massive. It is found inhydrothermal

antimonydeposits.

bent crystals

COMPOSITION Sulphide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Subconchoidal.LUSTRE/STREAK Metallic/Lead grey.HARDNESS/DENSITY 2 / 4.63.KEY PROPERTIES More common and lessdense than bismuthinite (below).

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COMPOSITION Sulphide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Metallic/Greyish black.HARDNESS/DENSITY 5.5 / 6.33.KEY PROPERTIES Perfect cleavage, unlike cubic minerals skutterudite CoAs2-3

and nickel-skutterudite (Ni,Co)As2-3.


ErythriteCo3(AsO4)2

.8H2O

The bright purplish pink colour of erythrite in a rockindicates the presence of cobalt, and it was known tominers as ‘cobalt bloom’. It typically occurs as flattenedprismatic crystals and powdery coatings in the oxidizedzones of cobalt-nickel-arsenic deposits. Widespread inoccurence, fine crystals come from Mount Cobalt in Australia.

COBALT-NICKEL

mines in the AtlasMountains, Morocco,are sources ofexceptional erythrite.

subadamantinelustre

COMPOSITION Arsenate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Sectile andflexible.LUSTRE/STREAK Subadamantine orpearly/Pale pink.HARDNESS/DENSITY 1.5–2.5 / 3.06.KEY PROPERTIES Colour, sectility.

COBALTITE is foundwith other sulphidesand quartz atTunaberg, Sweden.

CobaltiteCoAsS

Cobaltite crystals appear cubic, pyritohedral, octahedral, or combinations of these forms, and faces may be striated.Granular or massive aggregates occur. Although the shapesof crystals resemble pyrite, the colour is different. Cobaltiteis opaque, pale silvery grey, often tinted pink. It is found inhigh-temperature hydrothermal veins ordisseminated in metamorphic rocks.

SECTION SHOWN

chalcopyriteand othersulphides

combination of cubeand octahedron

flattened elongatecrystals

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typicallypurplishpink


DELICATE sprays ofmillerite crystals arefound in hardconcretions on coalmine dumps as here,in Limburg, Holland.

COMPOSITION Sulphide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Parting/Conchoidal.LUSTRE/STREAK Metallic/Light bronze-brown.HARDNESS/DENSITY 3.5–4 / 4.6–5.KEY PROPERTIES Bronze tarnish, notmagnetic like pyrrhotite (p.125).

COMPOSITION Sulphide.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Metallic/Greenish black.HARDNESS/DENSITY 3–3.5 / 5.5.KEY PROPERTIES Golden acicular crystalsmay be coated with green annabergite (above right).

PENTLANDITE is mined underground at Sudbury, Ontario,Canada.

Pentlandite(Fe,Ni)9S8

Pentlandite is always massive or granular, so distinct crystalsare not seen. It is opaque, metallic yellow in colour, tarnishesbronze, and is nearly always found mixed with pyrrhotite.Pentlandite typically occurs in basic and ultrabasic igneousintrusions. In the Sudbury region of Ontario, Canada, nickel

from an ancient meteorite is thought to haveenriched the ore. These deposits contain

enough pentlandite to make this the mostimportant ore of nickel.

SECTION SHOWN

MilleriteNiS

Millerite’s opaque golden crystals are fibrous or acicular andvery delicate. They can be free-standing as single crystals,tufts, matted groups, or radiating sprays, or, are beddedwithin other sulphidesand gangue minerals.This low-temperaturehydrothermalmineral is found incavities in limestonesand carbonate veins,

in nodulesand otherassociated

rocks withcoal deposits,

and in serpentinites.

acicularhabit

crystals inrock cavity

bronze tarnish

granular,mixed withpyrrhotite

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goldencrystals


GarnieriteMixed nickel silicates

Garnierite is a general name for mixed nickel-bearingsilicates, including népouite, a nickel serpentine. Crystals ofthis mineral are hexagonal, but garnierite normally occurs assoft foliated or earthy aggregates. The colour is pale tobright grass-green, typical of nickelsecondary minerals. It has a waxyor dull lustre. Garnierite is animportant ore of nickel, andresults from tropicalweathering of nickel-richultrabasic igneous rocks,to form laterite deposits.

GARNIERITE is a majorore of nickel minedaround Nouméa, on

the tropical islandsof New Caledonia.

softfoliatedmass

typicalapple-greencolour

SILVERY pink nickelinehas altered to applegreen annabergite inthis sample fromnorthern England.

COMPOSITION Arsenate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Subadamantine; pearly oncleavages; dull when earthy/Very pale green.HARDNESS/DENSITY 1.5–2.5 / 3.07.KEY PROPERTIES Apple-green mineral thatdoes not fizz in dilute HCl.

AnnabergiteNi3(AsO4)2·8H2O

Annabergite is normally bright apple-green, pale green, or, more rarely, grey. It is usually found as fibrous crusts,coatings, or earthy masses. Bladed or prismatic crystalscome from only a few places, notably Lavrion (Laurium)in Greece. Annabergite forms by alteration of pentlandite,millerite, nickeline (niccolite), and other nickel-bearingsulphides and arsenides. It is the ‘nickel bloom’ that is seen coating the walls of nickel mines.

powdery greencoating

rockmatrix

SECTION SHOWN

COMPOSITION Silicates.CRYSTAL SYSTEM Various.CLEAVAGE/FRACTURE Unknown/Splintery oruneven.LUSTRE/STREAK Waxy, dull/White to light green.HARDNESS/DENSITY 2–4 / 2.3–2.8.KEY PROPERTIES Apple-green; waxy lustre.

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THE LARGEST silvernugget in the worldwas extracted from the Smuggler mine inAspen, Colorado, USA, in 1894.

SilverAg

Natural crystals of the element silver are cubic, octahedral,or dodecahedral, but usually, this mineral is found as grainsand wiry, dendritic, lamellar, or scaly masses. Silver is opaqueand bright silvery white with a slightly pink tint, butreadily tarnishes grey or black on exposureto light and pollutants. A primaryhydrothermal mineral, silver alsoforms by alteration of othersilver-bearing mineralsin the oxidizedzone of oredeposits.silvery

white

COMPOSITION Sulphide.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Indistinct/Uneven.LUSTRE/STREAK Metallic/Black.HARDNESS/DENSITY 2–2.5 / 7.22.KEY PROPERTIES Distinguished from galena (p.96) by its imperfect cleavage and sectility.

AcanthiteAg2S

Acanthite usually crystallizes from argentite, the high-temperature form of silver sulphide, which has cubic oroctahedral crystals. On cooling, acanthite developsinternal monoclinic symmetry without altering crystalshape. It is opaque and greyish black. It occurs in

hydrothermal veins and secondaryveins of mineral

deposits.

ACANTHITE and silverwere obtained fromBulldog mine nearCreede, Colorado,USA, until it wasclosed in 1985.

crystal appearsto be cubic

uneven fracture

opaque,metallicgreyish black

COMPOSITION Element.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Hackly.LUSTRE/STREAK Metallic/Silvery white.HARDNESS/DENSITY 2.5–3 / 10.1–11.1. KEY PROPERTIES A ductile and malleable silvery white mineral that tarnishes easily.

wiry silver

dendriticcrystals

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ProustiteAg3AsS3

As its old name ruby silver suggests, proustite is translucentand red, but if it is exposed to light, it turns dull opaquegrey. Red proustite should be stored in the dark. Thestriated crystals are typically prismatic with rhombohedralor scalenohedral terminations, or form massive or granularaggregates. Proustite occurswith other silver minerals in hydrothermalveins and in thesecondary zone of silverdeposits.

PROUSTITE comes from Guanajuato,Mexico, one of fourmining districts that make Mexico theworld's largest producerof silver.

COMPOSITION Sulphosalt.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Distinct/Conchoidal to uneven.LUSTRE/STREAK Adamantine/Purplish red.HARDNESS/DENSITY 2.5 / 5.82.KEY PROPERTIES Distinguished from cuprite(p.111) by crystal shape.

COMPOSITION Sulphosalt.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Distinct/Conchoidal touneven.LUSTRE/STREAK Adamantine/Scarlet.HARDNESS/DENSITY 2–2.5 / 5.57.KEY PROPERTIES Lighter in colour andstreak than pyrargyrite (above).

PYRARGYRITE was oncemined and processedat the historic works in Rammelsberg,Saxony, Germany.

PyrargyriteAg3SbS3

The silver sulphosalt pyrargyrite is typically massive or granular, or occurs as prismatic crystals withrhombohedral, scalenohedral, or flat terminations,different at each end. In colour, it is translucent dark red,but turns opaque dull grey onexposure to light, so shouldalways be stored in thedark. Pyrargyrite formsin low-temperaturehydrothermal depositsand by alteration of other silver-bearing minerals.

six-sidedprismaticcrystal

striationson crystalface

translucent red

adamantinelustre

deep redcolour

lustrebecomesdull

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opaque greyafter exposureto light

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BANDS of galena,purple fluorite, creamydolomite, and whitequartz are visible inthis hydrothermal veinfrom the northPennines, England.

SECTION SHOWN

GalenaPbS

Composed of lead sulphide, galena is opaque and bright metallic grey when fresh, but it dulls on exposure to the environment. Its crystals are cubic,octahedral, dodecahedral, or combinations of these forms, and irregular, coarse, or fine crystalline masses arecommon. Galena is very common in lead-zinc-copperhydrothermal ore deposits worldwide, associated withsphalerite, chalcopyrite, and pyrite. It is also found incontact metamorphic rocks, but it is rarely found inpegmatites. Galena is both the principal ore of lead and

the main source of silver; it often containssilver as an impurity.

NOTE

Lead was oncewidely used in thehome and, also, inindustry but,because it is toxic, it now has limitedapplication. Galenais relatively insolubleand so it is not verytoxic, but someother minerals arehazardous ifingested and shouldbe kept away from children.

granularpyrite

combination of cube andoctahedron

brightmetalliclustre

cubiccrystal

perfect cleavage in threedirections at right angles

COMPOSITION Sulphide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Perfect,cubic/Subconchoidal.LUSTRE/STREAK Metallic/Lead grey.HARDNESS/DENSITY 2.5–2.75 / 7.58.KEY PROPERTIES A markedly heavy mineralwith perfect cubic cleavage.

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COMPOSITION Sulphosalt.CRYSTAL SYSTEM Monoclinic.

CLEAVAGE/FRACTURE Good/Brittle.LUSTRE/STREAK Metallic/Grey to black.HARDNESS/DENSITY 2.5 / 5.63.KEY PROPERTIES Crystals are usually moreslender than stibnite (p.192); not flexible likeboulangerite (Pb5Sb4S11).

COMPOSITION Sulphosalt.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Imperfect/Subconchoidal to uneven.LUSTRE/STREAK Brilliant to dull/Steel grey tonearly black.HARDNESS/DENSITY 2.5–3 / 5.83.KEY PROPERTIES Distinctive cog-wheel twins.

BournonitePbCuSbS3

Its popular name ‘cog-wheel ore’ alludes to the distinctivetwinned crystals that are common to bournonite.Untwinned crystals of this opaque lead grey mineral aretabular or short prismatic, and granular or massiveaggregates are also found. It forms in medium-temperature hydrothermal veins associatedwith galena, tetrahedrite, andother sulphide minerals.

shortprismaticcrystal

COG-WHEEL twins ofbournonite with pyritefrom a hydrothermalore deposit at BaiaSprie, Romania.

JamesonitePb4FeSb6S14

Jamesonite is normally found as acicular or fibrous crystalsforming columnar, radiating, plumose (feather-like), orfelt-like masses. It is opaque dark grey but often developsan iridescent tarnish. Jamesonite forms in low- or medium-temperature hydrothermal veins with other lead andantimony sulphides and sulphosalts.

RICH specimens ofjamesonite come fromthe antimony-leadmine at Port Isaac,

Cornwall,England.

metalliclustre

iron-stainedweatheredsurface slightly

iridescenttarnish

feather-likeaggregate of crystals

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NOTE

The adamantine lustre – named after the Greekfor diamond – is particularly bright. It is shownby certain transparent or translucent minerals,such as diamond (p.186) and cerussite (p.98),and gives an important clue to their identity.Opaque minerals with an equally brightappearance have a metallic lustre.

COMPOSITION Carbonate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Good/Conchoidal.LUSTRE/STREAK Adamantine/White.HARDNESS/DENSITY 3–3.5 / 6.55.KEY PROPERTIES Very dense; colourless orwhite with adamantine lustre; fizzes in dilute HNO3.

CerussitePbCO3

Cerussite is typically colourless, grey, or yellow. It formstabular or pyramidal crystals or twins, which may be star-shaped or in reticulated (net-like) masses. Fragileaggregates of randomly grown prismatic crystals known as jack-straw cerussite are also common. A product of the weathering of galena, cerussite is found in thesecondary zone of lead deposits. Sources of large crystalsinclude Tsumeb, Namibia, and the Broken Hill mine in

New South Wales, Australia.

colourless crystal

star-shapedtwin

jack-strawcerussite

tabularcrystal

THE KILLHOPE museumpreserves the history ofnorthern England’slead mines, wherecerussite is often foundin altered galena.

�twinned crystal

adamantinelustre


ANGLESITE takes itsname from the Isle ofAnglesey, Wales, siteof the Parys Mountaincopper mines.

AnglesitePbSO4

Anglesite is colourless, white, grey, yellow, or pale shades ofblue or green. It often occurs as well-formed crystals, but alsoas nodular, granular, or massive aggregates. Crystals can beprismatic, tabular, or equant, and striated along the length.This mineral forms by the alteration of galena and otherlead minerals in the secondaryzone of lead deposits.

LinaritePbCu2+(SO4)(OH)2

Bright azure-blue linarite occurs as tabular or prismaticcrystals, crystalline crusts, and massive aggregates. It formsby secondary alteration of lead and copper sulphides, andits association with lead minerals helps distinguish it fromazurite. Although quite widespread indistribution, linarite is usually only found in small quantities.

EXCEPTIONALLY large crystalsof linarite come from theMammoth-St Anthony and

Grand Reef mines,Arizona, USA.

adamantinelustre

COMPOSITION Sulphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Vitreous, subadamantine/Pale blue.HARDNESS/DENSITY 2.5 / 5.35.KEY PROPERTIES Turns white in dilute HCI.

green andblue copperminerals

greygalena

metallic greygalena

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pointed bluecrystal

blue crystalshavevitreouslustre

COMPOSITION Sulphate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Good/Conchoidal.LUSTRE/STREAK Adamantine, resinous orvitreous/White.HARDNESS/DENSITY 2.5–3 / 6.38.KEY PROPERTIES Very dense; colourless,yellow, white; does not fizz in dilute HNO3.

1 0 0 O R E M I N E R A L S

resinouscrystallinemass

THE VARIETY campylitecomes from CaldbeckFells in Cumbria,England, and hasrounded, barrel-shapedcrystals.

PyromorphitePb5(PO4)3Cl

Like mimetite, pyromorphite forms green, brown, yellow, or orange hexagonal prismatic crystals, drusy coatings, andglobular or botryoidal masses. It is often found as a

secondary mineral in lead deposits. Superb crystalscome from Germany, France, England, and

China, while the fine crystalgroups of Bunkers Hill

mine in Idaho, USA,make it a source of

specimens forAmerican

collectors.

PYROMORPHITE andcerussite result fromthe weathering ofgalena at Leadhills,Lanarkshire, Scotland.

COMPOSITION Phosphate.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Poor/Subconchoidal.LUSTRE/STREAK Subadamantine orvitreous/White.HARDNESS/DENSITY 3.5–4 / 7.04.KEY PROPERTIES Dense hexagonal crystals,often yellow–green.

CAMPYLITE

iron-richmatrix

green, six-sidedprismaticcrystals

COMPOSITION Arsenate.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Poor/Subconchoidal.LUSTRE/STREAK Subadamantine orresinous/White.HARDNESS/DENSITY 3.5–4 / 7.24.KEY PROPERTIES Dense hexagonal or barrel-shaped crystals.

MimetitePb5(AsO4)3Cl

Mimetite is colourless, yellow, orange, brown, or, lesscommonly, green. It forms hexagonal prismatic crystals,drusy coatings, and globular or botryoidal masses, and isnamed after the Greek word for imitator because it looks likepyromorphite. A common secondary mineral of lead

deposits, superb specimens come from Tsumeb,Namibia, and Santa Eulalia and San Pedro

Corralitos, Chihuahua,Mexico.

hexagonalcrystal

barrel-shapedcrystal

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O R E M I N E R A L S 1 0 1

CRYSTALS of vanadinitefrom the lead mine atMibladen, Morocco,are of a particularlystriking red colour.

VanadinitePb5(VO4)3Cl

Vanadinite has orange-red, brown, or yellow hexagonal crystals. Curious hollow crystals and fibrous or globular masses may be found. It is a secondary mineral in lead deposits, vanadium being leached from surrounding rocks.Endlichite’s composition is roughly midway betweenvanadinite and mimetite.

DescloizitePbZn(VO4)(OH)

The orange-red, brown, or nearly black colour, rather greasylustre, and high density help distinguish descloizite fromother minerals. Crystals may be equant, tabular, pyramidal,or prismatic, and often occur as coarse masses or drusycoatings. Descloizite can also be botryoidal, stalactitic,granular, or massive. It forms in the oxidized zone ofdeposits containing lead, zinc, and vanadium,where it occurs with other leadsecondary minerals.

DESCLOIZITE is one ofmany well-crystallizedsecondary minerals

obtained fromTsumeb,Namibia.

COMPOSITION Vanadate.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE None/Uneven toconchoidal.LUSTRE/STREAK Resinous tosubadamantine/White or yellow.HARDNESS/DENSITY 2.5–3 / 6.88.KEY PROPERTIES Colour and crystal shape.

ENDLICHITE

SECTION SHOWN

resinouslustre

rockmatrix

distinctlygreasylustre

mass oftabularcrystals

bright orangishred crystal

pale yellowprismaticcrystals

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COMPOSITION Vanadate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE None/Conchoidal touneven.LUSTRE/STREAK Vitreous to greasy/Orangeto brownish red.HARDNESS/DENSITY 3–3.5 / 6.2.KEY PROPERTIES Density, colour, and streak.

COMPOSITION Chromate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Distinct/Conchoidal or uneven.LUSTRE/STREAK Adamantine/Yellow to orange.HARDNESS/DENSITY 2.5–3 / 6.0–6.1.KEY PROPERTIES Dense, orange-red,prismatic crystals.


CROCOITE mixes withgreen lead minerals inthis specimen fromBeresovsk,Russia.

WulfenitePbMoO4

Small, platy, yellow, orange, or red crystals with a squarecross-section that are found in the oxidized zone of ahydrothermal lead deposit are most likely to be wulfenite.It also forms massive or granular aggregates. Fine, largecrystals come from Arizona, USA; other importantlocalities are in Austria, Namibia, and Slovenia.

THE RED CLOUD mine,in a canyon in theArizona hills, USA, isone of the world'smost famous localitiesfor wulfenite.

CrocoitePbCrO4

One of the most eye-catching of minerals, crocoite is alwaysbright orange to red. The prismatic or acicular crystals arenearly square in section, may be striated along the length,

and rarely show distinct terminations.They usually occur in radiating orrandomly intergrown clusters. Crocoite is found in the oxidized zones of leaddeposits in chromium-bearing rocks, as at Dundas, Tasmania, Australia.

breaks withunevensurface

radiatingacicularcrystals

COMPOSITION Molybdate.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Distinct/Subconchoidal to uneven.LUSTRE/STREAK Resinous orsubadamantine/White.HARDNESS/DENSITY 2.75–3 / 6.5–7.5.KEY PROPERTIES Shape and high density.

thin, square, tabularcrystals

manganese-rich matrix

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red, tabularcrystals

adamantinelustre

nearly adamantinelustre

long, red, prismaticcrystal


THE PRINCIPAL ore ofzinc, huge deposits ofsphalerite are mined inTennessee, USA.

SphaleriteZnS

Pure sphalerite is colourless and rather rare. Normally, ironis present and the colour varies from pale greenish yellowto brown and black, as the quantity of iron increases. It canalso be red, a variety known as ruby blende; blende is theold name for sphalerite. Sphalerite crystals are normallycomplex, combining tetrahedral or dodecahedral formswith other faces. Often, sphalerite is coarsely crystalline ormassive, or it forms banded, botryoidal, or stalactiticaggregates. Sphalerite is the main ore of zinc. It is a very

common mineral, and it is foundmainly in hydrothermal lead-zinc deposits insedimentary rocks.

transparent, paleyellow crystals

NOTE

Lustrous darkcrystals of sphaleriteare often mistakenfor cassiterite(p.134). Sphaleriteis less dense andhas a perfectcleavage, and bothminerals havedifferent geologicalassociations.Sphalerite is foundin hydrothermaldeposits withgalena (p.96).

brownmassivesphalerite

SECTION SHOWN

RUBY BLENDE

massivesphalerite

nearlyblack

crystal withcubic symmetry

transparent,lustrous redcrystals

COMPOSITION Sulphides.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Resinous toadamantine/Brownish yellow to white.HARDNESS/DENSITY 3.5–4 / 3.9–4.1.KEY PROPERTIES Perfect cleavage is notshown by yellow sulphur (p.88) nor red garnet.

brightadamantinelustre

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FRANKLINITE fromFranklin, New Jersey,USA, is oftenassociated withfluorescent minerals.

ZINCITE was onceextracted at Franklinand Sterling Hill, New Jersey, USA.

Franklinite(Zn,Mn2+,Fe2+)(Fe3+,Mn3+)2O4

Franklinite is nearly always opaque black, and its crystals areusually octahedral with rounded edges, or form granular ormassive aggregates. It is strongly or weakly magnetic. Itresults from high-temperature metamorphism of sedimentsrich in manganese, iron, and zinc. In New Jersey, USA, fine

crystals and beds of granularfranklinite are rich

enough to be an ore of zinc, butfranklinite is rare elsewhere

in the world.

COMPOSITION Oxide.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Resinous tosubadamantine/Yellow to orange.HARDNESS/DENSITY 4 / 5.66.KEY PROPERTIES Often associated withfluorescent minerals.

COMPOSITION Oxide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Parting/Uneven tonearly conchoidal.LUSTRE/STREAK Metallic, submetallic, ordull/Dark reddish brown.HARDNESS/DENSITY 6 / 5.05–5.22.KEY PROPERTIES Streak, fracture, and magnetism.

red, coarselycrystallinezincite

whitecalcite(fluorescespink underUV light)

granularzincite withfranklinite

submetallicblack grains

large,singlecrystal

octahedralcrystal

unevenfracture

Zincite(Zn,Mn2+)O

Rare natural crystals of zincite are pyramidal, pointed atone end and flat at the other. Mostly it occurs as cleavableor granular masses. It is orange, red, yellow, or green.Zincite forms by secondary alteration or metamorphism ofzinc deposits. It is a rare constitutent of volcanic ash. The

chimneys of zinc smelters are sources ofnon-naturally occuring, large

zincite crystals.

SECTION SHOWN

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SKARN deposits atFalun, Dalarna,Sweden are a source of gahnite and other zinc minerals.

GahniteZnAl2O4

Gahnite normally occurs as simple octahedral crystals andgranular or massive aggregates. It is dark blue, blue-green,grey, yellow, or brown. Gahnite is a minor constituent ingranites and granite pegmatites, and forms in medium- to high-grade metamorphic rocks,metamorphosed ore deposits, and skarns.It is also found in placer deposits.

COMPOSITION Oxide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Parting/Conchoidal.LUSTRE/STREAK Vitreous/Grey.HARDNESS/DENSITY 7.5–8 / 4.38–4.6.KEY PROPERTIES Octahedral crystals canlook like spinel (p.149) and hercynite(Fe2+Al2O4).

SmithsoniteZnCO3

Most smithsonite is spherular, botryoidal, stalactitic, massive,or earthy. Crystals are uncommon; they are prismatic,rhombohedral, or scalenohedral, and often have curved faces.Smithsonite can be of various colours including yellow,orange, brown, pink, lilac, white, grey, green, and blue. Itforms in the oxidized zones of zinc deposits and in adjacentcarbonate rocks. Like hermimorphite, itused to be known as calamine.

BRIGHTLY colouredcrystals and botryoidalmasses come fromTsumeb, Namibia.

COMPOSITION Carbonate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Nearly perfect,rhombohedral/Uneven to conchoidal.LUSTRE/STREAK Vitreous or waxy/White.HARDNESS/DENSITY 4–4.5 / 4.43.KEY PROPERTIES Does not fizz in cold, diluted HCl.

SECTION SHOWN

very dark bluecrystal, translucentwith vitreous lustre

botryoidalblue coating

earthy, whitesmithsonite

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crystals show cubicsymmetry

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HISTORIC lead and zinc mines on theisland of Sardinia,Italy, have yieldedbeautiful specimens of aurichalcite.

dulllustre

COMPOSITION Carbonate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Silky or pearly/White or pale blue-green.HARDNESS/DENSITY 1–2 / 3.96.KEY PROPERTIES Tufts softer than malachite(p.112) or rosasite (Cu,Zn)2(Co3)(OH)2.

Aurichalcite(Zn,Cu2+)5(CO3)2(OH)6

Aurichalcite is found as delicate pale green, bluish green,or pale blue acicular or lath-like crystals. These form tufts,spherical aggregates, and coatings, and are found in theoxidized zones of copper-zinc deposits.Aurichalcite comesfrom many places,but rarely in large quantities.

SECTION SHOWN soft crystals withpearly lustre

tufts of radiating,lathe-like crystals

iron-richgossanmatrix

COMPOSITION Carbonate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/None.LUSTRE/STREAK Pearly, silky, dull orearthy/White.HARDNESS/DENSITY 2–2.5 / 4.00.KEY PROPERTIES Fizzes in diluted HCl,fluoresces blue-white; associates with zinc ores.

pea-likepisoliticcrystal

HydrozinciteZn5(CO3)2(OH)6

Usually hydrozincite forms fibrous, stalactitic, or powderycoatings, and pisolitic, nodular, or massive aggregates. It is

colourless or white, or tinted othercolours by impurities. Under UVradiation, it fluoresces bluish white.Hydrozincite forms by the alteration

of sphalerite orsmithsonite in theoxidized zone of

zinc deposits.

HYDROZINCITE is seenencrusting the walls ofthe Andara zinc minein the Picos deEuropa, Spain.

rock matrix

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HemimorphiteZn4Si2O7(OH)2·H2O

Hemimorphite crystals are colourless, white, or yellow.They are thin, tabular, pointed at one end and straight atthe other, and are usually grouped in fan-shaped clusters.Botryoidal or chalky hemimorphite is also common and iswhite, grey, brown, green, or greenish blue. Widelydistributed in the oxidized zone of zinc deposits,hemimorphite used to be known as calamine,especially in the USA.

THE MINE at Andara,Spain, is a source ofhemimorphite, asecondary mineral of zinc deposits.

COMPOSITION Silicate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven,subconchoidal.LUSTRE/STREAK Vitreous, adamantine, orpearly/White.HARDNESS/DENSITY 4.5–5 / 3.47.KEY PROPERTIES Crystal shape and habit.rock matrix

THE MINERAL is foundin the ancient mines ofLavrion (Laurium),close to Cape Sounionin Greece.

COMPOSITION Arsenate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Good/Uneven.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 3.5 / 4.32.KEY PROPERTIES Crystal shape andfluorescence distinguish it from hemimorphite(below) and smithsonite (p.105).

AdamiteZn2(As4O)(OH)

Adamite forms honey-coloured, yellow, or yellowish greencrystals and crystalline crusts, which may fluoresce lemonyellow under UV light. It is a secondary mineral found inthe oxidized zone of zinc deposits. The most famouslocalities for adamite are Laurium in Greece, Tsumeb inNamibia, and Mapimí and elsewhere in Mexico.

globular aggregateof small crystals

iron-richorangematrix

tufts of palecrystals incavity

botryoidal aggregates of tiny crystals vitreous

lustre

tabularcrystals

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THIS SAMPLE of nativecopper is associatedwith red cuprite andgreen malachite. It isfrom Leicestershire,England.

CopperCu

Copper is one of the few metals that occurs naturally in‘native’ form, not bonded to other chemical elements. It isopaque and bright metallic salmon pink when fresh, butsoon turns dull brown. Most copper is found as irregular,flattened, branching, or dendritic masses. Crystals, which

are usually cubic or dodecahedral, are uncommon.Native copper forms in the

oxidized zone of copperdeposits, and is found

in basic andultrabasicigneous rocks.

pinkishbrowncolour

COMPOSITION Sulphide.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Indistinct/Conchoidal.LUSTRE/STREAK Metallic/Blackish grey.HARDNESS/DENSITY 2.5–3 / 5.5–5.8.KEY PROPERTIES Crystal shape, but canresemble rarer enargite (p.111).

matrix containsother coppersulphides

ChalcociteCu2S

An important ore of copper, chalcocite is opaque, darkmetallic grey, becoming dull on exposure to light. Crystalsare striated, and prismatic or tabular, or form pseudo-hexagonal twins. Most chalcocite is massive or granular. It occurs in hydrothermal and porphyry copper deposits.

RICH chalcocite‘blankets’ form wherewater alters primarycopper minerals.

COMPOSITION Element.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Hackly.LUSTRE/STREAK Metallic/Shiny pinkishbrown.HARDNESS/DENSITY 2.5–3 / 8.95.KEY PROPERTIES A pinkish brown metal that is easily bent.

bends veryeasily

hacklysurfacewhenbroken

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metalliclustre

conchoidal fracture

massivehabit

dendriticmass ofcrystals

pseudo-hexagonaltwins


BorniteCu5FeS4

This important ore of copper is opaque, metallic brownishred in colour, but develops an iridescent purple tarnish.Although it has orthorhombic symmetry, crystals are cubic,octahedral, or dodecahedral; granular or massiveaggregates are common. Bornite is foundin hydrothermal copper ore deposits,pegmatites, basic and ultrabasicigneous rocks, and skarns.

BORNITE in Arizona isassociated withPrecambian volcanic

activity and morerecent granite

intrusions.

metallicbrownish red on freshsurfaces

DISTINCT crystals andmassive aggregates ofcovellite come fromButte, Montana, USA.

CovelliteCuS

Opaque, with a bright metallic blue colour and purpleiridescence, covellite is easy to recognize. It is found ashexagonal platy crystals, foliated, or massive aggregates,and as a superficial coating on other copper sulphideminerals. It forms in the oxidized zone of copper sulphidedeposits in association with bornite, chalcocite, and chalcopyrite.

lamellarstructure

purpletarnish

covellitecrystals

COMPOSITION Sulphide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE None/Uneven, nearlyconchoidal.LUSTRE/STREAK Metallic/Greyish black.HARDNESS/DENSITY 3–3.25 / 5.06.KEY PROPERTIES Metallic brownish red,purple tarnish and pseudo-cubic symmetry.

COMPOSITION Sulphide.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Perfect, thin sheetsmay be flexible/Uneven.LUSTRE/STREAK Submetallic, dull orresinous/Shiny lead grey.HARDNESS/DENSITY 1.5–2 / 4.6.KEY PROPERTIES Blue with purple tarnish.

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massivebornite

crystalsappearcubic

metallicblue colour

purpletarnish

COMPOSITION Sulphide.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Poor/Uneven.LUSTRE/STREAK Metallic/Greenish black.HARDNESS/DENSITY 3.5–4 / 4.1–4.3.KEY PROPERTIES Brassy yellow colour,hardness, and tarnish distinguish this ‘fool’sgold‘ from real gold (p.87) and pyrite (p.124).


TETRAHEDRITE crystalsfrom Cornwall,England, are oftencoated in yellowchalcopyrite.

ChalcopyriteCuFeS2

Fresh chalcopyrite is opaque, brassy yellow, but onexposure it develops an iridescent tarnish. Its crystalsappear tetrahedral, and massive aggregates are common,

botryoidal masses less so. Chalcopyrite is a constituent of hydrothermal

sulphide deposits. It is foundas disseminated grains inigneous rocks and is an

important ore mineralin porphyry

copper deposits.More rarely it

occurs inmetamorphic

rocks.

THIS OPENCAST coppermine at Phalaborwa,South Africa, extractschalcopyrite, one of theprincipal ores of copper.

TetrahedriteCu10(Fe,Zn)2(Sb,As)4S13

The name comes from this mineral’s characteristictetrahedral crystals, although it also occurs as massive or

granular aggregates. This mineral isopaque, metallic grey, or nearly black,and it sometimes coats or is coated

by brassy yellow chalcopyrite. It is found in hydro-thermal

veins or contactmetamorphic rocks.

tetrahedralcrystal

massivechalcopyrite

brassytetrahedralcrystal

iridescenttarnish

COMPOSITION Sulphosalt.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Subconchoidal.LUSTRE/STREAK Metallic/Black to brown.HARDNESS/DENSITY 3–4.5 / 4.97.KEY PROPERTIES Tetrahedral crystals looklike its arsenic-bearing counterpart tennantite,which associates with other arsenic minerals.

white quartzcrystals

mixed sulphideminerals

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unevenfracture


HYDROTHERMAL veinsthat contain enargitein the Red Mountainmining district ofColorado, USA.

EnargiteCu3AsS4

Like many sulphide minerals, enargite has a bright metalliclustre and is opaque grey when fresh. On exposure to lightand pollutants, it turns dull black. Crystals are tabular orprismatic, commonly striated along the prism faces. Twins

are common, and may appear hexagonal.Enargite occurs in hydrothermal veindeposits, and exceptional crystals come

from Quiruvilca, Peru andButte, Montana, USA.

CupriteCu

1+

2OCuprite crystals are usually octahedral or cubic in shape,and massive or granular aggregates are common. Freshcuprite is translucent bright red, but exposure to light andpollutants can turn surfaces dullmetallic grey. An exception ischalcotrichite, a fibrous varietythat retains its red coloration.

Cuprite is an importantore of copper found

in the oxidizedzone of oredeposits.

RED crystals of cupritecome from Brisbee andother locations in

Arizona, USA.

COMPOSITION Oxide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Poor/Conchoidal oruneven.LUSTRE/STREAK Adamantine or sub-metallic/Brownish red, shiny.HARDNESS/DENSITY 3.5–4 / 6.14.KEY PROPERTIES Red; found with copper ores.

brightadamantinelustre

SECTION SHOWN

COMPOSITION Sulphide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Metallic/Greyish black.HARDNESS/DENSITY 3 / 4.45.KEY PROPERTIES Crystal shape, but can resemble more common chalcocite (p.108).

translucent red

perfectcleavage

CHALCOTRICHITE

red, hair-likecrystals

brightmetalliclustre �

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striatedgreycrystal


MalachiteCu2

2+(CO3)(OH)2

Malachite is the most common of the copper secondaryminerals and is always pale to dark green in colour.Crystals can be tabular but are usually prismatic oracicular, forming tufts and drusy coatings, which may bevelvety in appearance. Botryoidal aggregates of malachiteare also common, and when cut and polished, make abeautiful, banded, decorative stone. Malachite from theUral Mountains of Russia was extensively used to decoratethe palaces of the tsars, but today the malachite used in

jewellery comes from the copper belt of the Democratic Republic of Congo.

NOTE

True malachite crystals are typically small and slender, but malachite often formspseudomorphs, where it replaces a mineralwithout altering the crystal shape. Nodules are found containing azurite that is partiallyaltered to malachite, but the tabular shape of the azurite crystals (above right) is retained.

GREEN malachite andblue azurite occurtogether in the orange-brown goethite thatforms ‘gossan’ caps oncopper deposits.

botryoidalhabit

green incolour

COMPOSITION Carbonate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Subconchoidalor uneven.LUSTRE/STREAK Vitreous; silky iffibrous/Pale green.HARDNESS/DENSITY 3.5–4 / 4.05.KEY PROPERTIES Green; fizzes in diluted HCl.

dark and lightbands

silkylustre

spray ofcrystals

gossanmatrix

polishedsurface

SECTION SHOWN

vitreouslustre

radiatingfibrouscrystals

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COMPOSITION Carbonate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Vitreous/Pale blue.HARDNESS/DENSITY 3.5–4 / 3.77. KEY PROPERTIES Bright to deep bluemineral that fizzes in diluted HCl.

FINE crystals of azuritecome from the Touissitlead mine in the AtlasMountains, Morocco.

AzuriteCu3

2+(CO3)2(OH)2

Azurite crystals are complex, often tabular with wedge-shaped terminations, and can form rosette-shapedaggregates. Well-developed crystals aredark azure blue, but massive or earthyaggregates may be paler. Azurite iscommon in the oxidizedzones of copper depositsformed in carbonate rocks,such as limestone.

vitreouslustre

AtacamiteCu2

2+Cl(OH)3

Bright or dark emerald green crystals of atacamite aretypically prismatic or tabular, striated along the length, andhave wedge-shaped terminations. Fibrous, granular, ormassive aggregates also occur. Atacamite forms inthe oxidation zone of copper deposits in salt-rich, arid environments. It also forms involcanic fumaroles and in ocean-bottom black smoker deposits.

ATACAMITE formsreadily in aridenvironments, such asthis opencast coppermine in Peru.

COMPOSITION Halide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Adamantine orvitreous/Apple green.HARDNESS/DENSITY 3–3.5 / 3.74–3.78.KEY PROPERTIES Green mineral forms insalty arid places. Does not fizz in diluted HCl. light green

malachite

dark greencrystalsprismatic

crystal

thin,tabularcrystals

radiating acicularcrystal

associatedquartz

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blocky azure bluecrystals

SECTION SHOWN


BEAUTIFUL examples ofcyanotrichite come fromBisbee and elsewhere in theArizona copper belt, USA.

BrochantiteCu4

2+(SO4)(OH)6

Brochantite is emerald green or blue-green and formsacicular or fibrous crystals, usually in tufts, drusy coatings,and crusts. It is found in the oxidized zones of copperdeposits, especially those in arid regions of the world.Excellent examples of brochantite come from coppermines in Chile, the Democratic Republic of Congo, and Arizona, USA.

THIS COMMON

secondary mineral wasonce found in themetal mines ofCornwall, England.

fibrousgreencrystal

COMPOSITION Sulphate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Uneven/Uneven.LUSTRE/STREAK Silky/Pale blue.HARDNESS/DENSITY Not determined /2.74–2.95.KEY PROPERTIES Fine, fibrous sky bluecrystals.

COMPOSITION Sulphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven orconchoidal.LUSTRE/STREAK Vitreous/Pale green.HARDNESS/DENSITY 3.5–4 / 3.97.KEY PROPERTIES Bright green crystals that donot fizz in diluted HCl.

rock matrix

CyanotrichiteCu4

2+Al2(SO4)(OH)12·2H2O

Cyanotrichite is one of several copper minerals that are skyblue to azure blue in colour. Its name, from the Greek for‘blue’ and ‘hair’, describes it very well. Crystals are acicularor fibrous, and form tufts, velvety coatings and delicateradiating groups. Cyanotrichite is found in the oxidizedzone of copper sulphide deposits. The specimen belowcomes from Cap Garonne, Var, France.

vitreouslustre

sky blue, hair-likecrystals

iron-richgossan

SECTION SHOWN

greenbrochantite

brochantitewith blueazurite

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LIBETHENITE occurs as small, lustrouscrystals at VillaVicosa, Portugal.

LibetheniteCu2

2+(PO4)(OH)

Crystals of libethenite are usually short, prismatic withwedge-shaped terminations, or appear octahedral andrather rounded. It is dark emerald or olive green,sometimes nearly black. Libethenite is found as clusters ofcrystals, drusy coatings, and crusts in the oxidized zones ofcopper deposits, often associated with other phosphateminerals. It was first discovered at Libethen (nowL'ubietová), Slovakia, and small quantities occurin many places.

OliveniteCu2

2+(AsO4)(OH)

Olivenite gets its name from its typical olive green coloration,although it can also be grey, pale yellow, or white. Crystalsare prismatic or fibrous, and earthy or matted aggregates arecommon. If dense masses of crystals are banded like woodgrain, it is given the name wood copper. Olivenite is the most common copper arsenate mineralfound in the oxidized zones of hydrothermalcopper deposits.

THE DUMPS of Claramine in the BlackForest of Germanyhave yielded excellentspecimens of oliveniteand other minerals.

COMPOSITION Arsenate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Indistinct/Conchoidalor uneven.LUSTRE/STREAK Adamantine, vitreous orsilky/Yellowish green.HARDNESS/DENSITY 3 / 4.46.KEY PROPERTIES Colour and fibrous habit.

COMPOSITION Phosphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Imperfect/Conchoidalor uneven.LUSTRE/STREAK Vitreous or greasy/Green.HARDNESS/DENSITY 4 / 3.97.KEY PROPERTIES Colour and crystal shape;less fibrous than olivenite (below).

fibrousolivenite

rock matrix

short, prismatic,olive greencrystals

quartzmatrix

blackish greencolour

SECTION SHOWN

SECTION SHOWN

crystalsappearoctahedral

rockmatrix

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THE MINE at Tsumeb,Namibia has yielded anumber of superbdioptase crystals.

Chrysocolla(Cu,Al)2H2Si2O5(OH)4

.nH2O

Crystals of chrysocolla are acicular but uncommon, and itis usually found as fine-grained, sometimes botryoidalmasses. The colour is rich blue-green. Chrysocolla is acommon mineral in the oxidized zones of copper oredeposits, often intimately mixed with quartz or opal,

which makes it hard enough to bepolished as a semi-preciousgemstone. Fine chrysocolla comes

from the copper belt of Congoand from mines in Peru and

Arizona, USA.

DioptaseCu6Si6O18

.6H2O

Distinctive, vibrant, bluish green colour and its tendency tooccur as well-formed crystals makes dioptase very popularwith collectors. Crystals are blocky with rhombohedralterminations, or more rarely, elongate prisms. Dioptase canalso be granular or massive. It forms in the oxidized zonesof some copper deposits, and the most famous localities

are Tsumeb in Namibia, Renéville in Congo Republic,and Altyn-Tyube in Kazakhstan.

COMPOSITION Silicate.CRYSTAL SYSTEM Probably orthorhombic.CLEAVAGE/FRACTURE None/Conchoidal orbrittle.LUSTRE/STREAK Usually earthy/White orpale green.HARDNESS/DENSITY 2–4 / 1.93–2.4.KEY PROPERTIES Absence of crystals.

COMPOSITION Silicate.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Vitreous/Green.HARDNESS/DENSITY 5 / 3.28–3.35.KEY PROPERTIES Vibrant green colour andcrystal shape are distinctive.

associatedazurite

orange iron-rich matrix

CHRYSOCOLLA mixedwith other copperminerals is anornamental stone fromIsrael, known as Eilat stone.

bluishgreen

earthychrysocolla

conchoidalfracture

�short prismaticcrystals

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intense bluishgreen


MANGANITE occurswith other manganeseoxides and minerals,such as rhodonite(shown here) andrhodochrosite.

ManganiteMn3+O(OH)

Opaque, metallic dark grey or black manganite crystals areprismatic and striated lengthways. They typically have flator blunt terminations and are often grouped in bundles.Manganite can also be massive or granular and is thenhard to distinguish from other manganese oxides, such aspyrolusite. Usually, manganite forms in low temperaturehydrothermal veins, hot spring manganese deposits, andby alteration of othermanganese minerals.

HausmanniteMn2+Mn2

3+O4

Hausmannite is dark brown or black and is usuallygranular or massive. Well-formed crystals, although lesscommon, are more distinctive. They appear octahedral inshape, but often have additional faces. An ore ofmanganese, hausmanniteforms in hydrothermalveins. It also occurswhere manganese-richrocks have beenmetamorphosed.

AN ORE mineral of theKalahari manganesefield in South Africa,hausmannite formsmassive aggregates and fine crystals.

COMPOSITION Oxide.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Submetallic/Reddish brown.HARDNESS/DENSITY 5.5 / 4.84. KEY PROPERTIES Black octahedral-shapedcrystals, perfect cleavage, reddish brown streak.

COMPOSITION Oxide.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Metallic or submetallic/Darkreddish brown.HARDNESS/DENSITY 4 / 4.29–4.34.KEY PROPERTIES Black mineral distingishedfrom pyrolusite (p.118) by streak.

massive habit

crystalsappearoctahedral

bundles of crystals

flatterminations

submetalliclustre

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striationson faces


MANGANESE, iron, andcopper oxide mineralscolour Castle Rocks,Michigan, USA.

PyrolusiteMn4+O2

This common opaque black or dark grey mineral is usuallyfibrous or fine-grained; prismatic crystals are rare. It isfound as botryoidal masses, concretions, and as coatingsthat may be powdery or dendritic. Pyrolusite, a secondarymineral in manganese deposits, also forms in bogs, lakes,and shallow seas.

PYROLUSITE occurs inthe metamorphosedsedimentary rocks ofthe Green Mountains,Vermont, USA.

COMPOSITION Oxides and hydroxides.CRYSTAL SYSTEM Various.CLEAVAGE/FRACTURE None/Uneven.LUSTRE/STREAK Earthy/Usually black.HARDNESS/DENSITY Varies according tocomposition.KEY PROPERTIES Soft or hard black masses;no distinct crystals visible.

black dendriticpyrolusite

botryoidalhabit

WAD

blackmass

marlylimestone

Psilomelane, WadMixed manganese oxides

Black manganese oxides that lack distinct crystals aredifficult to tell apart, and the problem is greater when theyoccur in mixtures. Soft, unidentified manganese oxides areknown in general terms as wad, and when they are hard, as

psilomelane. In the past, psilomelane has alsobeen used as a name for

the barium manganeseoxide romanèchiteand it often containsthis mineral.

no obviouscrystals

hard, blackmass

soft powderymass

earthy habit

COMPOSITION Oxide.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Metallic to dull/Bluish black.HARDNESS/DENSITY 6–6.5 / 5.06.KEY PROPERTIES Black mineral,distinguished from other manganese oxidesby streak, crystal shape, or chemical analysis.

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THESE HYDROTHERMAL

veins of rhodochrosite,galena, and fluorite are in the roof of theSweet Home mine,Colorado, USA, andother localities.

RhodochrositeMg2+CO3

Rhodochrosite is usually pink, rarely brown, and formsrhombohedral or dog-tooth crystals like calcite. Bandedrhodochrosite comes from Argentina and Peru, and makesan attractive decorative stone, but the finest crystals arefrom the Sweet Home mine and elsewhere in Colorado,USA, and are bright cherry red.Rhodochrosite is found mainlyin hydrothermal depositswith black manganeseoxides and mineralssuch as fluorite,galena, andsphalerite.

NOTE

Manganese secondary minerals tend to be pinkor black. Minerals containing other chemicalelements can also be a particular colour. Coppersecondaries are blue or green; those of cobalt,lilac pink, and of nickel, apple green. Chromiumsecondaries are green or purple-red, while ironminerals are often yellow, brown, or green.

bands ofpink andred

quartz crystal

cherry redcrystals

rhombohedralcrystal

COMPOSITION Carbonate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect,rhombohedral/Uneven or conchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 3.5–4 / 3.7.KEY PROPERTIES Pink mineral that fizzes indilute HCl; does not fluoresce under UV light.

polished surface

pale silvery-yellow pyrite pink

rhodochrosite

vitreouslustre

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BRAUNITE is animportant manganeseore in SouthAfrica.

RhodoniteCaMn4Si5O15

Rhodonite is a pink manganese mineral and an attractivesemi-precious gemstone. It is usually found as coarse,cleavable or fine-grained compact masses veined with blackmanganese oxides. It can also be granular, but crystals are

much less common. Rhodonite comes frommanganese deposits formed by

hydrothermal, metamorphic, and sedimentary processes.

THIS RHODONITE

nodule contains blackmanganese oxides andbrown bustamite.

BrauniteMn

2+Mn

3+

6SiO12Not the most striking of minerals, braunite is an importantore of manganese. It is opaque, brown-black, or dark grey,and is typically granular or massive. Crystals are less

common and are pyramidal, nearlyoctahedral in shape. Braunite is

found with manganese oxidesand forms by metamorphism

or the weathering ofmanganese deposits.

submetalliclustre

COMPOSITION Silicate.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal to uneven.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 5.5–6.5 / 3.57–3.76.KEY PROPERTIES Harder than rhodochrosite(p.119) and does not fizz in dilute HCl.

massiverhodonite

crystal appearsoctahedral

mass of smallcrystals

tabular crystals

blackmanganeseoxides

polished pebble

COMPOSITION Oxide or silicate.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Perfect/Uneven tosubconchoidal.LUSTRE/STREAK Submetallic/Brownish blackto steel grey.HARDNESS/DENSITY 6–6.5 / 4.72–4.83.KEY PROPERTIES Shape, streak, and association.

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COMPOSITION Oxide.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Perfect/Subconchoidal.LUSTRE/STREAK Adamantine ormetallic/White or pale yellow.HARDNESS/DENSITY 5.5–6.0 / 3.79–3.97.KEY PROPERTIES Crystal form and absenceof magnetism.


BrookiteTiO2

Brookite has the same chemical composition as anatase(above) but has orthorhombic symmetry. Crystals are thinor thick, tabular, or less commonly, pyramidal or pseudo-hexagonal. It is always red- or yellow-brown, dark brownor black. Brookite occurs in alpine-type hydrothermalveins, in some contact metamorphic rocks, and as adetrital mineral in sedimentary deposits.

THE AREA aroundSnowdon, Wales, hasyielded thin tabularcrystals of brookite.

COMPOSITION Oxide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Indistinct/Subconchoidal.LUSTRE/STREAK Metallic, submetallic, oradamantine/White, greyish, or yellowish.HARDNESS/DENSITY 5.5–6 / 4.08–4.18.KEY PROPERTIES Colour and lack of cleavage.

AnataseTiO2

Elongate octahedral crystals are the most common habit of anatase. It also occurs as tabular and, rarely, prismaticcrystals. Anatase varies in colour from yellow- or red-brown to black, blue, grey, or lilac. It occurs, usually insmall quantities, in various igneous and metamorphicrocks, and as a detrital mineral in sediments.

THE FINEST crystals ofanatase are found inthe lens-shaped hydro-thermal veins typicalof the European Alps.

SECTION SHOWN

dipyramidalcrystal

elongate octahedralcrystals

metalliclustre

modifiedoctahedralcrystals

rockmatrix

�

�transparentlustre


NOTE

Like rutile crystals inquartz (p.143),inclusions in crystalsare very common,although most areusually microscopicin size. Bubbles ofgas or mineralisingfluid can sometimesform inclusions.

CHINA has become amajor supplier of rutilefor industry, withimportant mines in theHubei province.

RutileTiO2

This mineral is familiar to many people as the pale golden,acicular crystals trapped inside crystals of quartz. When notenclosed in quartz, it is usually darker in colour, yellowishor reddish brown, dark brown or black. It forms striated,prismatic crystals and massive aggregates. Twins are verycommon and may be geniculate (knee-shaped), cyclic, orreticulated (net-like). Rutile is an accessory mineral invarious igneous and metamorphic rocks, and occurs inalpine-type hydrothermal veins, and in some clasticsediments. Star-like sprays of rutile crystals, radiating from

the faces of hematite crystals, come fromNovo Horizonte, Bahia, Brazil

and elsewhere.

adamantinelustre

cyclic twin

striations along length of crystal

prismaticcrystal

rutilecrystals inquartz

COMPOSITION Oxide.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Good/Conchoidal touneven.LUSTRE/STREAK Adamantine orsubmetallic/Light brown or grey.HARDNESS/DENSITY 6–6.5 / 4.23.KEY PROPERTIES Colour, striations, twinning.

massiverutile

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PLACER deposits ofblack ilmenite sandsare dredged for thisimportant ore oftitanium.

IlmeniteFe2+TiO3Ilmenite is opaque and metallic grey-black but may weatherdull brown. It occurs as tabular crystals and granular ormassive aggregates. Intergrowths with hematite or magnetiteare common. Found in many igneous and high-grade

metamorphic rocks,economic deposits occur

in anorthosites andother layered

basic igneousrocks and asplacer sanddeposits.

TitaniteCaTiOSiO4

The crystals have a distinctive flattened wedge-shape, butthey can also be prismatic or form massive aggregates. Thecolour is normally black, brown, grey, green, or yellow.Titanite is found in metamorphic rocks suchas gneisses, schists, and marbles; in skarns,alpine-type hydrothermal veins, and as anaccessory mineral in acid and intermediateplutonic igneous rocks.

MANY famouslocalities for titanite, a mineral formerlyknown as sphene, arein the European Alps.

large,translucent,brown crystal

well-formedcrystal

metalliclustre

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good/Subconchoidal.LUSTRE/STREAK Adamantine orresinous/White.HARDNESS 5–5.5DENSITY 3.48–3.60.KEY PROPERTIES Wedge-shaped crystals.

rock matrix

small orangecrystals

wedgeshape

COMPOSITION Oxide.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Parting/Conchoidal.LUSTRE/STREAK Metallic orsubmetallic/Black or reddish brown.HARDNESS/DENSITY 5–6 / 4.72.KEY PROPERTIES Colour, crystal shape, andweak magnetism.

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thin, grey,tabular crystals


PyriteFeS2

A remarkably common mineral, pyrite is easy to distinguishfrom most other sulphides. It is opaque and pale silvery-yellow when fresh, turning darker and tarnishing withexposure. Crystals are cubic, octahedral, or twelve-sided‘pyritohedral’, and are often striated. Pyrite can also bemassive or granular, or form flattened discs or nodules ofradiating elongate crystals. Discs and nodules are found inmany kinds of sedimentary rocks, where pyrite can also fillor replace fossils. Crystals and crystalline masses of pyrite

occur in many igneous,metamorphic, and

sedimentary rocks,and extensively in

hydrothermaldeposits.

metallic, paleyellow cube

PYRITE crystals coverthis mould of anammonite shell inclay. The exampleshown here comesfrom Oxfordshire,England.

marl matrix

COMPOSITION Sulphide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Indistinctpartings/Conchoidal to uneven.LUSTRE/STREAK Metallic/Greenish black.HARDNESS/DENSITY 6–6.5 / 5.02.KEY PROPERTIES Crystal shape and colourpaler than chalcopyrite (p.110).

NOTE

Both pyrite and marcasite (above right) reactwith water vapour in the air, decomposing toform powdery yellow and white iron sulphates and sulphuric acid, which can burn holes in containers and labels. To prevent ‘pyritedecay’, specimens must be stored in a dry atmosphere.

octahedralcrystal

striations

radiatingcrystals

five-sidedface

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NODULES composed ofmarcasite crystals

come from thechalk depositsof France and

England.

MarcasiteFeS2

Marcasite is opaque and very pale silvery-yellow whenfresh, darkening and tarnishing after exposure. Crystals aretabular, pyramidal, or prismatic, or form spear-shapedtwins. It is found in granular, stalactitic, ormassive aggregates and ofteninfills or replaces fossils.Less widely distributed thanpyrite, marcasite is found insedimentary rocks.

PyrrhotiteFe1-xS (x=0.1 to 0.2)

Often seen mixed with other sulphide minerals in massiveor granular aggregates, pyrrhotite also occurs as tabular or platy hexagonal crystals, or rosette-shaped crystalclusters. It is opaque and bronze-yellow to pink-yellow in colour,readily tarnishing brown.Pyrrhotite forms in basic andultrabasic igneous rocks,pegmatites, high-temperaturehydrothermal veins, and lessoften in some metamorphicand sedimentary rocks.

GOLD prospecting hasrevealed deposits ofpyrrhotite and other

sulphide minerals inthe Alaska

Range, USA.

COMPOSITION Sulphide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Distinct/Uneven, brittle.LUSTRE/STREAK Metallic/Greyish black.HARDNESS/DENSITY 6–6.5 / 4.89.KEY PROPERTIES Crystal shape and paler colour distinguishes marcasite from pyrite (left).

COMPOSITION Sulphide.CRYSTAL SYSTEM Monoclinic and hexagonal.CLEAVAGE/FRACTURE Cleavage-likeparting/Uneven to subconchoidal.LUSTRE/STREAK Metallic/Greyish black.HARDNESS/DENSITY 3.5–4.5 / 4.58–4.65.KEY PROPERTIES Slight magnetism, crystalshape, and colour.

browntarnish

group ofhexagonalcrystals

spear-shapedtwin

pointedorthorhombiccrystals

chalkmatrix

SECTION SHOWN

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ArsenopyriteFeAsS

Light silvery grey arsenopyrite gives off the odour of garlic ifcrushed – a sign that it contains arsenic. Crystals are blockyor prismatic, often diamond-shaped in section. They areusually striated and often twinned. Massive or granularaggregates also occur. Arsenopyrite is a hydrothermalmineral often found in pegmatites and gold- or tin-bearing

veins. It also occurs in contact metamorphicsulphide deposits and other metamorphic rocks.

LARGE aggregates ofarsenopyrite crystalscome from mines atPanasqueira, Portugal.

IGNEOUS intrusions atSouth Africa’sBushveld Complex area rich source ofmagnetite.

MagnetiteFe2+Fe3+

2O4

Magnetite is a strong natural magnet. It is grey or black,weathering rusty brown. Crystals are octahedral, more rarelydodecahedral, and may be striated. Massive or granularaggregates are very common. Magnetite is an accessorymineral in many igneous and metamorphic rocks and a

detrital mineral in some sediments. Economicallyimportant deposits occur in layered basic and

ultrabasic igneous intrusions and banded ironstones.

diamond-shaped insection

granularmass ofmagnetite

SECTION SHOWN

blockycrystals

brightmetalliclustre

octahedralcrystal

metalliclustre

COMPOSITION Sulphide.CRYSTAL SYSTEM Monoclinic,

pseudo-orthorhombic.CLEAVAGE/FRACTURE Distinct/Uneven.LUSTRE/STREAK Metallic/Greyish black.HARDNESS/DENSITY 5.5–6 / 6.07.KEY PROPERTIES Silvery grey; less densethan iron arsenide (löllingite).

COMPOSITION Oxide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Cleavage-likeparting/Uneven.

LUSTRE/STREAK Metallic to dull/Black.HARDNESS/DENSITY 5.5–6.5 / 5.18.

KEY PROPERTIES Strongly magnetic, greyoctahedral mineral with a black streak.

siderite

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striationson surface


BANDED ironstoneformations are rich in hematite andimportant iron ores.

Hematiteα-Fe2O3

The names of the different varieties of hematite are verydescriptive. Opaque grey crystals with a mirror-like lustreare known as specular hematite. Crystals are complexrhombohedral, pyramidal, or tabular, and sometimes thinand platy; triangular striations are often seen. Rosette-shaped groups of crystals are known as eisenrose. Wherecrystals have grown tightly together to form compact

masses, hematite is always red. Redreniform masses are known as

kidney ore. Hematite is foundin volcanic fumeroles

and hydrothermalveins, in contactmetamorphic rocks,

banded ironstones,oolitic ironstones, andas an alteration productof other iron minerals.

reniformred mass

colourfultarnish onsurface

unevenbrokensurface

nearly cubiccrystals

KIDNEY ORE

SECTION SHOWN

COMPOSITION Oxide.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Cleavage-likeparting/Uneven or subconchoidal.LUSTRE/STREAK Metallic, submetallic, ordull/Cherry red or reddish brown.HARDNESS/DENSITY 5–6 / 5.26.KEY PROPERTIES Colour and streak.

NOTE

Whether in metallicgrey crystals orbrownish red masses,hematite has a brickred streak, whichaids identification.Goethite (p.128) canalso form botryoidalbrown masses butits streak is alwaysyellowish brown.

highly reflective

hexagonalplaty crystal

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SPECULAR HEMATITE


mass ofclose-packeddark browncrystals

COATINGS of limonite,where iron-rich rockshave been weathered,give this landscape itsrusty colour.

Goethiteα-Fe3+O(OH)

Goethite is an exceptionally common mineral. Crystals arebrown to nearly black. They are small, prismatic, andusually occur in tufts and drusy coatings. But mostgoethite is found as coatings and botryoidal, reniform,stalactitic, or massive aggregates. These can be brownish

yellow, reddish brown, or dark brown. Goethite is analteration product in iron-bearing deposits; present in

gossans, laterites, and many ironstones.

GOETHITE is one of theiron oxides extractedfrom the Kremikovtsiiron deposit at thismine in Bulgaria.

LimoniteMixed hydrated iron hydroxides

To modern geologists, limonite is a general name formixtures of the hydrated iron hydroxide minerals goethite,lepidocrocite, and akaganéite. Some hematite, clay, andother impurities may also be present. Limonite is yellow,brownish yellow or orange-brown with a yellowish brownstreak. It is fine-grained, massive, stalactitic, or powdery.

When analysed, many specimens of limonite turnout to be pure goethite.

COMPOSITION Oxide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Adamantine, dull, earthy, orsilky/Yellowish brown.HARDNESS/DENSITY 5–5.5 / 4.28.KEY PROPERTIES Yellowish brown streak; islike naturally occurring rust.

PEA IRON ORE

COMPOSITION Oxides.CRYSTAL SYSTEM Various.CLEAVAGE/FRACTURE None/Uneven.LUSTRE/STREAK Dull, earthy, orwaxy/Yellowish brown.HARDNESS/DENSITY Varies according tocomposition / 3.5–4.3.KEY PROPERTIES Colour, streak, and habit.

SECTION SHOWN

small, adamantineprismatic crystals

individualcrystals cannotbe seen

sphericularmasses

brownishyellowmass

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A COMMON mineral inhydrothermal veins,siderite is associatedhere with colourlessquartz and dark brown sphalerite.

DufréniteCaFe2+

2Fe3+10(OH)12(PO4)8

.4H2O

Dufrenite is found as crusts, spherical aggregates ofradiating crystals, and botryoidal coatings. These are oftencolour-banded, with the fresh centre being dark green orgreenish black and the weathered surface being olive greenor brown. Crystals are tabular and rather rounded, andoccur in sheaf-like aggregates. Dufrénite is found withother phosphate minerals mainly in the secondaryzone of iron-rich deposits.

DARK GREEN spherulesof dufrénite withyellow cyrilovite arefound in joints in thekaolinized granite ofCornwall, England.

COMPOSITION Phosphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous or silky/Green oryellowish green.HARDNESS/DENSITY 3.5–4.5 / 3.1–3.34.KEY PROPERTIES Habit, colour-banding, andassociation with other phosphates.

SideriteFe2+CO3

Siderite typically forms irregular crystalline masses orrhombohedral crystals with curved faces, so rarer elongate,prismatic crystals are highly prized. This common mineralis usually found in hydrothermal veins and in the oxidizedzones of iron ore deposits. Remarkably lustrous, dark brown crystalline masses have come fromthe rare cryolite-bearing pegmatite at Ivigtut, Greenland.

colourlessquartz

unusual botryoidalsiderite

pearlylustre

SECTION SHOWN

typicalrhombohedralcrystals

COMPOSITION Carbonate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect,rhombohedral/Uneven or conchoidal.LUSTRE/STREAK Vitreous or pearly/White.HARDNESS/DENSITY 3.75–4.25 / 3.96.KEY PROPERTIES Fizzes slowly in dilute HCl;usually darker brown than ankerite (p.152).

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dark browngoethite

botryoidal. olivegreen dufrénite


COMPOSITION Phosphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Splintery.LUSTRE/STREAK Vitreous; dull whenearthy/Colourless, turns blue or brown.HARDNESS/DENSITY 1.5–2 / 2.68.KEY PROPERTIES Indigo blue or greenmineral, dissolves in diluted HCl without fizz.

VivianiteFe2+

3(PO4)2·8H2O

Vivianite crystals are prismatic, blocky, or bladed, and occursingly or in radiating groups. While crystals are dark indigoblue or green and sometimes nearly black, this mineral oftenoccurs as coatings, nodules, and powdery masses, which area much lighter blue. Vivianite forms in the secondary zoneof ore deposits, and as a result of the weathering ofphosphate minerals in granite pegmatites. It also occurs inlake sediments and bog iron ores, and in other sedimentaryrocks rich in bone and organic matter. Here, the crystals canline cavities in fossil shells or form a blue coating on bones.The delicate-bladed crystals, shown below, fill cavities and

moulds left by fossil bivalves in an ironstone from Kerch, Ukraine.

NOTE

When vivianite is first exposed to light it maybe colourless, but it very quickly turns blue orgreen as ferrous iron changes to ferric iron.Continued exposure to light causes somecrystals to fracture and should be avoided.Blocky crystals are most likely to crack thanslender or thin-bladed ones.

earthy, paleblue vivianite

clay matrix

prismaticcrystals

vitreous lustre

POWDERY bluevivianite is found insedimentary clays, forexample, at this pit inOxfordshire, England.

SECTION SHOWN

transparent whencrystals are thin

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radiating blueand greenbladed crystals

crystals incavity leftby fossilshell


AN EXCAVATION of the Ting Tang mine dumps in Cornwall yielded good scorodite crystals.

ScoroditeFe3+AsO4·2H2O

Scorodite is violet, bluish green, leek green, grey, brown, or brownish yellow. Crystals may be tabular or shortprismatic but usually they are di-pyramidal, appearingoctahedral. Drusy coatings are common, but scorodite canalso be porous and earthy, or massive. It is formed by theweathering of arsenopyrite and otherarsenic-bearing minerals.

PharmacosideriteKFe3+

4(AsO4)3(OH)4·6–7H2O

Simple cubic crystals are unusual for phosphate andarsenate minerals, but they are typical of pharmacosiderite.It is olive green, grass green, yellow, brown, or red, andthe crystals are rarely more than a fewmillimetres across. It can bedeposited by hydrothermalsolutions but usually forms byweathering of arsenopyriteand other arsenic minerals.Brown crystals are difficult todistinguish from the rarer but related bariumpharmacosiderite.

THIS MINERAL is neverabundant, but it occurs

at many localitiesworldwide,

including theLake District,

England.

violet, tabularcrystals

arsenic-richmatrix

greencubiccrystals

COMPOSITION Arsenate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Imperfect/Subconchoidal.LUSTRE/STREAK Vitreous or resinous/White.HARDNESS/DENSITY 3.5–4 / 3.27.KEY PROPERTIES Unlike adamite (p.107),scorodite does not fluoresce under UV light.

COMPOSITION Arsenate.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Poor/Uneven.LUSTRE/STREAK Adamantine togreasy/White.HARDNESS/DENSITY 2.5 / 2.8.KEY PROPERTIES Cubic crystals that lack theperfect octahedral cleavage of fluorite (p.156).

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poor cleavage

quartzmatrix

GIBBSITE


JAMAICA is a majorsupplier of bauxite, a laterite rich inaluminium oxides and hydroxides.

GIBBSITEDIASPORE

BÖHMITE

BauxiteMixed aluminium oxides and hydroxides

Gibbsite, diaspore, and böhmite are all aluminium oxidesor hydroxides that occur in altered or metamorphosedaluminium-rich rocks but are most abundant, mixedtogether in a rock called bauxite. These are pale-colouredminerals, but bauxite also contains quartz, clays, hematite,and other iron oxides, so it is variably creamy yellow,orange, pink, and red in colour. It is by far the mostimportant ore of aluminium. Bauxite forms as extensivebut quite shallow deposits where aluminium-rich rockshave been heavily weathered in a humid tropicalenvironment. It may be nodular, pisolitic, or earthy in

structure. Bauxite is named after the depositsat Les Baux-de-Provence in France.

pisoliticstructure

earthyhabit

lamellarcrystals

minuteyellowcrystals

dull to earthylustre

red, where itis iron-rich

trace chromiumgives lilac colour

COMPOSITION Oxides and hydroxides.CRYSTAL SYSTEM Various.CLEAVAGE/FRACTURE None/Irregular.LUSTRE/STREAK Dull, waxy or earthy/White,yellow, or reddish brown.HARDNESS/DENSITY Varies according tocomposition.KEY PROPERTIES Colour; pisolitic habit.

NOTE

Aluminium is a strong, lightweight metal andan excellent conductor of heat. It is abundantin the crust of the Earth and present in manyminerals. The only important ore is bauxite;this is because too much costly energy isrequired to extract aluminium from otherminerals that contain the metal.

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COMPOSITION Phosphate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven orsubconchoidal.LUSTRE/STREAK Vitreous, resinous, or pearly/White.HARDNESS/DENSITY 3.5–4 / 2.36.KEY PROPERTIES Colour and habit.


TurquoiseCu2+Al6(PO4)4(OH)8·4H2O

The colour of turquoise varies from sky blue to green, andcrystals are rare. Nodules or fine-grained masses are moreabundant, and are found where copper-bearing surface waterreacts with aluminium-rich rocks. Turquoise is prized as agemstone. Material used in native American jewellery comes

from southwestern USA, while famoussources of Middle Eastern turquoise

include Neyshabur, Iran.

TURQUOISE crystals line small cavities in massive turquoise in this clay pit inCornwall, England.

COMPOSITION Phosphate.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal or smooth.LUSTRE/STREAK Dull or waxy/White or palegreenish blue.HARDNESS/DENSITY 5–6 / 2.86.KEY PROPERTIES Blue to green; massive.

nodularmass

WavelliteAl3(PO4)2(OH,F)3·5H2O

Globules, hemispheres, or flat aggregates of radiating acicularcrystals are typical of wavellite. This aluminium phosphatemineral is usually green but can also be white, yellow,brown, or black. Wavellite is nearly always a secondarymineral found lining joints and fractures in low-grademetamorphic rocks, in phosphate deposits, andwhere phosphate minerals havebeen weathered in granitesand granite pegmatites.

SECTION SHOWN

WAVELLITE has beenfound in a number of places in Ireland,including Kinsale.

joint inrock

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minutecrystals linethe cavity

radiatingcrystalswithvitreouslustre

massivehabit

unbroken spherule


crystals ofmuscovite mica

THIS RUINED enginehouse is in Cornwall,England, wherecassiterite was minedfrom Roman timesuntil the 20th century.

adamantineblack crystals

CassiteriteSnO2

Cassiterite is tin oxide and the only ore of tin. It is brownor black, rarely grey or white. Crystals are short prismaticand often twinned, or form botryoidal aggregates knownas wood tin. Cassiterite is found in granitic rocks and high-temperature hydrothermal veins, and in placer depositsderived from them. The main

sources are in China,Indonesia, and Peru.

twinnedcrystals

VARLAMOFFITE

HÜBNERITE

COMPOSITION Tungstate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Sub metallic/Brownish black.HARDNESS/DENSITY 4–4.5 / 7.12–7.58.KEY PROPERTIES Ferberite is opaque andnearly black; pure hübnerite is translucent,and red or reddish brown.

WolframiteMn2+WO4 (hübnerite) – Fe2+WO4 (ferberite)

The name wolframite is given to a mineral intermediate incomposition between hübnerite (manganese tungstate) andferberite (iron tungstate). It typically forms prismatic orbladed, opaque dark grey crystals and is found in graniticrocks and high-temperature hydrothermal veins. Wolframiteis an important ore of tungsten.

opaque grey

EXTENSIVE dumps ofwaste rock are broughtout from the wolframitemine at Panasqueira,Portugal.

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translucentred

yellow varietytin oxide

tabular crystal

sub-metalliclustre

COMPOSITION Oxide.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Poor/Uneven orsubconchoidal.LUSTRE/STREAK Adamantine; greasy onfractures/White to pale brown or grey.HARDNESS/DENSITY 6–7 / 6.98.KEY PROPERTIES More dense than sphalerite.


MOLYBDENUM ore often occurs withchalcopyrite. Both are extracted from this giant opencastmine at Bingham inUtah, USA.

massivemolybdenite

MolybdeniteMoS2

Molybdenite is a very soft, bluish grey, opaque, metallicmineral that forms tabular hexagonal crystals, foliatedmasses, scales, and disseminated grains. It is found in high-temperature hydrothermal veins with wolframite and scheelite, in porphyry ores, granites, and pegmatites, and in contactmetamorphic deposits.

ScheeliteCaWO4

Irregular masses of colourless, grey, orange, or pale brownscheelite can be difficult to spot, but they fluoresce vividbluish white under a short-wave UV light. Well-formedcrystals appear octahedral in shape and are found in high-temperature hydrothermal veins and greisens, and incertain contact metamorphic rocks. Fine examplescome from Chungju, South Korea, nearPingwu in Sichuan Province, China, aswell as in Bispberg, Sweden, andother European localities.

translucent palebrown

pseudooctahedralshape

COMPOSITION Tungstate.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Distinct/Subconchoidal or uneven.LUSTRE/STREAK Vitreous oradamantine/White.HARDNESS/DENSITY 4.5–5 / 6.1.KEY PROPERTIES Shape and fluorescence.

HIGH-TEMPERATURE

hydrothermal quartzveins contain scheeliteand wolframite inCumbria, England.

metallic,bluishgrey

cleavesinto thinsheets

tabularhexagonalcrystals

COMPOSITION Sulphide.CRYSTAL SYSTEM Hexagonal and trigonal.CLEAVAGE/FRACTURE Perfect/Flaky.LUSTRE/STREAK Metallic/Bluish grey.HARDNESS/DENSITY 1–1.5 / 4.62–4.73.KEY PROPERTIES Flexible, sectile, andgreasy; more dense than graphite (p.216),and has a blue tinge to colour and streak.

SECTION SHOWN

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quartzgrains

granitematrix


GEOLOGISTS prospectfor carnotite in thetuffs at Cook Inlet,Alaska, USA.

UraniniteUO2

Uraninite is black, sometimes tinted green or brown; fine modified cubic crystals are found in a few localities.Usually it is botryoidal or banded, or the massive varietyknown as pitchblende. Uraninite is found in granites,syenites, pegmatites, and in hydrothermal sulphide veins.Important deposits are also hosted in sandstones altered

by uranium- and vanadium-rich fluids, and in ancientconglomerates. Uraninite is highly radioactive, so

special precautions are needed to store and handle it safely.

THIS IMPORTANT ore ofuranium is mined atthe Rossing opencastmine in Namibia.

CarnotiteK2(UO2)2(VO4)2·3H2O

Bright yellow carnotite normally occurs as coatings andmassive or powdery aggregates. It is found in shallowsurface deposits formed by the weathering of rocks rich inuraninite and other uranium minerals. It also occurs in

sandstones that have been altered byvanadium- and uranium-

bearing solutions. Specialprecautions are neededto store and handle thisradioactive mineral.

COMPOSITION Oxide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Good,octahedral/Uneven to conchoidal.LUSTRE/STREAK Submetallic, greasy, ordull/Green or grey shining.HARDNESS/DENSITY 5–6 / 10.63–10.95.KEY PROPERTIES Highly radioactive mineral.

COMPOSITION Vanadate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/None.LUSTRE/STREAK Earthy, dull, or silky/Brightyellow.HARDNESS/DENSITY Softer than 2 / 4.7.KEY PROPERTIES Radioactive, powderyyellow mineral; crystals rarely seen.

black botryoidaluraninite

yellow uraniumsecondarymineral

powdery yellowradioactive coating

sandstone

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SECTIONSHOWN


BRIGHT yellow micro-crystals of autunite coat a joint in granitefrom Devon, England.

AutuniteCa(UO2)2(PO4)2·10-12H2O

Greenish or lemon yellow tabular crystals of autunite havea rectangular or octagonal outline. Coarse groups are found,but scaly coatings are more common. Autunite forms inthe oxidization zone of uranium deposits suchas uraninite-bearing igneous rocksand hydrothermal veins. It dehydrates to form meta-autunite. Both minerals fluoresceyellow-green under UV light and are radioactive.

TorberniteCu2+(UO2)2(PO4)2·8-12H2O

This radioactive mineral is bright mid-green, and occurs asisolated, square, tabular crystals, lamellar or sheaf-like crystalgroups, or scaly coatings. It is found in the oxidation zone ofdeposits containing uranium and copper, and is associatedwith other phosphate minerals.Exceptional crystals comefrom the DemocraticRepublic of Congo.

SMALL amounts oftorbernite and otheruranium mineralshave been discoveredin Cornwall, England.

COMPOSITION Phosphate.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Perfect, rathermicaceous/Uneven.LUSTRE/STREAK Vitreous or pearly/Pale yellow.HARDNESS/DENSITY 2–2.5 / 3.05–3.2.KEY PROPERTIES Radioactive tabular or platyyellow crystals.

SECTIONSHOWN

SECTION SHOWN

perfect cleavageinto thin layers

vitreous lustre(much duller inmeta-autunite)

tabular,octagonal insection

perfect mica-likecleavage

square, tabularcrystals

COMPOSITION Phosphate.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Perfect micaceous/Uneven.LUSTRE/STREAK Vitreous or waxy/Pale green.HARDNESS/DENSITY 2–2.5 / 3.22.KEY PROPERTIES Radioactive square greentabular crystals look like metatorbernite.

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iron-stainedrock matrix


Columbite-tantalite(Fe,Mn)(Nb,Ta)2O6 – (Fe,Mn)(Ta,Nb)2O6

Minerals at the columbite end of this coltan series areniobium-rich, and those at the tantalite end are tantalum-rich. Either iron or manganese is nearly always the othermajor element, in which case the name is prefixed ‘ferro’or ‘mangano’. Coltan minerals are brown or black,

massive, or form tabular or shortprismatic crystals. They occur

in granite pegmatites and indetrital deposits.

THESE ORES of niobiumand tantalum aremined from granitepegmatites in SouthDakota, USA.

thin tabularcrystal

typicalsubmetalliclustre

Pyrochlore(Ca,Na)2Nb2O6F

Pyrochlore is orange, brownish red, brown, or black. Crystalsare octahedral with modified faces, or in granular or massiveaggregrates. Pyrochlore forms in carbonatites, pegmatites,and is an accessory mineral in alkaline rocks. It alsoaccumulates in some detrital deposits. As it often containstraces of uranium and thorium, it may be radioactive.

COMPOSITION Oxide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Distinct/Subconchoidal or uneven.LUSTRE/STREAK Submetallic tovitreous/Red, brown, or black.HARDNESS/DENSITY 6–6.5 / 5.17–8.0.KEY PROPERTIES Dense, dark minerals.

two intergrownoctahedra

COMPOSITION Oxide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Indistinct/Uneven,splintery.LUSTRE/STREAK Vitreous to resinous/Yellowor brown.HARDNESS/DENSITY 5–5.5 / 4.45–4.9.KEY PROPERTIES Shape; slight radioactivity.

SECTION SHOWN

uneven surfacewhere broken modified

octahedron

THIS PYROCHLORE

comes from the Dande-Doma carbonatite in Zimbabwe.

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SECTION SHOWN

pyrimidal crystals

CARBONATITE fromKatete, northwesternZimbabwe, is a sourceof monazite.

unevenfracture

TRACE uranium inxenotime is used todate unfossiliferoussedimentary rocks suchas the Precambrianrocks in Kimberley,Western Australia.

Xenotime-(Y)YPO4

The yttrium phosphate xenotime-(Y) is brown, red, yellow,or grey, and crystals are prismatic or equant, often withpyramidal terminations. These may form radiating orrosette-shaped aggregates. Xenotime-(Y), an important ore

of yttrium, is an accessory mineral ingranitic and alkaline intrusive

igneous rocks, gneisses, alpine-type hydrothermal veins,

and clastic sediments. Itmay contain traces

of radioactiveuranium orthorium.

Monazite-(Ce)(Ce,La,Nd,Th)PO4

Cerium-rich monazite-(Ce) is the most common of the fourminerals known as monazite. It is brown, pink, or grey.Crystals are tabular, prismatic, or wedge-shaped, orform granular or massive aggregates. Sourcesinclude carbonatites, pegmatites, gneisses,and fissure veins, and monazite alsoaccumulates in rich sand deposits. Itis radioactive if thorium is present.

waxylustre

COMPOSITION Phosphate.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Good/Uneven orsplintery.LUSTRE/STREAK Vitreous or resinous/Palebrown or yellow.HARDNESS/DENSITY 4–5 / 4.4–5.1.KEY PROPERTIES Shape; slight radioactivity.

splinteryfracture

COMPOSITION Phosphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Distinct/Conchoidal or uneven.LUSTRE/STREAK Resinous, waxy, vitreous or adamantine/White or pale brown.HARDNESS/DENSITY 5–5.5 / 4.98–5.43.KEY PROPERTIES Crystal shape; radioactivity.

translucentbrown crystal

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�resinous lustre onbroken surface


THIS PEGMATITE

sample from Ytterby,Sweden, contains blackgadolinite-(Y) withbiotite and pale-coloured feldspar.

Gadolinite-(Y)Y2Fe2+Be2Si2O10

Gadolinite-(Ce) contains more cerium than yttrium but theopposite applies to the more common gadolinite-(Y). Itforms black, greenish black, or dark brown prismatic crystalsand massive aggregates, and has thin splinters that aretranslucent green. Gadolinite-(Y) is mainly found in granitesand granitic pegmatites. Traces of uranium and thorium canmake it slightly radioactive.

black massivegadolinite-(Y)

rock matrix

COMPOSITION Ring silicate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE None/Conchoidal or splintery.LUSTRE/STREAK Vitreous or greasy/Greenish grey.HARDNESS/DENSITY 6.5–7 / 4.36–4.77KEY PROPERTIES Translucent green splinters.

COMPOSITION Disilicate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Imperfect/Conchoidalto uneven.LUSTRE/STREAK Vitreous, resinous, orsubmetallic/Greyish brown.HARDNESS/DENSITY 5.5–6 / 3.5–4.2.KEY PROPERTIES Shape, colour, and lustre.

A DARK layer rich inbrown allanite-(Ce)from Namaqualand,South Africa.

Allanite-(Ce)(Ca,Ce,La)2(Al,Fe2+,Fe3+)3(Si3O12)(OH)

Allanite used to be called orthite; it is now divided intoallanite-(Ce), if cerium is dominant, and rarer allanite-(Y),if yttrium is dominant. Both can contain radioactive traceelements. Allanite-(Ce) forms tabular or prismatic brown or black crystals, but also occurs as bladed or massiveaggregates or embedded grains. It is found in granites,syenites, pegmatites, and certain metamorphic rocks.

Important localities are found in Norway,Sweden, Finland, and Greenland.

well-formedcrystals

ratherresinouslustre

mass of elongateblack crystals �

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conchoidalfracture

SECTION SHOWN


DARK chromite-richbands in anorthosite,a cumulate deposit at Dwars River,Eastern Bushveld,South Africa.

ChromiteFe2+Cr2O4

This most important ore of chromium, chromite is opaque black or dark brown and hasoctahedral crystals, often in granularor massive aggregates. It formslarge deposits in layeredbasic and ultrabasicigneous intrusions, and is preserved when theserocks are metamorphosedto form serpentinites.

PlatinumPt

This precious metal is opaque, silvery grey, and markedlydense. The cubic crystals are rarely seen, and platinumusually occurs as disseminated grains associated withchromium and copper ores in layered basic and ultrabasicigneous rocks. When these are weathered, platinumaccumulates as grains and nuggets in theresulting placer deposits. It oftenforms an alloy withiron and somay bemagnetic.

PLATINUM is mined atthe Merensky Reefdeposit of SouthAfrica, one of theworld’s most importantsources of this metal.

serpentinitematrix

COMPOSITION Oxide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Uneven.LUSTRE/STREAK Metallic orsubmetallic/Brown.HARDNESS/DENSITY 5.5 / 4.5–4.8.KEY PROPERTIES Weakly magnetic;resembles magnetite (p.126).

cube-shapedcrystal

platinumnugget

COMPOSITION Element.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Hackly.LUSTRE/STREAK Metallic lustre/Silverymetallic.HARDNESS/DENSITY 4–4.5 / 14–19.KEY PROPERTIES Very dense, silvery,malleable metal; lacks secondary alteration.

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granularchromite

roundedweatheredcrystals

The bulk of minerals that constitute rocks are not ores,although some have important uses in industry. This chapterfeatures minerals that are found in a wide range of rock types(pp.143–67), and those found mainly or exclusively insedimentary rocks (pp.168–75), igneous rocks (pp.176–96),and metamorphic rocks (pp.197–216). Talc, for example, isfound exclusively in metamorphic rocks, such as the cliffs ofKynance Cove, England (below). Hydrothermal minerals thatare neither ores nor their secondary minerals are alsoincluded in this chapter.

Rock-forming Minerals

MUSCOVITE SERPENTINEGYPSUMBERYL

R O C K - F O R M I N G M I N E R A L S 1 4 3

THE CAIRNGORM mountainsin Scotland are famous forthe yellowish brown quartzvariety called cairngorm.

QuartzSiO2

An exceptionally common mineral, quartz forms six-sidedcrystals with pyramidal terminations. The crystals are often prismatic, usually with striations at right angles tothe length. They can also be massive, or granular as incommon beach sand. Extremely fine-grained compactquartz is called chalcedony. The colour of quartz is highly variable, but mostly it is a colourless rock crystal or white. Purple amethyst and brown smoky quartz arerelatively common, but the natural yellow variety, citrine,is rare – most so-called citrine is actually heat-treatedamethyst. Rarer still are well-formed crystals of pink rosequartz, although rose quartz is more abundant in itsmassive form. Quartz is an essential constitutent ofgranites and granite pegmatites, quartzites, and manysandstones. It is frequently found in hydrothermal veins.

CITRINE TIGER EYE

clear quartzgrading intopurple amethyst

ROCK CRYSTAL

silky, fibrousappearance

striations conchoidalfracture

pink crystals

ROSE QUARTZ

pyramidalterminations

SMOKY QUARTZGEMSTONE

ROSE QUARTZGEMSTONE

COMPOSITION Oxide or silicate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE None/Conchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 7 / 2.65.KEY PROPERTIES A hard mineral withconchoidal fracture, usually with striationsacross prism faces.

NOTE

Orangish brown tiger-eye and blue hawk’s-eye(see Lustre, p.17) are both pseudomorphswhere quartz (p.143) has replacedriebeckite(p.163) asbestos. These varieties arecarved or cut into smooth-topped dome-shapedgems called cabochons; they have all the beautyof the blue asbestos without any of its dangers.

vitreous lustre

�

AMETHYST

1 4 4 R O C K - F O R M I N G M I N E R A L S

Chalcedony SiO2

Chalcedony is extremely fine-grained quartz and occurs in many geological environments. Commonly, it is white,brown, grey, or greyish blue, and is often botryoidal.Varieties include flint and chert, jasper, carnelian,chrysoprase, sard (translucent brown), prase (leek green),and plasma (dark green). Plasma with red hematite-richspots is called heliotrope or bloodstone. Agate is bandedchalcedony formed in gas bubbles in volcanic lava. Thebands are colourless, white, grey, greyish blue, brown,

yellow, pink, red, or black. When they are in flat parallellayers it is onyx, a name also given to creamy

white agate or when the bands are black andwhite. Sardonyx has bands of sard, and

in blue lace agate, they are light blue.Moss agate is not banded; it has

moss-like green chlorite,brown goethite, or black

manganese oxides in the chalcedony.

AGATES weather out ofthe basaltic rocks atAgate Beach on theisland of Haida Gwaiiin British Columbia,Canada.

typical applegreen colour

surface oftenbotryoidal

patchybrown and whitecolours

SECTION SHOWN

HELIOTROPE/BLOODSTONEJASPER

naturalwaxylustre

always translucentorange or red

red, brown,yellow, orgreen, andopaque

CARNELIAN

red, hematite-richspots

CHRYSOPRASE

�


flat, parallellayers

colourless, white, andbluish grey bands

naturalfracture banding best seen

on a polished surface

COMPOSITION Oxide or framework silicate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE None/Uneven.LUSTRE/STREAK Waxy to dull/White.HARDNESS/DENSITY 7 / 2.59–2.63.KEY PROPERTIES Shows no crystal faces; any ‘crystals' are pseudomorphs of other minerals.

NOTE

Vivid pink, green, blue, purple, red, or brownagates that are often seen for sale in shops areartificially dyed. Originally they were bluish greyin colour and most have come from basaltquarries in Brazil. Chalcedony can absorbartificial dyes very well although not all thebands in an agate are equally porous.

MOSS AGATE

SARDONYX BLUE LACE AGATE

translucentbrown sard

colourlesschalcedony

swirled, lacyappearance

dendriticgoethitelooks likemoss

polished stone

rough,unevensurface

AGATE

�

roundedshape oforiginal gasbubble


OpalSiO2

.nH2O

Opal is unusual among minerals in being amorphous, andnever forming crystals. Common opal is usually white,grey, pale brown, or yellow and has a waxy lustre. Bycomparison, precious opal is strikingly beautiful, showingflashes of vibrant colour as it is turned in the light. Mostprecious opal comes from Australia, but orange-red fireopal comes mainly from Mexico. Other varieties of opalinclude spherular pearly fiorite and the curious colourless,botryoidal hyalite. Fossil wood preserved in opal is calledwood opal. Opal solidifies from a gel under lowtemperature conditions in a variety of rocks. It is alsodeposited around fumaroles and hot springs.

THE OPAL-MINING

town of Coober Pedy,Australia, has grownaround the place wherea 14-year-old boy firstdiscovered preciousopal in 1915.

WOOD OPALFIORITE

pearlyspherules

COMPOSITION Silicate.CRYSTAL SYSTEM Amorphous.CLEAVAGE/FRACTURE None/Conchoidal.LUSTRE/STREAK Vitreous or waxy/White.HARDNESS/DENSITY 5.5–6.5 / 1.99–2.25.KEY PROPERTIES Crystals are never seen;the lustre is often waxy.

COMMON OPAL

conchoidalfracture

waxylustre

fossilwood

SECTION SHOWN

NOTE

Opal is made up of spherules of silicondioxide, which, in precious opal, are neatlystacked. The gaps between the spherules are the right size to break up white light intospectral colours, and, depending on the size ofthe spherules, they are able to flash certaincolours back to the viewer.

flashes ofcolour

orange red

translucentred mass

FIRE OPAL

�

translucent withflashes of blueand green


MULTICOLOURED gemsapphires can becollected from thegravels at Eldorado Barin Montana, USA.

CorundumAl2O3

Surprisingly, ruby and sapphire are gem varieties of thesame mineral, corundum. Commonly it is white, grey, orbrown, but gem colours include red ruby, orange-pinkpadparadscha, and colourless, blue, green, yellow, orange,violet, and pink sapphire. Crystals are rough and generallyhexagonal, either tabular, tapering barrel-shaped, ordipyramidal. It can also be massive or granular, and whenmixed with magnetite, it is the abrasive knownas emery. Corundum forms in syenites,certain pegmatites, and in high grademetamorphic rocks. It isconcentrated inplacer deposits.

NOTE

Red ruby is coloured by trace chromium. Theblue of sapphire is due to a little titanium andiron. All other gem colours are also calledsapphire but named with the colour as well, forexample pink sapphire, yellow sapphire, orcolourless white sapphire. Colours result fromtraces of iron, chromium, and other metals.

COMPOSITION Oxide.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE None, cleavage-likeparting/Conchoidal to uneven.LUSTRE/STREAK Vitreous to adamantine,sometimes pearly/White.HARDNESS/DENSITY 9 / 4.00–4.10.KEY PROPERTIES Hardness, crystal shape.

PADPARADSCHASAPPHIREGREENSAPPHIRE

YELLOWSAPPHIRE

WHITE SAPPHIRE

RUBY

COMMON CORUNDUMSAPPHIRE

colourmay bepatchy

vitreouslustre

drabgreyishbrown

pinkfeldspar

rockmatrix

tapering barrel-shaped crystals

rich redcolour

parting

�

RUBY


ChrysoberylBeAl2O4

Chrysoberyl is typically yellow, green, or brown, and formstabular or short prismatic crystals and heart-shaped orpseudohexagonal twins. It occurs in certain granitepegmatites, gneisses, mica schists, and marbles, and indetrital sands and gravels. Two varieties, alexandrite andcat’s eye, are especially prized as unusual gemstones. Cat’s eye contains parallel fibrous crystals of other minerals

which reflect light across the surface of a polishedgemstone, an effect known as chatoyancy.

THE MASVINGO graniterocks in Zimbabwe area modern source ofalexandrite chrysoberyl.

ALEXANDRITE GEMSTONES

brownish red (in tungsten light)

COMPOSITION Oxide.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Distinct/Uneven or conchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 8.5 / 3.75.KEY PROPERTIES A hard green mineral thatmay appear red under incandescent light.

NOTE

Chromium makes emeralds green in colourand rubies red, but it makes alexandrite lookgreen in daylight or fluorescent light, and redunder an ordinary tungsten light bulb. Firstdiscovered in the Ural Mountains of Russia,today alexandrite comes mainly from Lavra diHematita in Minas Gerais, Brazil.

CAT’S EYE

transparentwith vitreouslustre

greenishyellowtwinnedcrystal

green (in daylight)pseudohexagonalcrystal

band oflight

�

ALEXANDRITE


TOURISTS can try theirhand prospecting forspinel and sapphire ingem gravels at Anakie,Queensland, Australia.

LazuliteMgAl2(PO4)2(OH)2

Sky blue, greenish blue, and dark green are the typical coloursof lazulite, and crystals are pyramidal, tabular, granular, ormassive. It occurs in metamorphic rocks, such as schists andquartzites, and is found in the rocks bordering granitepegmatites and in alluvial deposits. Crystals from Georgia,USA, are nearly opaque, but those of the Yukon, Canada, and

from northwest Pakistan are translucent.

BLUE lazulite, seen herewith brown siderite, isthe official gemstone ofYukon, Canada.

SpinelMgAl2O4

Although most familiar as a blue, purple, red, or pinkgemstone, spinel can be other colours. Crystals are usuallyoctahedral and sometimes twinned, and granular ormassive aggregates are found. A minor constituent ofbasalts, kimberlites, peridotites, and other igneous rocks,spinel also forms in aluminium-rich schists andmetamorphosed limestones. Water-worn crystals come fromdetrital streamdeposits in SriLanka andMyanmar.

octahedralshape

COMPOSITION Oxide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None, cleavage-likeparting/Conchoidal, uneven or splintery.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 7.5-8 / 3.6–4.1.KEY PROPERTIES A red, blue, or violet gemmineral found as octahedral crystals.

COMPOSITION Phosphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Poor to good/Unevento splintery.LUSTRE/STREAK Vitreous to dull/White.HARDNESS/DENSITY 5.5–6 / 3.12–3.24.KEY PROPERTIES Name may be confusedwith lazurite (p.215), found in lapis lazuli.

roundedblue crystal

bi-pyramidalcrystal

quartzmatrix

�

appears octahedral

dull, sky bluecrystal

�

gem-like redwaterworncrystals


CalciteCaCO3

A particularly common mineral, calcite is colourless, white,or tinted by impurities. Scalenohedral, rhombohedral, and prismatic forms occur in combination. Crystals withshallow pointed ends are called nailhead spar but when

they are steeply pointed, they are dog-tooth spar.Clear rhombohedral cleavage fragments are Iceland

spar. Calcite is oftengranular or massive.A constituent of limestones,marbles, and othersedimentary andmetamorphicdeposits, it is alsofound in hydro-

thermalveins andigneousrocks.

CALCITE is the mineralthat most often formsstalagmites andstalactites in caves,and banded travertine depositsin hot springs.

COMPOSITION Carbonate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect,rhombohedral/Conchoidal but rarely seen.LUSTRE/STREAK Subvitreous/White.HARDNESS/DENSITY 3 / 2.71.KEY PROPERTIES Perfect rhombohedralcleavage and fizzes in dilute HCl.

ICELAND SPAR

transparent

cleavagerhomb

group ofdog-toothcrystals

NOTE

Light passing through crystals may be split intopolarized rays which, travelling at differentspeeds, are bent or refracted by differentamounts. This means that a double image isseen of an object viewed through the crystal.Calcite crystals shows this double refractionparticularly well.

translucentwithsubvitreouslustre

line appearsdouble throughcrystal

NAIL HEAD SPAR

steeppointedend

six-sided,pointedcrystal

six-sidedcrystals

DOG-TOOTH SPAR

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criss-crosslines showwhere itwouldcleave


THESE SPHERULES ofacicular aragonitecrystals on specularhematite and quartzwere found inCumbria, England.

AragoniteCaCO3

Although it has the same chemical composition as calcite,aragonite crystals are different. They are tabular, prismatic,or acicular, often with steep pyramidal or chisel-shapedends, and can form columnar or radiating aggregates.Multiple twinned crystals are common, appearinghexagonal in shape. Aragonite is deposited in marinesediments and constitutes the shells of many molluscs.Pearls are composed mainly of this mineral. It is found inevaporites, hot spring deposits, and in caves where it mayform coral-like aggregates, known as flos ferri. It is also

found in some metamorphicand igneous rocks.

COMPOSITION Carbonate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Distinct/Nearlyconchoidal.LUSTRE/STREAK Subvitreous/White.HARDNESS/DENSITY 3.5–4 / 2.95.KEY PROPERTIES Aragonite tends to alter to calcite (left). It fizzes in dilute HCl.

brown sideritecrystals

fragileand easilybroken

subconchoidalfracture wheretips havebroken off

transparentand colourlesscrystal

acicularcrystals

subvitreouslustre

NOTE

Aragonite is common in modern sediments but tends to be found only in younger rocks.This is because it is relatively unstable andover geological time, converts to calcite (left).The exterior form may still be that of aragonite,but a rhombohedral cleavage gives away itsnew identity.

modern shellcomposed ofaragonite

pseudohexagonalmultiple twinsFLOS FERRI

�

twisted,coral-likeaggregate


BROWN ankeritecrystals accompanychalcopyrite in thiscopper ore deposit.

DolomiteCaMg(CO3)2

Dolomite is a white, pale brown, or pink mineral. It formsrhombohedral crystals that often have curved faces orcluster in saddle-shaped aggregates. It can also be massiveor granular. The main constituent of dolomite rocks anddolomitic marbles, dolomite is alsopresent in hydrothermal veins,serpentinites, altered basicigneous rocks, and insome carbonatites.

FINE crystals ofdolomite accompanyrare sulphide andsulphosalt minerals atthe Lengenbach quarryin Binntal, Switzerland.

COMPOSITION Carbonate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect,rhombohedral/Subconchoidal.LUSTRE/STREAK Vitreous to pearly/White. HARDNESS/DENSITY 3.5–4 / 2.93–3.10.KEY PROPERTIES Fizzes only slightly in dilute HCl.

AnkeriteCa(Fe

2+,Mg,Mn)(CO3)2

Ankerite is brown, yellow, or buff-coloured, and is found asrhombohedral crystals, often with curved faces. These mayform saddle-shaped groups, and resemble the related minerals

dolomite (above) and siderite. Prismatic crystals alsooccur, and massive or granular aggregates are

common. Ankerite is a gangue mineralin hydrothermal ore deposits and

occurs in carbonatites, low-grademetamorphosed ironstones andbanded ironstone formations, andin some carbonate sedimentary

rocks.

COMPOSITION Carbonate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect,rhombohedral/Subconchoidal.LUSTRE/STREAK Vitreous or pearly/White.HARDNESS/DENSITY 3.5–4 / 2.86.KEY PROPERTIES Harder than calcite(p.150), and fizzes only slightly in dilute HCl.

rhombohedralcrystals

curved crystalface

pearlylustre

rhombohedralcleavage

amethystmatrix

�

�

light browncrystals

prismaticcrystals


FIRST discovered near Alston, Cumbria,barytocalcite comesfrom a number of old lead mines in northern England.

BarytocalciteBaCa(CO3)2

Barytocalcite occurs as striated prismatic or bladed crystalsand cleavable masses. It can be colourless, white, grey, pale yellow, or green, and it fluoresces under UV light.Barytocalcite forms in limestones that have been altered

by lead and zinc-bearinghydrothermal veins,

but it also occurs incarbonatites.

COMPOSITION Carbonate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Nearlyperfect/Uneven to subconchoidal.LUSTRE/STREAK Vitreous to resinous/White.HARDNESS/DENSITY 3.5 / 3.76.KEY PROPERTIES Often in tufts of radiatingwhite or green crystals, dissolves in dilute HCl.

StrontianiteSrCO3

The chemical element strontium was first discovered in the mineral strontianite from Scotland. This mineral istypically colourless, white, yellow, green, or grey. Crystalsare prismatic, often in acicular or fibrous radiating groups;it also forms columnar, granular or massive aggregates, orpowdery coatings. It occurs inhydrothermal veins and isfound in carbonatite deposits.

TINY spherules ofstrontianite crystalsoccur with celestine inlimestone cavities onthe south Wales coast.

COMPOSITION Carbonate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven tonearly conchoidal.LUSTRE/STREAK Vitreous or resinous/White.HARDNESS/DENSITY 3.5–4 / 3.66–3.71.KEY PROPERTIES Crystal shape differs fromother gangue minerals; dissolves in dilute HCl.

SECTION SHOWN

radiating groupof bladed crystals

sprays ofneedle-likecrystals

�

�

limestonematrix

translucentpale yellow

COMPOSITION Sulphate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous; pearly oncleavages/White.HARDNESS/DENSITY 3-3.5 / 3.97.KEY PROPERTIES Colourless, orange, or whitebut usually blue; less dense than baryte (right).


THIS CAVITY in aseptarian concretionfrom Oxfordshire,England, contains bluecelestine with brownand white calcite.

WitheriteBaCO3

Witherite crystals may be prismatic and they may bepyramidal, but they are always twinned, appearinghexagonal in shape. Striations run across prism faces. This mineral can also be fibrous, botryoidal, spherular,columnar, granular, or massive. It is usually white or grey.Most witherite comes from low-temperature hydrothermalveins, usually resulting from the alteration of baryte.

LARGE deposits ofwitherite used to bemined as an ore ofbarium in the northernPennines of England.

CelestineSrSO4

Celestine is often light blue, taking its name from the colourof the sky. Well-formed crystals are common and are tabular,blocky, bladed, or elongate pyramidal in shape. Aggregatesare fibrous, lamellar, granular, or massive. Celestine is usuallyfound in cavities and fractures inlimestones and other sedimentaryrocks, but may be present inhydrothermal veins andbasic igneous rocks.

COMPOSITION Carbonate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Distinct/Uneven.LUSTRE/STREAK Vitreous or resinous/White.HARDNESS/DENSITY 3–3.5 / 4.22–4.31.KEY PROPERTIES Dense; usually inhexagonal crystals; dissolves in dilute HCl;fluoresces blue-white under UV.

SECTION SHOWN

sulphur

vitreouslustre

six-sided crystal

massivehabit

light blue colour

distinctcleavage

�

�

blocky,orthorhombiccrystals

colourless crystals


LITTLE remains of thebaryte mine at ForceCrag in Cumbria,England, where baryteand lead ore weremined until 1990.

BaryteBaSO4

The most common barium mineral, baryte is generallycolourless, white, grey, blue, pink, yellow, or brown.Crystals are usually thin or thick tabular; less commonly,they are prismatic. Cockscomb or crested aggregates ofnearly parallel platy crystals are a distinctive habit of thismineral. It also occurs in rosette-shaped clusters ofcrystals, and banded, granular, or massiveaggregates. Baryte is found as a ganguemineral in low-temperature hydrothermalveins or results from the weathering ofbarium-rich limestones. It is anaccessory mineral insome igneous rocks.

sphalerite

cockscombmass of platy crystals

colour-zonedtabularcrystal

prismatichabit

COMPOSITION Sulphate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous to resinous/White.HARDNESS/DENSITY 3–3.5 / 4.5.KEY PROPERTIES A very dense mineral that may form distinctive cockscombaggregates.

NOTE

The chemical element barium has a highdensity so baryte always feels distinctly heavy.It is used to make heavy drilling muds for oiland gas rigs, and it is a filler used to addweight to paper and rubber. It is also opaqueto X-rays, and is used to make the ‘bariummeals’ used in medicine.

translucent,nearly opaque

tabular habit

�


FluoriteCaF2

In terms of colour, fluorite is a remarkable mineral. Mostcolours are found, often vibrant, and commonly as zones ofdifferent colours within a single crystal. Well-formed crystalsare common. These may be octahedral but are usually cube-shaped. Additional faces can form bevelled edges, and thecorners of cubic crystals are easily cleaved off. Colour-bandedcolumnar or massive fluorite, such as the purple Blue John ofDerbyshire, England, make beautiful but fragile ornamentalstones. Fluorite occurs mainly as a gangue mineral in lowtemperature hydrothermal deposits, and, as an accessorymineral in acid and intermediate intrusive rocks.

LARGE masses ofpurple fluorite can beseen in the roof of thismine at Foisches,Ardenne, France.

purplefluorite

iron-stainedcoating

banded whitefluorite

zones of purpleand green

cubiccrystals

COMPOSITION Halide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Perfect,octahedral/Subconchoidal or uneven.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 4 / 3.17–3.18.KEY PROPERTIES Cubic mineral withoctahedral cleavage; often fluorescent.

NOTE

The phenomenon of fluorescence – where asubstance glows under ultraviolet or otherradiation – is named after fluorite, which oftenshows this property well. Fluorite comes fromthe Latin ‘to flow’ because it is an importantflux, that is, it is used to lower other minerals’melting temperatures.

bevelled edgespink octahedral crystal

corner cleavedoff

BLUE JOHN

�


VIOLET-BLUE apatite,with yellow fluorite,topaz, and cassiterite,comes from Saxony,Germany.

colour-zonedcrystal

uneven wherebroken

transparent with vitreouslustre

well-formedhexagonal crystal

HYDROXYLAPATITE

COMPOSITION Phosphate.CRYSTAL SYSTEM Hexagonal or monoclinic.CLEAVAGE/FRACTURE Poor/Conchoidal touneven.LUSTRE/STREAK Vitreous or waxy; earthywhen massive/White.HARDNESS/DENSITY 5 / 3.1–3.25.KEY PROPERTIES Softer than beryl (p.179).

NOTE

Bones and teeth are composed largely ofapatite, and rocks rich in fossil bones and teethare of huge interest to palaeontologists. Fossilscan be composed of a number of differentminerals, most commonly calcite, aragonite,quartz, or opal. Sometimes well-formed crystalscan be found in cavities in fossils.

waxylustre

ApatiteCa5(PO4)3x or Ca5(PO4,CO3)3x where x= F,OH,Cl

Apatite is the general name for a group of calciumphosphate minerals, the most common of which isfluorapatite. Others include hydroxylapatite, chlorapatite,carbonate-fluorapatite, and carbonate-hydroxylapatite.Apatite minerals occur as hexagonal or tabular crystals, and as nodular, globular, reniform, granular, or massiveaggregates. They can be colourless, white, pink, yellow,green, blue, violet, brown, and black. Apatites are accessoryminerals in nearly all igneous rocks, and large crystalscome from pegmatites and hydrothermalveins. They occur in marbles, skarns,and other metamorphic deposits. Insedimentary rocks, rich depositsoccur as phosphorites, derivedfrom fossil bones and teeth,coprolites, and other organic debris.

siderite

CHLORAPATITE

�


ZirconZrSiO4

Zircon is reddish brown, green, yellow, blue, grey, orcolourless. Controlled heating of brown zircon gives thesky blue and golden yellow stones preferred by jewellers.Zircon usually occurs as short, square prismatic crystalswith pyramidal terminations, or as irregular grains. Tracesof uranium and thorium can make it slightly radioactive. A common accessory mineral in many igneous andmetamorphic rocks, zircon is also a particularly durabledetrital mineral in sedimentary deposits. Much quality

zircon comes from placer deposits. It is mined inVietnam, Thailand, Myanmar, Sri Lanka, and elsewhere in southeast Asia.

ZIRCONS collected fromHanging Rock inVictoria, Australia,have been analysed fortheir trace elementcomposition and age of formation.

COMPOSITION Silicate.CRYSTAL SYSTEM Tetragonal or metamict.CLEAVAGE/FRACTURE Indistinct/Conchoidal.LUSTRE/STREAK Vitreous, adamantine, orresinous/White.HARDNESS/DENSITY 7.5 / 4.6–4.7.KEY PROPERTIES Crystal shape; harder thanvesuvianite (p.210).

black biotite

naturalcolour

NOTE

In some minerals containing even traces ofuranium or thorium, radioactive decay breaksdown the crystal structure. These ‘metamict’minerals become amorphous and some oftheir properties change. Metamict zircon isless dense than fresh zircon and often has agreasy lustre.

heat treated

reddish browncrystal hasvitreous lustre

plagioclasefeldspar

SECTION SHOWN

swan andpolished

GRAVEL

waterworn,roundedsurface

�

typicaltetragonalcrystal

HEAT TREATED BLUEGEMSTONE


COMPOSITION Silicate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Imperfect/Conchoidal.LUSTRE/STREAK Vitreous to resinous/White.HARDNESS/DENSITY 6.5–7 / 3.28–4.39.KEY PROPERTIES Rather rounded poorlyformed yellow-green crystals with aconchoidal fracture.

FORSTERITE

NOTE

Olivine was called topaz by ancient authorsbecause it was found on Topazius, an island inthe Red Sea now called Zebirget. This namewas only restricted to the modern topaz(p.178) in 1747. Gem quality olivine is calledperidot and today, superb crystals come fromthe North-West Frontier of Pakistan.

dark brown(zinc- andmanganese-rich)

tabularcrystals

typical yellowishgreen colour

granularmass

pale tabularcrystals

NODULES of greenperidotite composed ofcoarse granular olivine

are found in dark-coloured basalts onthe volcanic islandof Lanzarote.

OlivineMg2SiO4 (forsterite) to Fe2+

2SiO4 (fayalite)

Olivine is the name of a group of silicate minerals, and mostolivines have compositions ranging between magnesium-bearing forsterite and iron-bearing fayalite. Generally adistinctive yellowish green, they can also be yellow,white, grey, or brown. Rare well-formed crystals aretabular with wedge-shaped terminations, and may bestriated lengthwise. Usually crystals are poorlyformed; discrete grains and granular or massiveaggregates are most common. Olivine occurs inmafic and ultramafic igneous rocks and is themain constituent of peridotites. It also formsby the metamorphism of iron-rich sedimentsand impure limestones. Tephroite, amanganese olivine occurs in skarns andmetamorphosed manganese deposits. FAYALITE

distinctly resinouslustre

rounded,ill-formedcrystal

�

peridotgemstone


MuscoviteKAl2AlSi3O10(OH)2

Muscovite is colourless, grey, pale pink, or green. When richin chromium it is bright green and called fuchsite. As withall the micas, muscovite forms pseudohexagonal crystalswhich have a perfect, micaceous basal cleavage, splitting into thin, colourless, transparent sheets. In igneous rocks,

muscovite is a constituent ofgranites and granite pegmatites.Some pegmatites contain hugecrystalline masses. Muscovite is a key constituent of

metamorphic rocks, such asphyllites, muscovite-schists, and

some gneisses. It is also found inmicaceous sandstones.

THIS GRANITE

pegmatite containsclusters of muscovitecrystals with quartzand microcline.

SECTION SHOWN

flaky crystals

glass-like,vitreous lustre part of a

large crystal

cleaves intoflexiblesheets

COMPOSITION Silicate, mica group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect,micaceous/None.LUSTRE/STREAK Vitreous, pearly, or silkylustre/White.HARDNESS/DENSITY 2.5–4 / 2.77–2.88.KEY PROPERTIES Pale; splits into thin sheets.

NOTE

Muscovite has a much higher melting pointthan glass and can form large crystals,sometimes metres across, which cleave intothin, transparent sheets. This makes it suitablefor blast furnace windows, and other situationsin which glass would melt. India is the principalsupplier of sheet muscovite for industry.

greyishpinkcrystals

six-sided pseudo-hexagonal crystal

bright green

FUCHSITE

�


BIOTITE and quartzcrystals are seen in

this pegmatite fromNamaqualand, South Africa.

BiotiteDark mica without lithium

This dark brown or black mica is found as pseudohexagonalcrystals, lamellar or scaly aggregates, or disseminatedgrains. Biotite is a key constituent of many igneousand metamorphic rocks, including granites,nepheline syenites, schists, and gneisses. It is also found in potassium-richhydrothermal deposits, and some clasticsedimentaryrocks.

COMPOSITION Silicate, mica group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect,micaceous/None.LUSTRE/STREAK Adamantine; pearly oncleavage/White.HARDNESS/DENSITY 2.5–3 / 2.7–3.3.KEY PROPERTIES Dark; splits into thin sheets.PSEUDOHEXAGONAL

CRYSTAL

flexible, thinsheets

PhlogopiteKMg3AlSi3O10(OH)2

Phlogopite crystals are tabular or prismatic, six-sided, andoften tapered along the length. As with all micas, phlogopitecleaves easily into thin sheets. Lamellar or scaly masses arecommon. Phlogopite is red-brown, yellow-brown, green, or colourless. It is found in metamorphosed dolomites anddolomitic limestones, and in ultrabasic igneous rocks.Huge crystals of thisimportant insulatingmaterial come fromthe Kovdor depositon Russia’s Kolapeninsula.

RICH deposits ofapatite are associatedwith phlogopite at theDorowa phosphate

mine, Zimbabwe.

COMPOSITION Silicate, mica group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect,micaceous/None.LUSTRE/STREAK Pearly; submetallic oncleavage/White.HARDNESS/DENSITY 2–3 / 2.78–2.85.KEY PROPERTIES Paler than biotite (above).

submetalliclustre

long, taperingcrystal

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LARGE brown enstatitecrystals are seen herein a metamorphosedbasic pegmatite.

AegirineNaFe3+Si2O6

Prismatic crystals of aegirine are often striated along thelength, and have steep or blunt terminations. They can beacicular or fibrous, forming attractive radiating sprays. This

pyroxene mineral is usually green or green-black,less commonly brown. Most aegirine is

found as a characteristic constituent ofsyenites, carbonatites, and otheralkaline rocks. To a lesser extent, itoccurs in schists, granulites, and othermetamorphic rocks. The fine largecrystals shown here come from theMount Malosa alkaline massif insouthern Malawi.

AEGIRINE occurs withmany rare minerals inthe nepheline syenite ofMont St Hilaire in Quebec, Canada.

COMPOSITION Silicate, pyroxene group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good, crossing atabout 90°/Uneven.LUSTRE/STREAK Vitreous or resinous/Whiteor yellowish grey.HARDNESS/DENSITY 6 / 3.50–3.60.KEY PROPERTIES Pyroxene cleavage; colour.

darkgreencrystals

large,green,prismaticcrystal

EnstatiteMg2Si2O6

Enstatite, an orthopyroxene, forms white, grey, green, orbrown prismatic crystals and columnar, fibrous, or massiveaggregates. It is important in basic and ultrabasic igneousrocks, and occurs in granulites and othermetamorphic rocks. Most so-calledhypersthene is nowconsidered to beenstatite.

metallicbronzelustreBRONZITE

COMPOSITION Silicate, pyroxene group.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Good; crossing atabout 90°/Uneven.LUSTRE/STREAK Vitreous or sub-metallic;pearly on cleavages/White or pale grey.HARDNESS/DENSITY 6 / 3.50–3.60.KEY PROPERTIES Pyroxene cleavage.

potassiumfeldspar

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vitreous lustre


COMPOSITION Silicate, amphibole group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good; crossing at 56°and 124°/Uneven.LUSTRE/STREAK Vitreous/Blue-grey.HARDNESS/DENSITY 6 / 3.28–3.34.KEY PROPERTIES Blue asbestos causes fatallung disease; inhalation must be avoided.

RiebeckiteNa2(Fe2+

3Fe3+2)Si8O22(OH)2

Riebeckite is a greyish blue to dark blue amphibole. It isfound as prismatic crystals and massive or fibrous aggregatesin alkaline granites and syenites, their volcanic equivalents,and in some schists. It can also occur as blueasbestos, known to geologists as crocidolite.Riebeckite asbestos is formed where bandedironstones have been metamorphosed.

THE BLUE-GREY flecks inthis microgranite, fromthe island of AilsaCraig in Scotland, arecrystals of riebeckite.

CROCIDOLITE

deepgreyishbluecolour

amphibolecleavage

SECTION SHOWN

blue, fibrousasbestos

LARGE hornblendecrystals make up muchof this pegmatiteboulder at Glenbuchatin Aberdeenshire,Scotland.

typical six-sided crystal

COMPOSITION Silicate, amphibole group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good; crossing at 56°and 124°/Uneven or splintery.LUSTRE/STREAK Vitreous/White or grey.HARDNESS/DENSITY 5–6 / 3.10–3.30.KEY PROPERTIES Colour, prismatic habit, andamphibole cleavage.

HornblendeCa2[x4(AlFe3+)](Si7Al)O22(OH)2 (x= Fe2+ or Mg)

The hornblende series has compositions between ferro-hornblende and magnesiohornblende. Hornblendes aregreen, brown, or black. Crystals are prismatic, and usuallydispersed through a rock, but can also form columnar,bladed, or massive aggregates. Hornblendes are importantminerals in granodiorites, diorites, trachytes, amphibolites,hornblende schists, andmany other rocks.

amphibolecleavage

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long, blackprismaticcrystals

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COMPOSITION Silicate, feldspar group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal touneven.LUSTRE/STREAK Vitreous; pearly oncleavages/White.HARDNESS/DENSITY 6 / 2.56–2.62.KEY PROPERTIES The nature of its twinning.


PIKES PEAK, Colorado,USA, is the world’smost famous localityfor the amazonitevariety of microcline.

Sanidine(K,Na)AlSi3O8

Sanidine is colourless, white, or cream, and has crystalsthat are usually tabular with a square cross-section.Twinned crystals are also common. Sanidine is a potassium

feldspar that forms at hightemperatures in volcanic rocks such

as rhyolites, phonolites, andtrachytes. It forms spherular

masses of acicular crystals inobsidian, and also occurs

in eclogites and contactmetamorphic rocks.

SANIDINE, as found inVesuvius, Italy, is morelikely to be transparentthan other potassiumfeldspars.

MicroclineKAlSi3O8

Microcline is often found as twinned crystals or massiveaggregates. It is white, cream, pink, or pale brown, butamazonite is an attractive, bluish green variety. Microclineis a potassium feldspar that occurs in acid and intermediateintrusive rocks, some schists and gneisses, and as grains in sediment.

COMPOSITION Silicate, feldspar group.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect, at rightangles/Uneven.LUSTRE/STREAK Vitreous; pearly oncleavages/White.HARDNESS/DENSITY 6–6.5 / 2.54–2.57.KEY PROPERTIES Cross-hatched (tartan) twins.

AMAZONITE

nearly opaquebluish green

blocky,prismaticcrystal

tartantwinning

large, translucentcrystal

trachytematrix

square cross-section

whitealbite

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�pegmatite

smokyquartz

MOONSTONE CABOCHON


translucentprismaticcrystal

associatedcleavelanditealbite

OrthoclaseKAlSi3O8

Orthoclase is usually white, cream, buff, or pink, and forms blocky prismatic crystals. Carlsbad twins are mostcommon but other kinds occur. Orthoclase is a potassiumfeldspar that forms at medium to low temperatures. It isabundant in intrusive felsic rocks, such as granites, granitepegmatites, and syenites. It also occurs in high-grademetamorphic rocks and as coatings and grains insedimentary deposits. Adularia is a very distinctive varietyof orthoclase. It forms in low-temperature, alpine-typehydrothermal veins as simple, colourless crystals that arerhombohedral in section, and complex star-shaped twins.Adularia that shows a bluish white play oflight is known as moonstone.

ADULARIA CARLSBAD TWIN MOONSTONE

THIS SMALL cavity in granite fromNorthern Irelandcontains orthoclase,quartz, and beryl.

COMPOSITION Silicate, feldspar group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal to uneven.LUSTRE/STREAK Vitreous; pearly oncleavages/White.HARDNESS/DENSITY 6–6.5 / 2.55–2.63.KEY PROPERTIES The nature of its twinning.

NOTE

Orthoclase, microcline, sanidine, anorthoclase,and the plagioclase albite are collectivelyknown as alkali feldspars. The composition ofanorthoclase is between albite and sanidine.

cavity inpegmatite

bluish whiteplay of light

diamond-shaped ends

milkycolour

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roughcrystalface


WELL-FORMED crystalsof andesine, oftentwinned, are found inthe blue porphyries ofthe Estérel Mountainsin Var, France.

PlagioclaseNaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)

Albite is the sodium end of this compositional series offeldspars and anorthite is the calcium end. In between, arevarieties oligoclase, andesine, labradorite, and bytownite.Most plagioclases are colourless, white, grey, green, orbrown. They form tabular or prismatic crystals or can bemassive or granular. Simple twins are common, butlamellar plagioclase twinning is distinctive, seen as slenderdark and light bands on some faces. A platy, transparentvariety of albite found in pegmatites is called cleavelandite.Albite also occurs in granites, syenites, and alpine-typehydrothermal veins. Oligoclase and andesine are dominantin diorite and andesite. Labradorite and bytownite arefound in gabbro, dolerite, basalt, and anorthosite. Anorthite

is most often found in metamorphic rocks. Otherplagioclases may also be metamorphic or

form detrital grains in sedimentary rocks.

ANORTHITE

vitreous,pale browncrystals

unevenfracture

NOTE

Perthites are feldspars that have separated intothin potassium- and sodium-rich layers as theycool. Light may be reflected off boundariesbetween layers to give beautiful colour effects.The schiller in moonstone (p.165) and blueiridescence shown by Norwegian larvikite(p.49) are due to perthitic structures.

naturalfractures inmineral

tabular,tricliniccrystals

COMPOSITION Silicate, feldspar group.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal touneven.LUSTRE/STREAK Vitreous, pearly oncleavages of albite/White.HARDNESS/DENSITY 6–6.5 / 2.60–2.76.KEY PROPERTIES Shows lamellar twinning.

�creamywhitecolour

BYTOWNITE

OLIGOCLASE

ANDESINE

massive habit

conchoidalfracture

coarsegranularmass

sparkling flecks ofhematite or copper

polishedsunstonecabochon

plagioclasetwinning

simple twinnedcrystal

shows multicolouredflashes when turnedin light

translucent withvitreous lustre

grey whereno flashesshow

OLIGOCLASE SUNSTONE

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polishedlabradoritecabochon

coarsecrystallinemass

LABRADORITE

1 6 7


HIGH-PRESSURE waterjets wash kaolinite outof the granite in thechina clay pits ofCornwall, England.

IlliteK0.65Al2Al0.65Si3.35O10(OH)2

Illite is one of the most common clay minerals, although itis now classed as a member of the mica group. It occurs asvery fine-grained aggregates. Individual hexagonal crystalscan only be seen using an electron microscope. It is white,

but impurities may tint it grey and otherpale colours. Illite is most commonly

found in sedimentary rocks and isthe most abundant clay mineral

in shales and clays.

CLAY deposits rich inillite are used to makebricks. This brickpit is in Stewartby,Bedfordshire, England.

KaoliniteAl2Si2O5(OH)4

China clay is the more familiar name of kaolinite, which isused in porcelain manufacture. It is pure white and occurs asminute, hexagonal plates clustered into very fine-grainedmasses. Thick deposits of kaolinite result from hydrothermalalteration and weathering of aluminium silicates, forexample, feldspars in granites. Kaolinite is alsopart of the clay in other sedimentary rocks.

COMPOSITION Silicate, mica group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/None.LUSTRE/STREAK Dull lustre/White.HARDNESS 1–2.DENSITY 2.6–2.9.KEY PROPERTIES Fine-grained; important insedimentary clay rocks.

COMPOSITION Silicate.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect/None.LUSTRE/STREAK Pearly to dull, earthy/white.HARDNESS/DENSITY 2–2.5 / 2.61–2.68.KEY PROPERTIES Fine-grained, white clayfound in weathered granites (p.44) andgranite pegmatites (p.46) replacing feldspars.

SECTION SHOWN

SECTION SHOWN

pale, earthymass

LITHOMARGE

feldspar altered to kaolinite

dull, purewhitemass

very compactstructure

quartz

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COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Dull, earthy/White.HARDNESS/DENSITY 1–2 / 2–3.KEY PROPERTIES A fine-grained clay mineralthat swells as it absorbs water; found inaltered volcanic rocks.


THESE BENTONITE hillsin the Petrified ForestNational Park,Arizona, USA, havebeen stained by ironand manganese oxides.

Montmorillonite(Na,Ca)0.3(Al,Mg)2Si4O10(OH)2·nH2O

This clay mineral occurs in fine-grained, massive aggregates,or in globular clusters of microscopic, scaly, tabular crystals.It is usually white, pink, or buff. Montmorillonite is themain constituent of fuller’s earth or bentonite, a claydeposit formed by alteration of volcanic tuffs. Once usedfor cleaning (fulling) fleeces, it is now extracted as drillingmud for oil rigs.

AluniteKAl(SO4)2(OH)6

Alunite is usually granular or massive but can be fibrous orform tabular or cube-shaped crystals. It is colourless, white,or tinted grey, yellow or red-brown. Alunite forms wherealuminium-rich rocks, such as volcanic ash deposits, havebeen altered by sulphur-rich fluids.The sulphur is derived from volcanicactivity or the breakdown of pyritein overlying rocks. Large depositsof alunite, called alumstone, areformed in some locations.

TAIWAN is an importantsource of alum.Clusters of well-formedcrystals come fromChinkuahshih mine

in Taipeh Co.

COMPOSITION Sulphate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Vitreous, pearly on somefaces/White.HARDNESS/DENSITY 3.5–4 / 2.6–2.9.KEY PROPERTIES Does not fizz in dilute HCl like calcite (p.150).

BENTONITE

SECTION SHOWN

massive,creamy whitealunite

dull,earthysurface

tiny clustersof scalycrystals

quartzcrystal

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looks likemarl orlimestone

COMPOSITION Silicate, chlorite group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Unknown.LUSTRE/STREAK Pearly/Green to grey.HARDNESS/DENSITY 2–3 / 3.0–3.4.KEY PROPERTIES Brownish green chloritefound in sedimentary ironstones.

COMPOSITION Silicate, mica group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect,micaceous/Not seen.LUSTRE/STREAK Dull or glistening/Green.HARDNESS/DENSITY 2 / 2.4–2.95.KEY PROPERTIES A flaky green mineral thatis found in sedimentary rocks.


LOCAL chamosite oolitemakes an attractivebuilding stone in thearea around Banbury,Oxfordshire, England.

Glauconite(K,Na)(Fe3+,Al,Mg2)(Si,Al)4O10(OH)2

Bluish or yellowish green glauconite normally occurs as flakes, grains, pellets, or massive aggregates. Formedfrom iron-bearing micas and clays that are present in

marine sediments rich in organic matter,glauconite is found disseminated in

impure limestones, siltstones, andsandstones. Greensand rocks are

particularly rich in this mineral.

FAMILIAR to artists asthe pigment ‘greenearth’, glauconite wasonce quarried in thesehills by Verona, Italy.

Chamosite(Fe,Al,Mg)6(Si,Al)4O10(OH)8

Often resembling glauconite in appearance, chamositetends to be more yellowish or brownish green. It occurs as ooliths (grains composed of minute radiating crystals)and as scaly, foliated, orgranular masses. Chamositeis a common constituent ofironstones formed frommarine sediments richin organic matter. Itreadily alters toorange-browngoethite.

dulllustre

rich bluishgreen mass

brownishgreen

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fossilizedshellfragment

tiny greenchamosite ooliths


EPSOMITE is dissolvedin salt lakes; it may be

deposited alongtheir shorelinesduring the drysummer season.

EpsomiteMgSO4·7H2O

Epsom salts is the more familiar name for this hydratedmagnesium sulphate mineral. Epsomite is colourless,white, pale pink, or green; its crystals are prismatic,but rarely seen. Usually, it occurs as crusts, powderyor woolly coatings, or as botryoidal or reniformmasses. Epsomite is found in evaporite deposits andcoats the walls of mine workings and caverns indolomite and in dolomitic limestone.

HaliteNaCl

Culinary rock salt is actually a mineral called halite. Mosthalite is colourless, white, grey, orange, or brown, but itcan be bright blue or purple. It usually occursas cubic crystals and granular, compact, orcoarse crystalline masses. Halite isfound in evaporite deposits thatform by the evaporation of saltpans and land-locked seas.

JAGGED deposits ofhalite form the curious

Devil’s Golf Coursein Death Valley,California, USA.

COMPOSITION Halide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Perfect, cubic/Conchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 2–2.5 / 2.17.KEY PROPERTIES Brittle and water-soluble;feels greasy in a damp atmosphere.

COMPOSITION Sulphate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Vitreous or silky/White.HARDNESS/DENSITY 2–2.5 / 1.68. KEY PROPERTIES Dehydrates to form white powder in dry air; dissolves easily in water.

SECTION SHOWN

orange crystallinemass

powdery whendehydratedwoolly mass

of fibrouscrystals

powderymass

browngranularhalite

colourlesscubiccrystals

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COMPOSITION Sulphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Vitreous to waxy; pearlyon cleavages/White.HARDNESS/DENSITY 2.5–3 / 2.75–2.85.KEY PROPERTIES Water-soluble, crystalsare commonly replaced by other minerals.


ABUNDANT glauberiteand gypsum areformed in the mudflats of Death Valley,California, USA.

SylviteKCl

Sylvite is usually colourless or white, but may be coloured byimpurities. Crystals are cubic, octahedral, or combinations ofthese forms, but crusts and columnar, granular, or massiveaggregates are more common. It is found in thick beds

mixed or interbedded with halite and otherevaporite minerals. It also forms in volcanic

fumaroles and caves. Sylvite is very water-soluble and needs protection from

moisture in the air.

SYLVITE is extractedfrom thick evaporitedeposits in this potashmine in New Mexico, USA.

GlauberiteNa2Ca(SO4)2

Glauberite crystals are steep tabular or pyramidal, and may have rounded edges. Often they turn out to bepseudomorphs where glauberite has been replaced byanother mineral, such as calcite or gypsum. Glauberite iscolourless, pale yellow, or grey, and the surface may alter to white, powdery sodium sulphate. Itforms mainly in evaporite deposits andin volcanic fumeroles.

COMPOSITION Halide.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Perfect,cubic/Uneven.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 2 / 1.99.KEY PROPERTIES Water-soluble but notbrittle; fragments turn wax-like when crushed.

pointed tabularcrystals

GLAUBERITEPSEUDOMORPH

crystals in theshape ofglauberite

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pinkishtint

octahedralface

rare well-formedcrystals

transparent andcolourless


THIS extraordinarycurled ‘ram’s horn’crystal of gypsum is in a cave in Mexico.

GypsumCaSO4

.2H2O

Many people call gypsum selenite, a name more correctlyused for the transparent variety. This mineral is colourless orwhite, or tinted other colours by impurities. Crystals aretabular, lens-shaped, or prismatic. Fish-tail or swallow-tailcontact twins are common. When gypsum occurs in closelypacked, parallel fibrous crystal it is known as satin spar, butwhen it is compact and fine-grained, it is alabaster. Mostgypsum forms in marine evaporite deposits, associated withanhydrite and halite. It is found as crystals and rosette-shaped aggregates in sands and clays, and occurs involcanic fumeroles and in altered sulphide ore deposits.

NOTE

Desert rose is the name given to rosette-shapedclusters of rounded gypsum crystals that form indesert sands, incorporating sand grains as theygrow. They are common in Algeria and Tunisia.Baryte (p.155) also forms sand crystals whichare more dense. Those of calcite (p.150) arerhombohedral and easy to distinguish.

SATIN SPAR

DESERT ROSE pearly lustre

radiatingcrystals

colourless,translucent

swallow-tailtwin

parallelfibrouscrystals

curved, lens-shapedcrystals

inclusion ofclay

tabularcrystal

COMPOSITION Sulphate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Splintery.LUSTRE/STREAK Subvitreous orpearly/White.HARDNESS/DENSITY 2 / 2.32.KEY PROPERTIES Very common, easilyscratched, does not fizz in dilute HCl.

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packed full ofsand grains


BORAX is mined fromevaporite deposits atDeath Valley,California, USA.

BoraxNa2B4O5(OH)4

.8H2O

A hydrated sodium borate, borax is an important source ofboron for industry. It occurs as flattened prismatic crystalsor as massive aggregates, and is colourless, white, grey, palegreen, or pale blue. Because borax is water-soluble, it is

only found in arid regions of the world. Itforms where water – enriched in boron

from local volcanic activity – hasevaporated from inland lakes.

Borax readily dehydrates in air, turning to chalky

white tincalconite.

COMPOSITION Borate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Vitreous to resinous orearthy/White.HARDNESS/DENSITY 2–2.5 / 1.71.KEY PROPERTIES Water-soluble; dehydrates,turning powdery white.dull on

surface

coating ofwhitetincalconite

flattenedcrystals

COMPOSITION Sulphate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect, nearlycubic/Uneven to splintery.LUSTRE/STREAK Pearly or vitreous togreasy/White to pale grey.HARDNESS/DENSITY 3–3.5 / 2.98.KEY PROPERTIES Harder than gypsum (p.173).

AnhydriteCaSO4

Less common than gypsum, anhydrite is colourless, pale blue, violet, white, grey, pink, or brown. Crystals are tabular or equant, or in granular, fibrous, or massiveaggregates. It is an important constituent of evaporite

deposits and often forms by the dehydration ofgypsum. Anhydrite is often a constituent of

cap rocks above salt domes that act asreservoirs for natural oil. It also occurs

in volcanic fumaroles and in sea-floor hydrothermal ‘chimneys’.

perfectcleavage isnearly cubic

pearlylustre

broken crystalshowstransparentborax

ANHYDRITE is seenhere mixed withgypsum in anevaporite deposit in Cyprus.

coarsely crystallinemasses

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COMPOSITION Borate.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous, silky orsatiny/White.HARDNESS/DENSITY 2.5 / 1.95.KEY PROPERTIES Slightly water-soluble; lessdense than satin spar gypsum (p.173).

COMPOSITION Borate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven tosubconchoidal.LUSTRE/STREAK Vitreous to adamantine/White.HARDNESS/DENSITY 4.5 / 2.42.KEY PROPERTIES Harder than other borateminerals except boracite (Mg3B7O13Cl).


SPOIL heaps mark theborate mines at DeathValley, California,USA; colemanite ismined here.

ColemaniteCaB3O4(OH)3

.H2O

An important source of boron, colemanite is colourless,white, yellowish white, or grey and occurs as shortprismatic or equant crystals, in nodules, or as granular or coarse massive aggregates. It forms by alteration ofborax and ulexite in playa lake deposits in arid regions.Fine crystals come fromCalifornia, USA, andlarge depositsoccur in Turkey.

UlexiteNaCaB5O6(OH)6

.5H2O

A borate mineral with a number of curious habits, ulexite iscolourless or white and is found in cotton wool-like masses,and in dense veins of parallel fibres. These behave likenatural fibre-optics, transmitting light from one endof the crystals to the other. It also occursin radiating or compact aggregates ofcrystals. Ulexite occurs withcolemanite and borax inplaya lake deposits.

THE UYUNI salt flats near Potosí, Bolivia are an importantcommercial source

of ulexite.

transmits lightthrough crystal

naturalfibreoptics

mass ofparallelfibres

equanthabit

transparentcrystals

usuallycolourlessor white

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vitreous lustre

silky lustre


AMBLYGONITE andmontebrasite are foundin the Black Hills ofSouth Dakota, USA.

CRYOLITE formscrystalline masseswith sphalerite andsiderite (shown here),and other sulphidesand halides.

Amblygonite-montebrasiteLiAl(PO4)(F,OH) amblygonite - LiAl(PO4)(OH,F)

In amblygonite, fluorine is more abundant than hydroxyl.The opposite is true in montebrasite at the other end ofthis compositional series. Both can be white or palecoloured. Blocky, poorly formed crystals are uncommon,coarse crystalline masses being more typical. Theseminerals occur in granite pegmatites.

COMPOSITION Phosphates.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect todistinct/Uneven to subconchoidal.LUSTRE/STREAK Vitreous; pearly oncleavages/White.HARDNESS/DENSITY 5.5–6 / 2.98–3.11.KEY PROPERTIES Four cleavage directions.

massive,whitetransparent

yellow crystalfragment

COMPOSITION Halide.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Partings/Uneven.LUSTRE/STREAK Vitreous to greasy; pearlyon certain faces/White.HARDNESS/DENSITY 2.5 / 2.97.KEY PROPERTIES Seems to disappear whenin water as its refractive index is close to water.

CryoliteNa3AlF6

Cryolite is usually colourless or white. It occurs rarely as nearlycubic crystals and most commonly as coarse granular ormassive aggregates. The only major deposits were at Ivigtut,Greenland, where it occurred in layers and veins in pegmatitesand granites. These were used as a flux for aluminium smeltingbut now synthetic cryolite isused instead. cleavage-like

partings

rathergreasylustre

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brown siderite

nearlycubiccrystals


THIS MINERAL isassociated with galenaand other manganeseminerals inmetasomatic ore deposits.

Spessartine Mn3

2+Al2(SiO4)3

This manganese-bearing garnet is orange, red, brown, orblack. Crystals are dodecahedral or trapezohedral, or ingranular or massive aggregates. Fine gem spessartine comesfrom granite pegmatites. It also occurs in granites,rhyolites, and metasomatic deposits.

THE FIRST petalitespecimens came fromUtö, Sweden.

COMPOSITION Silicate, garnet group.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Uneven toconchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 7–7.5 / 4.19.KEY PROPERTIES Brownish reddodecahedral crystals without cleavage. SPESSARTINE GEMSTONE

SPESSARTINE GEMSTONE

uneven fracture

SECTION SHOWN

coarsecleavablemass

cut facets

cabochon

PetaliteLiAlSi4O10

Petalite is colourless, white, grey, yellow, or pale pink andusually occurs as coarse cleavable masses or massiveaggregates. Rare crystals are tabular or rather elongate.Petalite is found in granite pegmatites.It is a minor ore of lithium, andirregular crystals of colourlesspetalite from Minas Gerais, Brazil are cut as gemstones.

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Subconchoidal.LUSTRE/STREAK Vitreous; pearly/White.HARDNESS/DENSITY 6.5 / 2.41–2.42.KEY PROPERTIES Single perfect cleavageunlike quartz (pp.143–44) or feldspars.

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dodecahedralcrystal

vitreous lustre


TopazAl2SiO4(F,OH)2

Most topaz is colourless or pale blue, but it is the yellow toorange-brown gemmy crystals from Minas Gerais in Brazilthat are better known. Other colours include brown, pink,and red. Inferior-coloured gemstones may be irradiatedand/or heat-treated to turn them pink or sky blue. Topazcrystals are short or long prismatic, with wedge-shapedterminations, or they form columnar, granular, or massiveaggregates. Topaz is a hydrothermal mineral normallyfound in granites, granite pegmatites, and rhyolites, indetrital deposits and, occasionally, in high grademetamorphic rocks.

SMALL but beautifullyformed topaz crystalscome from the roundedgranite hills of theMourne Mountains inNorthern Ireland.

blue and colourless waterworn pebbles

naturalcolour

orange-browncolour

cut faces

platy, transparentalbite feldspar

NOTE

For a mineral to be classed as a gemstone ithas to be beautiful, durable, and rare. Whengem-bearing rocks, such as pegmatites (p.46),are broken down over time in streams andrivers, topaz and other durable minerals mayaccumulate in gem gravels. Waterworn crystals can be cut to make fine gemstones.

COMPOSITION Silicate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Singleperfect/Subconchoidal or uneven.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 8 / 3.49–3.57.KEY PROPERTIES Basal cleavage plane oftenvisible inside crystals and waterworn pebbles.

COLOURLESS GEMSTONE

YELLOW GEMSTONE

typical wedge-shapedtermination

transparentcrystal withvitreous lustre

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PEGMATITES from theNorth West Frontierprovince of Pakistanyield exceptionalaquamarine andmorganite crystals.

COMPOSITION Silicate.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Imperfect/Conchoidalto uneven.LUSTRE/STREAK Vitreous or resinous/White.HARDNESS/DENSITY 7.5–8 / 2.62–2.97.KEY PROPERTIES Forms hexagonal crystalslike apatite (p.157) but is much harder.

NOTE

The most important sources of emerald are inthe Andes, in an area of Colombia, north ofBogotá. Unusually, Colombian emerald is hydro-thermal and hosted in sedimentary limestones(p.32) and shales (p.31). Unflawed naturalemerald is rare and very valuable. Inexpensive,gem-quality ‘emerald’ is usually synthetic.

COMMON BERYL EMERALD

BerylBe3Al2Si6O18

Two beautiful gem varieties of the mineral beryl areintense green emerald, coloured by trace chromium, andsea green to sky blue aquamarine, which contains traceiron. Others include pink morganite, yellow heliodor,colourless goshenite, and a raspberry red, manganese-bearing beryl from the Wah Wah Mountains of Utah, USA.Common beryl is usually pale green or white. This mineraloccurs as hexagonal prismatic or tabular crystals with flator pyramidal terminations, or as massive,columnar or granular aggregates. Most beryl is found in granites, granite pegmatites, andrhyolites, but can occur in metamorphic rockssuch as schists.

MORGANITE

mass of hexagonalcrystals

transparent sky blue

iron-stainedcoating

pink andtabular

rich greencolour

nearly opaquecrystal

pyramidaltermination

HELIODOR

AQUAMARINE

typical oblongcut stone

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aquamarinegemstone

emeraldgemstone


GRANITE and pinkorthoclase make up atourmalinized granitenamed luxullianite,after a village inCornwall, England.

EuclaseBeAlSiO4(OH)

Beryllium-bearing euclase is usually light or dark green orblue, but it can also be colourless or white. It is found asprismatic crystals with slanted terminations. This attractivegem mineral forms by the alteration of beryl in pegmatites,and is found in alpine veins. Exquisite colourless and bluecolour-zoned crystals come from Karoi in Zimbabwe.

EUCLASE was firstdiscovered at OuroPreto in Minas Gerais,Brazil, better knownfor its topaz crystals.

Schorl NaFe2+

3Al6(BO3)3Si6O18(OH)4

Schorl is black and like other tourmalines, forms six-sidedprismatic crystals, with alternate narrow and wide faces,giving a roughly triangular cross-section. Acicular andfibrous crystals and massive aggregates are also found.

Schorl occurs mainly in granites andgranite pegmatites, and in high-

temperature hydrothermal veins.

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Vitreous, pearly oncleavages/White.HARDNESS/DENSITY 7.5 / 2.99–3.10.KEY PROPERTIES Crystal shape; cleavagediffers from topaz (p.178).

COMPOSITION Silicate, tourmaline group.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE None/Uneven toconchoidal.LUSTRE/STREAK Vitreous to resinous/White.HARDNESS/DENSITY 7 / 3.18–3.22.KEY PROPERTIES Black and, unlikeamphiboles and pyroxenes, has no cleavage.

rich bluecolour

colourlessprismaticcrystal

perfectcleavage

massivehabit

alwaysblack

striations arecommon

alteredpegmatite

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shortprismaticcrystal


THE ISLAND of Elba,Italy, gave its name tothe mineral elbaite,discovered in granitequarries on the island.

COMPOSITION Silicate, tourmaline group.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE None/Uneven toconchoidal.LUSTRE/STREAK Vitreous to resinous/White.HARDNESS/DENSITY 7 / 2.90–3.10.KEY PROPERTIES Distinctive crystal shape,lack of cleavage, and possible colour zoning.

NOTE

Tourmaline crystals are pyroelectric, whichmeans that when the crystals are heated, theends of a crystal develop opposite electricalcharges. This will attract dust and is thereason why elbaite gemstones that are left in awarm shop window or display case, develop adirty halo over time.

ElbaiteNa(Al1.5Li1.5)Al6(BO3)Si6O18(OH)4

Tourmalines are gem minerals renowned for their beautifulcolours. Pink tourmaline is the variety rubellite; whenblue, it is indicolite, and when colourless, it is achroite.Elbaite can be all these colours, and green, yellow, or orangetoo. It can even have zones of different colours within asingle crystal. The forms and habits of elbaite are like thoseof schorl. Elbaite is found in granites and granite pegmatites,and also occurs in high-temperature hydrothermal veins

and certain metamorphic rocks.

BICOLOURED ELBAITEGEMSTONE

YELLOW ELBAITEGEMSTONE

greenoutside

pegmatitematrix

rich pinkcolour

triangularcross-section

pink in themiddle

RUBELLITE

WATERMELON TOURMALINE

vitreouslustre

pink crystalis green atbase

yellowcolour

SECTION SHOWN

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BLUE INDICOLITEGEMSTONE


ZINNWALDITE is one of a large number ofminerals to come fromthe tin-bearing granitesat Greifenstein,Saxony, Germany.

LepidoliteLight-coloured lithium-bearing mica

Pale-coloured micas containing substantial lithium arecalled lepidolite. They are usually violet-pink and formhexagonal cleavable crystals and coarse to fine-grainedmasses. Lepidolite is found in granite pegmatites withelbaite tourmaline, spodumene, and amblygonite.Excellent crystals andunusual botryoidalaggregates comefrom MinasGerais, Brazil.

LOOK for the geologicalhammer to gauge thesize of these rosettes of lepidolite in apegmatite outcrop inSouth Dakota, USA.

ZinnwalditeDark-coloured lithium-bearing mica

Any dark-coloured mica that contains lithium is known aszinnwaldite. The colour is typically brown or greyishbrown, and crystals are six-sided with a perfect micaceouscleavage. These can form rosettes or fan-like groups, orlamellar or scaly masses. Zinnwaldite is found in greisens,

granite pegmatites, and associated high-temperature hydrothermal veins.

COMPOSITION Silicate, mica group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect,micaceous/None.LUSTRE/STREAK Pearly or vitreous/Colourless.HARDNESS/DENSITY 2.5–4 / 2.80–2.90.KEY PROPERTIES Pink mica that splits intopaper-thin flexible sheets.

COMPOSITION Silicate, mica group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect,micaceous/None.LUSTRE/STREAK Vitreous or pearly/Colourless.HARDNESS/DENSITY 2.5–4 / 2.90–3.02.KEY PROPERTIES Chemical analysisdistinguishes it from other darker micas.

botryoidalhabit

hexagonalcrystals

pearly lustre

split intothinsheets

violet-pinkin colour

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HUNDREDS of tons ofspodumene wereextracted from apegmatite at theHarding mine in New Mexico, USA.

SpodumeneLiAlSi2O6

Two varieties of spodumene are cut as gemstones. Rarehiddenite is light emerald green. More abundant is lilac orpink kunzite, which often occurs as superb transparentcrystals. Most spodumene is not so gem-like but is stillvalued, as the main ore of lithium. Typically colourless,light green, pinkish brown, and other pale colours, it mayfluoresce yellow, orange, or pink under UV light. Crystalsare prismatic, usually flattened and striated along thelength, or in massive aggregates. This pyroxenemineral comes mainly from granite pegmatiteswhere it can form crystals several metres long. Morerarely, it occurs in aplites and gneisses.

NOTE

Kunzite crystals maybe deep violet,violet, or colourless,depending on theangle from whichthe crystal isviewed. Thisproperty is calledpleochroism.

deep striations

pink kunzitegemstone

flattenedprismaticcrystals

appearsopaque

COMPOSITION Silicate, pyroxene group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good, crossing atabout 90°/Uneven or subconchoidal.LUSTRE/STREAK Vitreous; pearly oncleavage/White.HARDNESS/DENSITY 6.5–7 / 3.03–3.23.KEY PROPERTIES Often pleochroic.

KUNZITE

crystal withsubconchoidalfracture

light greenhiddenitegemstone

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HIDDENITE

COMPOSITION Silicate.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Submetallic, pearly, orgreasy/Golden yellow.HARDNESS/DENSITY 3 / 3.2–3.4.KEY PROPERTIES Brittle, mica-like mineral,often in star-shaped crystal groups.


IMPORTANT localitiesfor eudialyte includeMount St Hilaire,Canada, and theIlimaussaq intrusion, Greenland.

Astrophyllite(K,Na)3(Fe2+,Mn)7Ti2Si8O24(O,OH)7

At first, the lustrous yellow, brown, or reddish browncrystals of astrophyllite may look like mica. They aretabular, bladed, or acicular with a perfect basal cleavage,often grouped in radiating star-like clusters. Astrophylliteforms mainly in nepheline syenites, alkaline granites, and

their pegmatites. More rarely it occurs ingneisses and metasomatic deposits.

STAR-SHAPED

astrophyllite aggregatesare found at variouslocations in the areaaround Pike’s Peak,Colorado, USA.

EudialyteNa15(Ca,Ce)2(Fe2+,Mn2+)ZrSi8O22(OH,Cl)2

Eudialyte is best known as a deep pink mineral, but it canalso be brown or yellow. It is usually found as irregularmasses, more rarely as stubby or long prismatic crystals.Eudialyte is an important constituent ofmany nepheline syenites,alkaline granites, andtheir pegmatites.

COMPOSITION Silicate.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous to dull/White.HARDNESS/DENSITY 5–6 / 2.74–3.10.KEY PROPERTIES Pink mineral found innepheline syenites (p. 49) and alkaline graniticrocks with the sodium amphibole arfvedsonite.

blackarfvedsonite

radiating reddishbrown crystals

bladedcrystals

stubbycrystals

perfectcleavage

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submetalliclustre

dull pink, massiveaggregates

SECTION SHOWN


DARK augite crystalsand pale labradoritein a gabbro pegmatiteat Manacle Point onthe Lizard Peninsulain Cornwall, England.

Augite(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

Augite is a clinopyroxene and is dark green, black, or brown.Typical crystals are short prismatic with a square oroctagonal cross-section, but augite can also be granular ormassive. Augite is an essential component of basalts andgabbros, and is common in pyroxenites and other basic and

ultrabasic igneous rocks.It also occurs in

intermediaterocks, such asandesite.

PyropeMg3Al2(SiO4)3

Pyrope is magnesium aluminium garnet. The rich redcolour may have a purple, pink, or orange tinge, or tend toblack. Crystals are normally dodecahedral, trapezohedral,or form granular or massive aggregates. Unlike othergarnets, pyrope is most common as a mineral of ultrabasicigneous rocks, such as eclogites and diamond-bearingkimberlites, and detrital deposits derived from them.

KIMBERLITE, fromdiamond mines inSouth Africa, containspyrope garnets, as wellas diamonds.

COMPOSITION Orthosilicate, garnet group.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Conchoidal.LUSTRE/STREAK Vitreous lustre/White.HARDNESS/DENSITY 7–7.5 / 3.58.KEY PROPERTIES Dodecahedral crystals thatmay be easily confused with almandine(p.207) and other garnets.

COMPOSITION Silicate, pyroxene group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good; crossing atabout 90°/Uneven to conchoidal.LUSTRE/STREAK Vitreous, resinous, ordull/Grey-green.HARDNESS/DENSITY 5.5–6 / 3.19–3.56.KEY PROPERTIES Dark colour; cleavage.

dark colouredand nearlyopaque

conchoidalfracture

resinousbrowncrystal

SECTION SHOWN

rich redcolour

gem-like roundedgrains

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goodcleavages atright angles

volcanictuff


DiamondC

Not only is diamond remarkably hard and durable, it canalso show great beauty in the form of ‘fire’, where lightreflected inside a gemstone is split into its spectral colours.Crystals are octahedral or cubic, often with curved faces.Only the most flawless, colourless crystals and those rareand valuable stones with a clear colour are cut asgemstones. Fancy diamonds can be golden-yellow, blue,pink, red, green, orange, and violet.Most diamonds are yellow or brownand is used as an industrial abrasive,as are dark irregular masses calledboart or bort, and fine-grained blackcarbonado. Diamond comes fromkimberlites and lamproites, andbecause it is so durable, it is alsoobtained from placer deposits suchas river and beach sands.

GRAVEL-FILLED potholesat the Kleinsee diamonddiggings, South Africa,are a source of alluvial diamonds.

COMPOSITION Element.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Perfect/Conchoidal.LUSTRE/STREAK Adamantine togreasy/White.HARDNESS/DENSITY 10 / 3.51.KEY PROPERTIES Brilliant adamantine lustreand extreme hardness are distinctive.

NOTE

Diamond and graphite (p.216) are bothcomposed of carbon atoms. In diamond, they are evenly distributed and bonded closelytogether, making it the hardest of minerals. Ingraphite, they are arranged in layers, with strongbonds across layers but weak ones betweenlayers. This makes graphite soft and slippery.

curved faceon anoctahedralcrystal

weatheredkimberlite

cubic crystal withadamantine lustre

SECTION SHOWN

YELLOW GEMSTONE

COLOURLESS GEMSTONE

GREY-GREEN GEMSTONE

OCTAHEDRAL CRYSTAL

CUBIC CRYSTAL

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COMPOSITION Silicate.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Poor/Subconchoidal.LUSTRE/STREAK Greasy or vitreous/White.HARDNESS 5.5–6.DENSITY 2.55–2.66.KEY PROPERTIES Often has greasy lustre;never found with quartz (p.143).


THE LARGE pale-coloured crystals seenhere are nepheline in asyenite at Spitskop,South Africa.

Nepheline(Na,K)AlSiO4

Nepheline is an important rock-forming mineral but is notalways easily identified. Crystals are prismatic but rarelywell-formed and this mineral usually occurs as grains ormassive aggregates. It is colourless, white, grey, yellow, orbrown, often with a distinctly greasy lustre. Nepheline is

a feldspathoid mineral that forms insilica-poor, alkaline rocks. It is

also found in sodium-rich basalts

and tuffs.

SodaliteNa8Al6Si6O24Cl2Sodalite is nearly always massive or in disseminated grainsso dodecahedral crystals are rarely seen. Blue sodalite is apopular decorative stone, but this mineral can also becolourless, grey, pink, and other pale shades. It fluorescesbright orange under UV light. Sodalite is a feldspathoidand occurs in alkaline igneous rocks, debrisejected from volcanoes, and inmetasomatized calcareous rocks.

WHITE dodecahedralcrystals of sodaliteoccur in limestoneblocks ejected fromMount Vesuvius, Italy.

COMPOSITION Silicate.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Poor/Uneven orconchoidal.LUSTRE/STREAK Vitreous or greasy/White.HARDNESS/DENSITY 5.5–6 / 2.27–2.33.KEY PROPERTIES Fluoresces orange; lackslazurite’s bright blue streak (p.215).

SODALITE CABOCHON

crystals

translucentwith avitreous lustre

massivehabit

round polishedsurface

slightly greasylustre

typical massivesodalite

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CRYSTALS

COMPOSITION Silicate.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Distinct/Uneven toconchoidal.LUSTRE/STREAK Vitreous or greasy/White orbluish white.HARDNESS/DENSITY 5.5–6 / 2.44–2.50.KEY PROPERTIES Colour and fluorescence.


AMONG the richestsources of nosean are the ancientvolcanoes of the Eifeldistrict, Germany.

HaüyneNa6Ca2Al6Si6024(SO4)2

Haüyne, pronounced ‘how-een’, is usually bright blue. Itmay fluoresce orange or pink under longwave UV light.Crystals are dodecahedral or octahedral but are relativelyuncommon. Mostly haüyne occurs as irregular rounded

grains and crystalline masses. It is afeldspathoid, and occurs in phonolites and other leucite- or nepheline-rich

igneous rocks.

VIVID blue grains ofhaüyne can be seen inthis lava block takenfrom Laacher See, Eifeldistrict, Germany.

NoseanNa8Al6Si6O24(SO4).H2O

Nosean is also a feldspathoid and is colourless, white, grey,greyish brown, or blue. Crystals are rarely more than a fewmillimetres across and are dodecahedral or twinned withsix-sided prisms. Disseminated crystals or grains are foundin phonolites and related volcanic rocks that are low in

silica, and in somevolcanic bombs.

COMPOSITION Silicate.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Indistinct/Uneven toconchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 5.5 / 2.30–2.40.KEY PROPERTIES Crystal shape and lack offluorescence under UV.

translucentblue grains

irregularshape

tinycrystalsin cavity

SECTION SHOWN

SECTION SHOWN

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glassysanidinefeldspar


EXCELLENT crystalscome from Mount

Vesuvius and MonteSomma, Italy.

LeuciteKAlSi2O6

Leucite is colourless, white, or grey. At high temperatures, it is cubic and forms trapezohedral crystals. This form is preserved as the mineral cools and developstetragonal symmetry. Good crystals arecommon; leucite is found more rarelyas disseminated grains. A zeolite,leucite is a constituent ofpotassium-rich and silica-poorbasic lavas, and sometimesmakes up nearly the entirerock composition.

AnalcimeNaAISi2O6·H2O

Like leucite, analcime is usually found as colourless, white, orgrey trapezohedral crystals. It can also be pink, pale green, or yellow, and form granular or compact aggregates.Analcime is a zeolite that forms byhydrothermal alteration in jointsand vesicles in silica-poor basic andintermediateigneous rocks.

DISTINCTIVE crystals ofanalcime occur inaltered igneous rocks of

Cornwall, England.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Cubic, tetragonal,orthorhombic, or monoclinic (pseudocubic).CLEAVAGE/FRACTURE None/Conchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 5–5.5 / 2.24–2.29.KEY PROPERTIES Trapezohedral crystalsfound in cavities and joints in rocks.

colourlessanalcime

coatingjoint in rock

trapezohedralcrystal

more crystalsscatteredthrough rock

basaltmatrix

whitetranslucentcrystal

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Tetragonal(pseudocubic).CLEAVAGE/FRACTURE Poor/Conchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 5.5–6 / 2.45–2.50.KEY PROPERTIES Trapezohedral crystalsdisseminated in rock.

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COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous to pearly/White.HARDNESS/DENSITY 5–5.5 / 2.20–2.26.KEY PROPERTIES Looks like mesolite (below)and scolecite (right), but found in pillow lavas(p.63) and some intrusive rocks.


A FINE-GRAINED

mixture of mesoliteand thomsonite foundon Northern Ireland’sAntrim coast, isknown as antrimolite.

NatroliteNa2[Al2Si3O10]·2H2O

Natrolite is colourless, white, pale yellow, or pink, andcrystals are prismatic, usually with a square cross-section.They can form fibrous, granular, or compact aggregates.

Natrolite is typically found with otherzeolites in cavities in marine-

deposited basalts and trachytes. It also occurs in altered syenites,aplites, and dolerites.

SODIUM-RICH zeoliteoften occurs in pillowlavas, which arebasalts depositedin sea water.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous; silky iffibrous/White.HARDNESS 5 DENSITY 2.26.KEY PROPERTIES Square-sectioned crystals.

MesoliteNa2Ca2[Al6Si9O30]·8H2O

Intermediate in composition between natrolite and scolecite,this zeolite mesolite forms as colourless, white, or pale-coloured slender prismatic or fibrous crystals. These can formtuffs, radiating aggregates, or compact masses. Most mesoliteis found in cavities in basalts, where delicate glassy prisms can occur with stilbite, heulandite, and green apophyllite.

SECTION SHOWN

hair-likecrystalbasalt

crystalsare squarein section

gasbubble

prismaticcrystals withcalcite

vitreous lustre

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FINE sprays of scolecitecome from Tertiarybasalts at Berufjördur,in Iceland but it hasalso been detected in other igneous rocksin Iceland.

ScoleciteCa[Al2Si3O10]·3H2O

Like natrolite and mesolite, scolecite occurs as a white orcolourless acicular prismatic crystals. These are generallystriated and are often V-shaped, rather than square, in cross-section, which is a result of twinning. They typicallyform radiating sprays, fibrous masses, or massive aggregates.Scolecite – a zeolite – is found mainly in cavities in basalts.

ThomsoniteCa2Na[Al5Si5O20]·6H2O

Thomsonite belongs to the zeolite group and is colourless,white, pink, yellow, or brown. Crystals are tabular, blocky,bladed, or radiating acicular. Globular, botryoidal, ormassive aggregates are found and can haveattractive concentric colour banding. Althoughmost common in amygdaloidal basalts,thomsonite also occurs in some pegmatites and contact metamorphic rocks.

THOMSONITE crystalsfrom County

Antrim, NorthernIreland, are

typicallytabular.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous; silky iffibrous/White.HARDNESS/DENSITY 5–5.5 / 2.25–2.29.KEY PROPERTIES V-shaped terminations orstriations.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven orsubconchoidal.LUSTRE/STREAK Vitreous lustre/White.HARDNESS/DENSITY 5–5.5 / 2.23–2.39.KEY PROPERTIES Elongate crystals may bebladed.

tabularcrystals

basalt

crystal has V-shapedcross-section

colourless squareapophyllite crystalson tip

vitreouslustre

radiatingcluster ofcrystals

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acicularhabit

red crystalsin basalt


FINE crystals of stilbitecome from the Bay of Fundy, Nova Scotia, Canada.

Heulandite(Ca0.5,Na,K)9[Al9Si27O72]·~24H2O (heulandite-Ca)

Heulandite-Ca is the most common of four minerals calledheulandite. All four look the same, forming coffin-shaped,tabular crystals and granular or massive aggregates. They arecolourless, white, pink, red, yellow, or brown. Heulandite

occurs with other zeolites and apophyllites incavities in basalts and other volcanic rocks.

HEULANDITE is foundin cavities in basalt atVictoria Falls on theborders of Zimbabweand Zambia.

Stilbite(Ca0.5,Na,K)9[Al9Si27O72]·28H2O

Stilbite crystals are thin and tabular, but they often occur indistinctive wheatsheaf- or bow tie-shaped clusters, or inspherular aggregates. The crystals are colourless, white, pink,red, yellow, or brown. The two minerals of this name,stilbite-Na and stilbite-Ca, appear identical. Stilbite occurs

with other zeolites in cavities inbasalts, and andesites, and is

found in some metamorphicand sedimentary rocks too.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous, pearly oncleavage/White.HARDNESS/DENSITY 3.5–4 / 2.19.KEY PROPERTIES Wheatsheaf-, or bow tieshaped crystal clusters with a pearly lustre.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Subconchoidalto uneven.LUSTRE/STREAK Vitreous, pearly/White.HARDNESS/DENSITY 3.5–4 / 2.10–2.20.KEY PROPERTIES Elongate, coffin-shapedcrystals, widest at centre.

tabularcrystal

pearly oncleavage

bow tie clustersof crystals

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crystalwidest atcentre

pearly lustre


AMYGDULES in basaltlava flows of Quairang,Isle of Skye, Scotland,contain chabazite andother zeolites.

Phillipsite(K,Na,Ca0.5,Ba0.5)4-7[Al4-7Si12-9O32]·12H2O

Phillipsite, another zeolite, occurs as twinned prismaticcrystals that often form spherular aggregates. It is usuallycolourless or white, but may be tinted yellow or red. Thethree minerals of this name, phillipsite-Ca, -K, -Na, are notdistinguishable by eye. Most phillipsite is found with otherzeolites in cavities in basalt, but it can also occurin some sedimentary deposits.

A FAMOUS locality forphillipsite is the Capodi Bove on the AppianWay near Rome, Italy.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Distinct/Uneven.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 4–4.5 / 2.20.KEY PROPERTIES Spherular aggregates, but may resemble other zeolites in basalt (p.61).

Chabazite(Ca0.5,Na,K)4[Al4Si8O24]·12H2O

Crystals of chabazite are normally rhombohedral, nearlycubic, or twins, which may be hexagonal and rounded. The crystals are colourless, white, yellow, pink, or red.Chabazite is now considered to be three minerals that look alike – chabazite-Ca, -K, and -Na. Likeother zeolites, it occurs in cavitiesin basalts, andesites, and othervolcanic rocks. It is alsofound in hydrothermalore veins andbedded tuffs.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Distinct/Uneven.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 4–5 / 2.05–2.20.KEY PROPERTIES Nearly cubic crystals thatdo not fizz in dilute HCl; found mainly in volcanic rocks.

SECTION SHOWN

rhombohedronis nearly cubic

whitetranslucentcrystals

basalt

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vitreouslustre

tinyspherules

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous, pearly oncleavages, dull when dehydrated/White.HARDNESS/DENSITY 3–4 / 2.23–2.41.KEY PROPERTIES Readily dehydrates to formthe variety leonhardite.


FORMS in the lavas ofthe Krafla volcano inIceland, at relativelylow temperatures of 100–230°C.

HarmotomeBa2(NaKCa0.5)(Al5Si11O32).12H2O

The barium zeolite harmotome is found as slightlyelongate tabular crystals and simple or complex twins. It is normally colourless, white, or grey but may be tintedyellow or pink. Unlike other zeolites, it occurs more often

in hydrothermal ore deposits than cavities inbasalt and other volcanic rocks. Some of

the finest crystals come from Strontian,Scotland, and the Harz

Mountains, Germany.

WHITE harmotome andpurple amethyst linean amygdule from IdarOberstein, Germany.

LaumontiteCa4[Al8Si16O48].18H2O

Laumontite is a white, grey, pink, brown, or yellow zeolite.Crystals are square prisms with steep oblique terminations,but radiating fibrous or massiveaggregates are more often seen.Laumontite is found in cavities in igneous rocks depositedby low-temperaturehydrothermal solutionsand can form thicksedimentary deposits.

COMPOSITION Silicate, zeolite group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Distinct/Uneven tosubconchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 4.5 / 2.41–2.47.KEY PROPERTIES Distinctive, blocky crystal shape.

vitreous lustre

thicktabularcrystal

SECTION SHOWN

LEONHARDITE

crumblydehydratedlaumontite

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steep obliqueend

perfectcleavage

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COMPOSITION Silicate.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous, pearly on somefaces/White.HARDNESS/DENSITY 4.5-5 / 2.33–2.37.KEY PROPERTIES Large well-formed, square-sided crystals associated with zeolites.


APOPHYLLITE and afine assemblage ofzeolite minerals comefrom the volcanic

cliffs of the Isle ofSkye, Scotland.

ApophylliteKCa4Si8O20(F,OH).H2O – KCa4Si8O20(OH,F).H2O

An abundance of large transparent or translucent crystals,which are green, pink, colourless, or white, makeapophyllite very popular with collectors. These aresquare-sided striated prisms with flat ends thatmay appear cubic, or can have steep pyramidalterminations. Apophyllite – a general namefor minerals of the fluorapophyllite-hydroxyapophyllite series – is commonlyfound with zeolites in cavities in basalt.

PrehniteCa2Al2Si3O10(OH)2

Crystals of prehnite are pale green, white, or yellow, andtabular or prismatic, but they are rarely seen. Usually theyform fan-like, spherular, or botryoidal aggregates which tend to be more deeplycoloured. Prehnite is generally found with calcite and zeolitesin cavities in basic volcanicrocks. A common mineral,localities for exceptionalspecimens include LeBourg d’Oisans, France.

PREHNITE was firstdiscovered in thebasalts of the Cape

of Good Hope,South Africa.

COMPOSITION Silicate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Good/Uneven.LUSTRE/STREAK Vitreous or slightlypearly/White.HARDNESS/DENSITY 6–6.5 / 2.80–2.95.KEY PROPERTIES Usually green andbotryoidal, and found in volcanic rocks.

yellowish green,spherular aggregate

pyramid

pearlylustre

flat ends tocrystals

square-sidedcrystal

transparentwith vitreouslustre

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THIS AMYGDULE

contains hard whiteokenite and a singlecrystal of calcite ongrey chalcedony.

CavansiteCa(V4+O)Si4O10·4H2O

Stunning peacock blue spheres and rosette-shaped clusters ofprismatic cavansite crystals come from quarries near Poona(Pune), India. Here they are found associated with zeolites incavities in altered andesite and basalt. Until the discovery ofthis deposit, cavansite was very rare. Its name is taken fromthe first letters of its principal chemical constituents,calcium, vanadium,and silicon.

THE DECCAN basaltflows in India are hostto the world’s finestcavansite crystals.

OkeniteCa5Si9O23·9H2O

Okenite from the Mumbai and Poona (Pune) regions of Indialook remarkably like white fur balls but the slender crystalsare brittle and fragile. When found at other localities, itswhite or creamy white crystals are usually fibrous but can bebladed. It occurs in amygdules in basalt associated with

zeolites, calcite, quartz, and palegreen spherules of the silicatemineral gyrolite.

COMPOSITION Silicate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Good/Conchoidal.LUSTRE/STREAK Vitreous/Blue.HARDNESS 3–4DENSITY 2.21–2.31.KEY PROPERTIES Distinguished from copperminerals by association with zeolites.

COMPOSITION Silicate.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect/Splintery.LUSTRE/STREAK Vitreous to pearly/White.HARDNESS 4.5–5DENSITY 2.28–2.33.KEY PROPERTIES White balls of fur-likecrystals found in cavities in basalt (p.61).

fragile,whitehair-likecrystals

SECTION SHOWN

drusy coatingof heulandite

SECTION SHOWN

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rossette-shaped massof blue crystals


thin whitebladedcrystals

vitreouslustre

TremoliteCa2Mg5Si8O22(OH)2

Tremolite, an amphibole mineral, is white becoming palegreen as it grades towards the composition of actinolite.Traces of manganese may tint it pink or violet. It is found asbladed crystals, fibrous, or granular aggregates,and as asbestos. It can also be massive andfine grained when, like actinolite, it isknown as nephrite jade. Tremolite isfound in low-grade schists and inimpure dolomitic limestones.

TREMOLITE forms by metamorphism, likeduring the formation

of the EuropeanAlps.

COMPOSITION Silicate, amphibole group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect, crossing at56° and 124°/Uneven or splintery.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 5–6 / 2.99–3.03.KEY PROPERTIES Crystals show amphibolecleavages; less dense than jadeite jade (p.199).

fibrous crystals

NEPHRITE JADE

ELONGATE bladed orfibrous green crystalsgive the metamorphicmineral actinolite adistinctive appearancein the field.

ActinoliteCa2(Mg,Fe2+)5Si8O22(OH)2

Green actinolite occurs as bladed, acicular, or fibrouscrystals often in bent masses, or as asbestos. Massive fine-grained actinolite and tremolite are both called nephritejade. Actinolite forms a compositional series with tremolite and is found in greenschists, blueschists, and other low- to medium-grademetamorphic rocks.

compact,fine-grainedactinolite

green-bladedcrystal

talcschist

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COMPOSITION Silicate, amphibole group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good, crossing at 56°and 124°/Uneven or splintery.LUSTRE/STREAK Vitreous or silky/White.HARDNESS/DENSITY 5–6 / 3.03–3.24.KEY PROPERTIES Crystals show amphibolecleavages; less dense than jadeite jade (p.199). �

polishedsurface

COMPOSITION Silicate, chlorite group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect,micaceous/None.LUSTRE/STREAK Pearly, greasy, or dulllustre/Pale green.HARDNESS/DENSITY 2–2.5 / 2.6–3.02.KEY PROPERTIES Colour and cleavage.

KAMMERERITE


THE MOUNTAINS ofPiedmont in Italy, seenhere across Lake Orta,contain glaucophane-bearing blueschists.

Clinochlore(Mg,Al2)6(Si,Al)4O10(OH)8

Clinochlore is a common chlorite; forms dark green foliated,granular, or scaly masses, disseminated grains, and taperingpseudohexagonal crystals. It is the main mineral in chlorite

schists and occurs in serpentinites, marbles,amphibolites, and other metamorphic rocks,and in hydrothermal deposits. The chrome-bearing variety kämmererite is violet-pink, andfound in chromium-bearing deposits.

THIS BOULDER ofclinochlore with garnetis from Saas-Fee,Valais, in the PennineAlps of Switzerland.

GlaucophaneNa2(Mg3Al2)Si8O22(OH)2

A grey or greyish blue amphibole mineral, glaucophane isfound in slender prismatic, acicular or fibrous crystals, andin granular masses. It is the mineral that is characteristic ofblueschist metamorphic rocks. It alsooccurs in greenschist rocks and in some eclogites. Importantlocalities are in the ItalianAlps, Japan, andCalifornia, USA.

COMPOSITION Silicate, amphibole group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect crossing atabout 56° and 124°/Conchoidal to uneven.LUSTRE/STREAK Vitreous to pearly/Blue-grey.HARDNESS/DENSITY 6 / 3.08–3.22.KEY PROPERTIES Blue-grey crystals inschists showing amphibole cleavages.

prismatic crystals

foliatedmass

typical darkgreen colour

violet-pinkcoating

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�SECTION SHOWN

fuchsite (avariety ofmuscovite)

SECTION SHOWN

monocliniccrystals


MOST jadeite is aconstituent of botheclogites and schists,such as these at AsSifah, Oman.

COMPOSITION Silicate, pyroxene group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Good but rarelyseen/Splintery when massive.LUSTRE/STREAK Subvitreous, pearly oncleavages/White.HARDNESS/DENSITY 6–7 / 3.24–3.43.KEY PROPERTIES Denser than nephrite jade.

NOTE

Another member of the pyroxene group thatoften occurs with jadeite and glaucophane(left) is omphacite (Ca,Na)(Mg,Fe,Al)Si2O6.Omphacite is found in blueschists (p.71) and it is also the main green constituent ofeclogites (p.71). Omphacite is nearly alwaysgranular or massive in habit.

brown weatheredsurface

fine-grained,tough carving-stone

pale greenishwhite

polishedslab

translucent greencabochon

orientalcarving

JadeiteNa(Al,Fe3+)Si2O6

Massive jadeite, the most highly prized form of jade, is atough, even-grained translucent stone, ideal for carving.When bright emerald green, it is called imperial jade, butjadeite can be many other colours, including white, yellow,violet, and other shades of green. Weathered surfaces areoften brown. Jadeite can also be fibrous, and veryrarely occurs as prismatic or platy crystals, usuallyin cavities in the massive stone. A pyroxenemineral, jadeite is found in high-pressure blueschistrocks, metamorphic rocks, and in eclogites.Important gem deposits are in the Uru River area ofMyanmar and the Motagua Valley, Guatemala.

rough lilacmass

IMPERIAL JADE

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EPIDOTE can be seencoating joints in thesealtered andesite fromSomerset,England.

AnthophylliteMg7Si8O22(OH)2

This amphibole mineral is generally found as aggregates ofbladed or fibrous crystals but can also be massive or occuras asbestos. It is grey, purplish brown, or yellowish brown.Although only found in magnesium-rich metamorphicrocks, anthophyllite can form in rich deposits. It occurs

in amphibolites, schists, gneisses,metaquarzites, metamorphic

ironstone formations, and granulites.

THESE rosette-shaped masses ofanthophyllite in thisboulder in South Africaare very typical of this mineral.

EpidoteCa2Al3(Fe3+,Al)Si3O12(OH)

Typically, epidote is yellowish to brownish pistachio green,but it can also be rather grey or yellow. Epidote crystals areprismatic and often striated. Aggregates are fibrous,granular, or massive. Epidote is found in metamorphic

rocks, such as greenschists andamphibolites. It also forms by

alteration of plagioclases inother rocks.

COMPOSITION Silicate, amphibole group.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect, crossing atabout 56° and 124°/Splintery.LUSTRE/STREAK Vitreous, pearly oncleavage/White or grey.HARDNESS/DENSITY 5.5–6 / 2.9–3.5.KEY PROPERTIES Colour and cleavages.

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous, pearly orresinous/Colourless or grey.HARDNESS/DENSITY 6–7 / 3.38–3.49.KEY PROPERTIES Pistachio-green colour,prismatic crystals, and basal cleavage.

radiatingcrystals

perfectamphibolecleavage

yellowishgreen, slendercrystals

striatedprismaticcrystals

perfectcleavage

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purplish brown

vitreouslustre


PiemontiteCa2(Al,Mn3+,Fe3+)3Si3O12(OH)

Piemontite is a manganese silicate closely related toepidote (left), that has a distinctive purplish red colour.Crystals are prismatic or bladed, and fine examples comefrom St Marcel, Val d’Aosta, in the Piedmont regionof Italy where it was first discovered. Itcommonly occurs as grains and granularaggregates, and forms in low- to medium-grade metamorphic rocks, metasomaticdeposits, low-temperaturehydrothermal veins, andsome altered igneous rocks.

AT ANDROS in Greece,piemontite is foundwith the closely relatedmineral androsite-(La).

SECTION SHOWN

rather contortedprismatic crystals

purplishred

TALC is commonlyfound in the

serpentinite on theLizard Peninsula,

Cornwall,England.

COMPOSITION Sheet silicate.CRYSTAL SYSTEM Triclinic or monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven,sectile, and flexible.LUSTRE/STREAK Pearly, greasy, or dull/White.HARDNESS/DENSITY 1 / 2.58–2.83.KEY PROPERTIES Soft, greasy; like theigneous silicate pyrophyllite.

TalcMg3Si4O10(OH)2

Talc is white, brown, or green, and forms foliated, fibrous,or compact masses. It is exceptionally soft, and often has apearly lustre. It is found in talc schists, forms hydrothermalveins in serpentinites, and results frommetamorphism of siliceousdolomites. Talc is the mainingredientof talcumpowder.

pale green,foliatedcompact

massive talccarving

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous/Red.HARDNESS/DENSITY 6–6.5 / 3.46–3.54.KEY PROPERTIES Distinctive purplish red, translucent to nearly opaque, rock-forming mineral.

pearlylustre

SOAPSTONE LION

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perfectmicaceouscleavage


SerpentineTypically, Mg3Si2O5(OH)4

Resembling snake’s skin in appearance, serpentine is agroup of white, yellowish, or grey-green magnesiumsilicate minerals. Usually they are mixed but sometimesindividual members can be distinguished. Antigorite isfound as platy, bladed, or fibrous aggregates. Chrysotile

consists of three minerals with different crystalsymmetry – ortho-, para-, and clinochrysotile.

Its crystals are fibrous and occur as veins ofasbestos. Lizardite occurs as fine-grained

massive aggregates. Serpentine mineralsare the main constituents of

serpentinites; rocks formed by thealteration and metamorphism of

ultrabasic igneous rocks.

THIS SMALL, polishedslab from Italy showsthe snake’s skinappearance that givesserpentine its name.

COMPOSITION Silicates.CRYSTAL SYSTEM Varies with mineral.CLEAVAGE/FRACTURE Perfect/Splintery;chrysotile crystals mat together.LUSTRE/STREAK Dull, waxy or silky/White.HARDNESS/DENSITY 2.5–3.5 / 2.53–2.65.KEY PROPERTIES Chrysotile fibres mat whenrubbed together; hazardous if inhaled.

ANTIGORITESECTION SHOWN

NOTE

Rocks rich in serpentine minerals makeattractive ornamental stones, and are oftenused in the cladding of public buildings.Ornaments and jewellery are made from atranslucent serpentine called ‘bowenite’ or‘new jade’ but this is inferior to true jade(p.197 and p.199) in hardness and value.

very finegrain

silky asbestos

green mixtureof serpentineminerals

CHRYSOTILE

LIZARDITE

platy mass

vein ofchrysotile

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MagnesiteMgCO3

Magnesite rarely occurs as distinct rhombohedral or six-sidedprismatic crystals. More typical are coarsely crystalline,granular, fibrous, and earthy masses. Magnesite forms as analteration product of peridotites, serpentinites, talc schists,and other metamorphic rocks. Rich deposits make it an oreof magnesium. Occasionally, it is a primary mineral incarbonatites and sedimentary evaporite deposits.

MAGNESITE fromSnarum in Norwayforms veins in greenlizardite serpentine.

COMPOSITION Carbonate.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect,rhombohedral/Conchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 3.5–4.5 / 3.KEY PROPERTIES Does not fizz in cold diluteHCl; rare in igneous and sedimentary rocks.

SECTION SHOWN

typical coarselycrystalline mass

perfect rhombohedralcleavage

BruciteMg(OH)2

Brucite is white, pale green, pale blue, grey, or brown.Crystals are tabular, often in platy or foliated aggregates,but fine large crystals have been collected. In the varietynemalite, they can also be fibrous. Brucite is found inserpentinite, but it also occurs in low-temperaturehydrothermal veins in marbles andchlorite schists.

BEDS of serpentinite,that host veins ofbrucite, are found in the ShetlandIslands, Scotland.

perfectcleavage

flakymass

long fibrouscrystals

NEMALITE

pearly greensurface

COMPOSITION Hydroxide.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE Perfect/Flaky, splintery.LUSTRE/STREAK Waxy, pearly on cleavagesurfaces/White.HARDNESS/DENSITY 2.5 / 2.39.KEY PROPERTIES Soft, sectile, but lessgreasy to the touch than talc (p.201).

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COMPOSITION Silicate, garnet group.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Uneven toconchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 6.5–7 / 3.77–3.81.KEY PROPERTIES Small, vivid green crystalsin chromium-rich host rock.


GEM-QUALITY benitoitecomes from the Dallasgem mine in thesouthern Diablo Range,San Benito county,California, USA.

UvaroviteCa3Cr2(SiO4)3

Uvarovite is calcium chromium garnet and is bright emeraldgreen. Its crystals are dodecahedral or trapezohedral, or arein massive or granular aggregates. It forms by hydrothermalalteration of chromite-rich serpentinite and is also found incertain skarns and metamorphosed limestones in the Ural

Mountains, Russia, and Outokumpu, Finland. It rarely forms crystals large enough

for jewellery.

GREEN coatings ofuvarovite garnet comefrom chromium mines in the UralMountains, Russia.

BenitoiteBaTiSi3O9

The rare gem mineral benitoite is normally sapphire blue,but can be colourless, white, or pink. Crystals aretriangular and tabular or pyramidal, or form star-shapedtwins. Nearly all benitoite comes from California, USA.Here, crystals are concealed in natrolite veins cutting aglaucophane schist within a large serpentinite deposit.

COMPOSITION Silicate.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Poor/Conchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 6–6.5 / 3.65.KEY PROPERTIES Softer than sapphire (p.147)with unusual triangular crystals and blue,fluorescence under shortwave UV light.

coating ofemerald greencrystals

vitreous lustre

curious triangularshaped crystals

BENITOITE GEMSTONE

natrolite

sapphireblue

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vitreouslustre

dodecahedralcrystal

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic.

CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous/Greyish white.HARDNESS/DENSITY 6.5 / 3.21–3.38.KEY PROPERTIES Different crystal forms tothose of the polymorph zoisite (p.206), but arerarely visible.

COMPOSITION Silicate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Good/Uneven,subconchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 6.5–7.5 / 3.13–3.16.KEY PROPERTIES Square prismatic crystals;may have cross-shaped dark inclusions.


CHIASTOLITE crystalsare clearly visible inthis outcrop of spottedslate near Threlkeld inCumbria, England.

AndalusiteAl2SiO5

Crystals are prismatic, square in section, and creamy-white,pink, buff, or grey. It can be columnar or massive, but is mostfamiliar as the variety chiastolite, which contains cross-shapedinclusions of dark carbonaceous matter. It is a polymorph ofaluminium silicate, forming at low temperatures, and occurs inslates, schists, and other low-grademetamorphic rocks, and rarely ingranites and pegmatites.

CLINOZOISITE formswhere crustal platescollide to createmountain chains.

CHIASTOLITE

ClinozoisiteCa2Al3Si3O12(OH)

Crystals of clinozoisite are striated and prismatic, or ingranular, fibrous, or massive aggregates. Clinozoisite isoften pale yellow, pink, or red but may becolourless, grey, or green. It is found inlow- to medium-grade regionallymetamorphosed rocks, and incontact metamorphosedand metasomatizedcalcium-richsediments.

andalusite

squareprismaticcrystal

pinkprismaticcrystals

radiatingfibrouscrystals

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�associatedquartz

carbonaceousmatter


TANZANIA is the source of precious bluetanzanite and brightgreen chrome-zoisite.

ruby

SECTION SHOWN

striatedcrystals

ZoisiteCa2Al3Si3O12(OH)

Most zoisite is grey, white, light brown, or pale greenishgrey. It is found as deeply striated prismatic crystals,disseminated grains, and columnar or massive aggregates.Zoisite typically occurs in medium-grade schists, gneisses,and amphibolites resulting from metamorphism ofcalcium-rich rocks. It also forms in eclogites. A lilac-blueprecious gem variety of zoisite, discovered in the MerelaniHills of Tanzania in 1967, is called tanzanite. Tanzania is

also the source of a zoisiteamphibolite containingruby crystals in vividgreen chrome-zoisiteand black hornblende.

CHROME-ZOISITE

perfectcleavage

GEMSTONE

NOTE

Manganese-rich thulite is a mottled pink, semi-precious variety of zoisite. It was named in1823 after Thule, which was an ancient namefor the far north of Europe. It was firstdiscovered in Telemark, Norway, and it is thenational stone of that country. As a gemstone,it is usually cut into cabochons.

cut face

TANZANITE

COMPOSITION Silicate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven orconchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 6–7 / 3.15–3.36.KEY PROPERTIES Like clinozoisite (p.205),the different crystal forms are rarely visible.

THULITE CABOCHON

conchoidalfracture

�chromium-richzoisite


CHLORITOID iscommonly present in the slates aroundTintagel, England.

Chloritoid(Fe2+,Mg,Mn)2Al4Si2O10(OH)4

Chloritoid looks very similar to chlorite. Crystals appearhexagonal and form rosette-like clusters, or more oftenoccur as disseminated scales and foliated or massiveaggregates. In colour, it is dark grey, greenish grey,or greenish black. Chloritoid formsin low- to medium-grade regionallymetamorphosed rocks such as micaschists and phyllites. It also formsin hydrothermal veins and byhydrothermal alteration of lavasand other rocks.

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic and triclinic.CLEAVAGE/FRACTURE Perfect/Brittle.LUSTRE/STREAK Pearly on cleavagesurfaces/White, greenish, or greyish.HARDNESS/DENSITY 6.5 / 3.46–3.8.KEY PROPERTIES Brittle, non-flexible; harderthan clinochlore chlorite (p.198).

AlmandineFe3

2+Al2(SiO4)3

Iron aluminium garnet is called almandine. It is alwaysred, often with a pink or violet tinge, and sometimesnearly black. Crystals often have well-developed faces and are dodecahedral, trapezohedral, or more complexforms; massive aggregates and rounded grains are alsofound. Almandine, the most common of the garnets, isfound in mica schists and gneisses, hornfelses, granites,eclogites, and in detrital sediments.

THIS ALMANDINE iswith kyanite in acoarse-grained gneissfrom Namaqualand,South Africa.

ALMANDINE GEMSTONE

SECTION SHOWN

mica schist

well-formedcrystal withcubic symmetry

foliated mass of dark green crystals

perfect, mica-like cleavage

translucentpinkish red

COMPOSITION Silicate, garnet group.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Subconchoidal.LUSTRE/STREAK Vitreous to resinous/White.HARDNESS/DENSITY 7–7.5 / 4.31.KEY PROPERTIES Pink to red dodecahedralor trapezohedral crystals.

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THE FIRST descriptions ofkyanite were of crystalsfrom the schists of Zillertalin the Austrian Alps.

Staurolite(Fe,Mg,Zn)3-4(Al,Fe)18 (Si,Al)8O48H2-4

Staurolite is reddish brown, yellowish brown, or nearlyblack. It normally occurs as prismatic crystals, which arehexagonal or diamond-shaped in section, often with roughsurfaces. Cross-shaped penetration twins are common. It

forms by regional metamorphism ofargillaceous (or clay) rocks,

and is found in medium-grade schists and gneisses.

STAUROLITE is oftenassociated with kyanite, as in this muscovite schistfrom St Gotthard,Switzerland.

KyaniteAl2SiO5

Blue, white, and green are the usual colours of kyanite, and these are generally mixed or zoned within a singlecrystal. The elongate, flat, bladed crystals are often bent;hardness is markedly greater across a crystal than along its length. Kyanite forms at temperatures between those of its polymorphs andalusite and sillimanite. It occurs inmica schists, gneisses, and associated hydrothermal quartzveins and pegmatites.

COMPOSITION SilicateCRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Perfect and distinctcleavages at 90°/Splintery.LUSTRE/STREAK Vitreous to pearly/Colourless.HARDNESS/DENSITY 5.5 along crystal, 7across crystal / 3.53–3.65.KEY PROPERTIES Bladed, blue crystals.

muscovite schist

cross-shapedtwin

shades of blue

bladedcrystals

triclinicprismaticcrystals

vitreouslustre

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic, pseudo-orthorhombic.CLEAVAGE/FRACTURE Distinct/Nearlyconchoidal.LUSTRE/STREAK Vitreous to dull/Pale grey.HARDNESS/DENSITY 7–7.5 / 3.74–3.83.KEY PROPERTIES Brown; cross-shaped twins.�

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pseudo-orthorhombiccrystals


PURPLISH BLUE

cordierite, as in thisSouth African gneiss,shows strong violet-blue/pale blue/yellow pleochroism.

CordieriteMg2Al4Si5O18

Cordierite is blue, violet, grey, or brown, and it ispleochroic (different colours at different angles underlight). Crystals are short and prismatic, but it is usuallygranular or massive. Mainlyfound in metamorphicrocks, it also occurs in contaminatedigneous rocks andas detrital grainsin sediments.

SillimaniteAl2SiO5

Usually colourless, white, or grey, sillimanite can also be otherpale tints. It occurs as prismatic or acicular striated crystalswith square cross-section, or in fibrous mats. Sillimanite isthe high-temperature polymorph of aluminium silicate and forms by high-grade metamorphism of aluminium-richrocks. It is found in hornfelses, sillimanite schists, and gneisses, and in detrital sediments derived from these rocks.

FOUND IN the region ofBrandywine Springs Park,sillimanite is the officialmineral of the State ofDelaware, USA.

COMPOSITION Silicates.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Distinct/Conchoidal.LUSTRE/STREAK Vitreous/Colourless.HARDNESS/DENSITY 7–7.5 / 2.60–2.66.KEY PROPERTIES Pleochroic; it can resemble blue quartz, but has distinct cleavage.

COMPOSITION Silicate.CRYSTAL SYSTEM Orthorhombic.CLEAVAGE/FRACTURE Perfect/Uneven.LUSTRE/STREAK Vitreous to silky/Colourless.HARDNESS/DENSITY 6.5–7.5 / 3.23–3.24.KEY PROPERTIES Prismatic white crystals inhigh-grade metamorphic rocks that do notform rounded radiating aggregates.

vitreouslustre

distinctcleavage

GEMSTONE FORM

mass of whiteacicular crystals

violet colour

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CRYSTALS are found inlimestone blocksejected from thedormant volcano nearVesuvius in Italy.

AxiniteCa2Fe2+Al2BO(OH)(Si2O7)2 (ferro-axinite)

The name axinite refers to ferro-axinite, rarer manganoaxinite,tinzenite, and very rare magnesioaxinite. All have flattenedaxe-shaped crystals but may be granular or massive. Axinites

are typically clove brown, but can be grey,pink, blue, or when manganese-

bearing, yellow. They are mostcommonly found in contact or

regional metamorphic rocks,skarns, and alpine-type

hydrothermal veins.

WELL-CRYSTALLIZED

axinite is found inskarn deposits aroundSt Just near LandsEnd, Cornwall,England.

VesuvianiteCa19(Al,Mg,Fe)13Si18O68(OH,O,F)10

Vesuvianite, formerly known as idocrase, occurs as pyramidalor prismatic crystals, and columnar, granular, or massiveaggregates. Yellow, green, and brown are the most commoncolours, but copper-bearing cyprine is greenish blue. Usually found in metamorphosed impurelimestones and skarndeposits, vesuvianite also occurs in serpentinitesand someigneousrocks.

COMPOSITION Silicate.CRYSTAL SYSTEM Triclinic.CLEAVAGE/FRACTURE Good/Uneven toconchoidal.LUSTRE/STREAK Vitreous/White.HARDNESS/DENSITY 6.5–7 / 3.18–3.31.KEY PROPERTIES Axe-shaped crystals,commonly clove brown.

characteristicclove-browncolour

CYPRINE

vitreouslustre

alpine-typevein

finetetragonalcrystalsCOMPOSITION Silicate.

CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTUREPoor/Subconchoidal or uneven.LUSTRE/STREAK Vitreous toresinous/White.HARDNESS/DENSITY 6–7 / 3.32–3.43.KEY PROPERTIES Crystal shape; hardness.

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distinctiveaxe shape

square cross-section


THE SERPENTINITES

at the foot of theMatterhorn inSwitzerland are one ofthe best known sourcesof andradite garnet.

NOTE

Garnets are rarely of pure composition. Pyrope,almandine, and spessartine belong to thepyralspite series; uvarovite, grossular, andandradite belong to the ugrandite series. Inter-mediate chemical compositions are commonbut only within a series, so most andraditescontain some chromium or aluminium.

crystals aredodecahedral

yellowish topaz-like colour massive

habit

cluster ofblack opaquecrystals

DEMANTOIDGEMSTONE

TOPAZOLITEGEMSTONE

AndraditeCa3Fe2

3+2(SiO4)3

Andradite is calcium-iron garnet, and like other garnets it typically forms dodecahedral or trapezohedral crystals, and granular or massive aggregates. The colour ofandradite can be very variable. Melanite is black andtitanium rich, and forms in alkaline igneous rocks.Topazolite is brownish yellow and is found in chloriteschists and serpentinites. Bright green demantoid iscoloured by trace chromium and comes mainly fromserpentinite deposits in the Ural Mountains of Russia.Other shades of yellow, green, brown, or reddish brown are more common, and most andradite forms as a result of contact or regional metamorphism of impure limestones.

cutfaces

brightgreen

COMPOSITION Silicate, garnet group.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Uneven toconchoidal.LUSTRE/STREAK Adamantine, resinous ordull/White.HARDNESS/DENSITY 6.5–7 / 3.8.KEY PROPERTIES Crystal shape and colour.

MELANITE

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TOPAZOLITE

dodecahedral crystal


COMPOSITION Silicate, garnet group.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE None/Uneven orconchoidal.LUSTRE/STREAK Vitreous or resinous/White.HARDNESS/DENSITY 6.5–7 / 3.59.KEY PROPERTIES Dodecahedral ortrapezohedral crystals.

NOTE

Tsavolite (also called tsavorite) was discoveredin Tanzania in 1967. The same gem depositwas tracked over the border and mined in the Tsavo National Park in Kenya. Now,Madagascar and Pakistan are also sources oftsavolite. The intense green colour is due totraces of chromium and vanadium.

TSAVOLITE GEMSTONE HESSONITE GEMSTONE

TSAVOLITE

grains ofgrossular

rounded pinkcrystals

bright greenirregular mass

conchoidalfracture

orange-browndodecahedralcrystals

GrossularCa3Al2(SiO4)3

Grossular is calcium-aluminium garnet. Its name, from theGreek for gooseberry, alludes to its rounded dodecahedral ortrapezohedral crystals and typical yellow, green, or honeycolour. When orange-brown, it is called hessonite orcinnamon stone and these fine crystals come from Sri Lanka,Canada, and Italy. East Africa yields the precious emeraldgreen variety tsavolite. Grossular may also be granular ormassive. It forms in impure calcareous rocks that haveundergone regional or contactmetamorphism and in some schists andserpentinites.

THE CHLORITE

clinochlore oftenaccompanies thehessonite variety ofgrossular, as shown here.

�impuremarble

HESSONITE


COMPOSITION Silicate, pyroxene group.CRYSTAL SYSTEM Monoclinic.CLEAVAGE/FRACTURE Distinct, crossing atabout 90°/Uneven to conchoidal.LUSTRE/STREAK Vitreous or dull/White, grey,or greyish green.HARDNESS/DENSITY 5.5–6.5 / 3.22–3.38.KEY PROPERTIES Light colour; cleavage.

NOTE

Gemstones of stardiopside show afour-rayed star effectwhen a beam of lightcrosses the stone.This property iscalled asterism.

often lightcoloured

SECTION SHOWN

hessonitegrossulargarnet

vitreouslustre

bright greencrystals

violet-bluelamellar mass

pyroxene cleavages cross nearly at rightangles

STAR DIOPSIDE

DiopsideCaMgSi2O6

Diopside, a clinopyroxene, is usually colourless, green,brown, or grey. It is violet-blue in manganese-bearingviolan, and vibrant green in chromium-rich chromediopside. Crystals are prismatic, usually nearly square insection. They can form columnar, lamellar, granular, ormassive aggregates. Most diopside is metamorphic, found inmarbles, hornfelses, schists, gneisses, and skarns. More rarelyit occurs in peridotites, kimberlites, and other igneous rocks.

THIS TYPICAL skarnassemblage, whichshows green diopsidewith calcite andbiotite, comes fromOntario, Canada.

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VIOLAN CHROME DIOPSIDE


WELL-FORMED

wollastonite crystalsare found inmetamorphosedlimestone blocksejected from MonteSomma, Italy.

WollastoniteCaSiO3

Wollastonite is usually white or grey but impurities cantint it other colours. Crystals are tabular, bladed, or fibrous,and normally occur in coarse masses, or in radiating,plumose, or massive aggregates. Most wollastonite comesfrom thermally metamorphosed siliceous limestones andfrom skarn deposits. Less often, it forms in carbonatites and alkalineigneous rocks.

COMPOSITION Silicate.CRYSTAL SYSTEM Monoclinic or triclinic.CLEAVAGE/FRACTURE Good to perfect,crossing at about 90°/Uneven.LUSTRE/STREAK Vitreous; pearly oncleavages/White.HARDNESS/DENSITY 4.5–5 / 2.86–3.09.KEY PROPERTIES Pyroxene-like cleavage.

vitreous lustre

cleavages cross atalmost right angles

COMPOSITION Silicate, tourmaline group.CRYSTAL SYSTEM Trigonal.CLEAVAGE/FRACTURE None/Uneven toconchoidal.LUSTRE/STREAK Vitreous to resinous/Whiteto light brown.HARDNESS/DENSITY 7 / 3.03–3.18.KEY PROPERTIES Often brown; no cleavage.

DraviteNaMg3Al6(BO3)Si6O18(OH)4

Dravite is the sodium magnesium member of the tourmalinegroup of minerals, and is usually brown but can be black,dark red, or green. Crystals are prismatic, usually with arather rounded triangular cross-section and different-shapedends. Dravite can also be granular or massive. Generally,

dravite forms in metamorphosed limestones and isonly very rarely found in pegmatites.

DRAVITE is found onManhattan Island, animportant locality forthis mineral in the Stateof New York, USA.

vitreous lustre

large prismaticcrystal

granulardravite

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whitebladedcrystals


THE HIGH mountainsof Badakhshan inAfghanistan have beena rich source of lapislazuli for thousands of years.

Lazurite(Na,Ca)8Al6Si6O24[(SO4),S,Cl,(OH)]2

Since ancient times, lazurite has been highly prized for itsexquisite blue coloration as the principal mineral in therock known as lapis lazuli. Lazurite is a member of thefeldspathoid group. It is always deep or vibrant blue, and it was once the source of the artist’s pigment ultramarine.Most lazurite is massive or in disseminated grains, anddistinct crystals – which are usually dodecahedral – aremuch sought after. Lapis lazuli forms by contactmetamorphism of limestones. At its finest, this rockconsists of lazuritespeckled with goldenpyrite, but white calciteand other feldspathoidsare normally present too.

polished surface

whitecalcite

dodecahedralcrystals withdull lustre

COMPOSITION Silicate.CRYSTAL SYSTEM Cubic.CLEAVAGE/FRACTURE Imperfect/Uneven.LUSTRE/STREAK Vitreous to dull/Bright blue.HARDNESS/DENSITY 5–5.5 / 2.38–2.45.KEY PROPERTIES Bright blue streak; doesnot fizz in dilute HCl like azurite (p.113).Should not be confused with lazulite (p.149).

NOTE

The best lazurite crystals come fromBadakhshan Province in Afghanistan, which isalso the source of the many lapis lazulispecimens in old collections said to be fromPersia (now Iran). The stone was traded, butnot mined, in Persia. Other deposits of lapislazuli are in the USA, Chile, and Russia.

LAPIS LAZULI CABOCHONS

POLISHED LAPIS LAZULI ROUGH LAPIS LAZULI

SECTION SHOWN

ULTRAMARINE

rich bluecolour

goldenpyritegrains

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COMPOSITION Silicate, scapolite group.CRYSTAL SYSTEM Tetragonal.CLEAVAGE/FRACTURE Distinct, splintery orfibrous/Uneven to conchoidal.LUSTRE/STREAK Vitreous, pearly orresinous/White.HARDNESS/DENSITY 5–6 / 2.50–2.78.KEY PROPERTIES Splintery cleavage.


MINED at Borrowdale,in the Lake District,England, graphite isused to make the ‘lead’in lead pencils.

Scapolite3NaAlSi3O8.NaCl – 3CaAl2Si2O8.CaCO3

The scapolite group of minerals has compositions rangingfrom sodium chloride-bearing marialite to calciumcarbonate-bearing meionite. Crystals are prismatic with

flattened pyramidal terminations or form massiveor granular aggregates. They are colourless, white,grey, yellow, green, pink, or other colours. Scapolite,once known as wernerite or dipyre, is found inregionally metamorphosed rocks, skarns, altered

basic and ultrabasic igneous rocks,and debris from volcanoes.

SCAPOLITE is present in this outcrop atMavuradonha,Zimbabwe.

GraphiteC

Graphite, like diamond, is composed of pure carbon, buthas very different properties. It is very soft, opaque, darkgrey or black, and is normally found as grains and scaly,foliated, or massive aggregates. Crystals are hexagonal and platy with triangular striations. Graphite forms by themetamorphism of organic material in sediments and

occurs in schists andmarbles. It is a rare

constitutuent ofmeteorites andigneous rocks.

splinteryfracture

COMPOSITION Element.CRYSTAL SYSTEM Hexagonal.CLEAVAGE/FRACTURE Perfect/Flaky, sectile,and greasy to touch.LUSTRE/STREAK Metallic, dull, earthy/Shinyblackish grey.HARDNESS/DENSITY 1–2 / 2.09–2.23.KEY PROPERTIES Very soft and greasy to touch.

ratherresinous lustre

bright, metalliclustre

pyramidalend tocrystal

foliated masswith perfectcleavage

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prismaticcrystals

vitreouslustre

G L O S S A R Y 2 1 7

GlossaryFor illustrations of many of theterms defined here, andadditional definitions, see thegeneral introduction (pp.8–19).Words in italic are definedelsewhere in the glossary.

ACICULAR Needle-shaped.

ACID A class of igneousrocks with the highestproportion of silica (SiO2),including granite andrhyolite.

ADAMANTINE A particularlybrilliant lustre as shown bydiamond.

ALKALINE A class of igneousrocks rich in sodium- andpotassium-bearingminerals; it includes syeniteand phonolite.

ALPINE-TYPE VEINS Low-temperature hydrothermalveins typical of theEuropean Alps, that containa characteristic assemblageof minerals.

ALTERATION The change ofrocks at or near the Earth’ssurface, which may resultin the formation of new,secondary minerals.

ALUMINOSILICATE A mineralcontaining proportions ofboth aluminium andsilicon.

AMPHIBOLES A complexgroup of at least 65 rock-forming silicate oraluminosilicate minerals,which typically formelongate crystals.

ASTERISM The star-shapedplay of light caused byorientated microscopiccrystal inclusions; best seenin minerals cut ascabochons.

BASIC A class of igneousrocks with a low proportionof silica (SiO2), includingbasalt and gabbro.

BEDDING A flat structure insedimentary rocks, that isthe product of a period ofsedimentation.

BLACK SMOKER DEPOSITSHydrothermal mineralsdeposited from volcanicvents along geologicallyactive mid-ocean ridges atthe bottom of the sea.

BOTRYOIDAL Like a bunchof grapes.

CABOCHON Gemstone cutwith a smooth, curved,polished surface.

CARBONIFEROUS Ageological period, 299 to359.2 million years ago.Named after its abundantcoal deposits.

CHLORITES A group of ninehydrated aluminosilicateminerals with a sheet-likestructure and perfectcleavage; most containmagnesium and/or iron.

CLAST (adj. CLASTIC) A grainin a sedimentary rock;usually a larger one thanthose in the surroundingmatrix.

CLEAVAGE In rocks, a flatlayer produced by deforma-tion or metamorphism,along which the rock splits;in minerals, the flat planewhere the crystal structureis weakest, along which acrystal tends to split.

CRETACEOUS A geologicalperiod, 65.5 to 145.5million years ago. Namedafter the Latin for chalk,one of its characteristicrocks.

DEFORMATION ROCKSRocks produced bystretching, squashing, orfracturing of a rock duringEarth movements.

DENDRITIC Tree-likebranching.

DETRITAL A type of sedimentthat has settled in water orbeen deposited by wind.

DEVITRIFICATION Theprocess of turning fromglass into a crystallinemineral.

DRUSY COATING Coating ofmany small, often parallel,well-formed crystals.

DYKE A vertical sheet ofigneous rock; a minorintrusion (see alsointrusive).

EQUANT Describes a crystalthat is roughly equal sizesin all directions.

EXTRUSIVE Igneous rocksformed at the Earth’ssurface from lava.

FELDSPARS A group of 16silicate or aluminosilicateminerals; the most

important contain calciumor sodium (plagioclase) orpotassium (potassiumfeldspar) and are majorconstituents of rocks.

FELDSPATHOIDSAluminosilicate mineralsthat occur instead offeldspars in rocks low insilica (SiO2). They are neverfound in the same rocks asquartz.

FLOW BANDING A structurewithin an igneous rockproduced by drag duringthe final stages ofcrystallization of a lava.

FLUORESCENCE Glowing of a mineral when exposed toultraviolet or otherradiation, caused by themineral’s chemicalcomposition or by traces ofcertain chemical impurities.

FLUX A substance used topromote the melting of anore or metal and removeimpurities.

FOLDING The bending ofbedding or banding duringdeformation of a rock.

FRACTURE For rocks, thedestruction of rock materialduring deformation; forminerals, the appearance ofa broken surface which isnot a cleavage or parting.

GANGUE A mineral in anore deposit such as ahydrothermal vein, that isnot itself an ore.

GARNETS A group of 15silicate minerals with cubicsymmetry that formdodecahedral ortrapezohedral crystals.

GOSSAN An iron-richresidue at the top of an ore deposit left after oreshave been leached away by downward-percolatingwater.

GRADED BEDDING A type of bedding where the grainsare sorted by larger grainssettling before smallerones.

GROUNDMASS The fine-grained portion of anigneous rock, in whichlarger crystals are set.

HIGH GRADE Rocks formedat the highest temperaturesand pressures duringmetamorphism.

2 1 8 G L O S S A R Y

HYDROTHERMAL A processby which mineralscrystallize out of circulating,very hot water rich indissolved chemicals; amineral formed by thismechanism.

INTERMEDIATE A class ofigneous rocks between acid and basic types.

INTRUSIVE Igneous rocksformed beneath the Earth’ssurface. Large bodies aremajor intrusions; smallerbodies, minor intrusions.

LAMELLAR In thin, flatsheets.

LOWER PALAEOZOIC Ageological period, 416 to542 million years ago.

MAGMA Molten rock – thesource of igneous rocks.

MASSIVE In rocks, a textureshowing little variation; inminerals, a sample notshowing any crystal shape.

MATRIX In rocks, the finerportion of a detritalsedimentary rock in whichlarger grains areembedded; in minerals, theunderlying mineral or rockon which a crystal hasgrown.

METASOMATISM A kind ofmetamorphism involvingthe introduction of newmaterials from outside, forexample liquids or gasesfrom a magma body.

MICAS A group of 41 rock-forming aluminium silicateminerals with a sheet-likestructure and single perfectbasal cleavage.

MODIFIED CRYSTAL A crystalwith a particular form, butin which additional facesare also present.

ORE A mineral extractedfrom the ground as aneconomic source of a metal.

OXIDISED ZONE The area ofan ore deposit above thewater table, wherecirculating water hasaltered sulphide ores toform oxygen-bearingsecondary minerals.

PARTING A flat plane alongwhich a mineral tends tosplit, that is not a cleavage.May be caused byfracturing or twinning.

PHENOCRYST A large crystalin a porphyritic rock.

PLACER DEPOSITS Economicdeposits where dense,durable minerals have beenconcentrated in sands andgravels by rivers or seas.

PLEOCHROISM When thecolour of a crystal differsdepending on itsorientation.

PORPHYRITIC An igneoustexture consisting of largercrystals (phenocrysts), in amass of finer crystals (thegroundmass).

PORPHYRY ORE DEPOSITSPorphyritic rocks containingfinely dispersed oreminerals.

PRECAMBRIAN The oldestgeological period on Earth,earlier than 542 millionyears ago; rocks of thisperiod are usuallymetamorphic.

PRIMARY ORES Metal-bearing minerals that havenot been altered by theaction of rain or ground-water, or any other agency.

PSEUDOMORPH A mineralthat replaces another butretains the original’s outercrystal shape.

PYRITOHEDRON A formcomposed of twelvepentagonal faces; typicallyshown by the mineralpyrite.

PYROCLASTIC ROCK A rockformed from solid material,such as ash, ejected from avolcano.

PYROXENES A group of 21rock-forming silicateminerals which typicallyform elongate crystals.Clinopyroxenes havemonoclinic symmetry;orthopyroxenes haveorthorhombic symmetry.

RENIFORM With a kidney-like appearance.

SCALENOHEDRON Fortrigonal minerals, a formcomposed of twelveunequal-sided triangles.

SCHILLER A milky or bluishplay of light seen when amineral is turned in thelight; shown best by stonescut as cabochons.

SCREE Loose piles offragmented rocks, forexample on mine tips,mountain sides, and at thebase of cliffs.

SECONDARY MINERALSMinerals formed by theaction of rain or ground-water, or other alterationprocesses, on primary ores.

SECONDARY ZONE Part of anore deposit in whichsecondary minerals form;often enriched wheremetals leached from abovehave been redeposited bypercolating water.

SECTILE Can be cut with a knife.

SERIES A complete range ofchemical compositionspossible between two fixedend-members. Forexample, the plagioclaseseries albite (NaAlSi3O8) toanorthite (CaAl2Si2O8).Sometimes known as a‘solid solution’ series.

SHEAR ZONE Part of a rockbody that has beenstretched along a narrow,flat zone.

STRIATIONS Fine, parallelgrooves and ridges on thesurface of a crystal.

TERMINATIONS Faces makingup the ends of a crystal.

THRUST A fault, a crack inthe Earth’s crust alongwhich there has beenmovement, oriented at alow angle to the horizontal.

TOURMALINE A group of 11boron-bearing silicateminerals having a ring-likestructure and trigonalsymmetry.

TWIN A crystal with one or more parts that havegrown as mirror images ofeach other, joined along acrystal face edge or aninternal plane.

ULTRABASIC A class ofigneous rocks with the leastamount of silica (SiO2); itincludes dunite andpyroxenite.

VESICULAR A term todescribe lavas that are fullof gas bubble holes, orvesicles.

ZEOLITES A group of 83hydrated aluminosilicateminerals in which the otherelements present arepredominantly sodium,potassium, and calcium.They are commonly foundin altered basic rocks.

I N D E X 2 1 9

IndexAacanthite 94achneliths 67achroite 181acid tests 18actinolite 197

nephrite jade 197adamite 107, 131adularia 165aegirine 162agate 145agglomerate 62, 64

carbonatite 64alabaster 42, 173albite 166alexandrite 148allanite 140almandine 185, 207

gemstone 207alnoite 54alum shale 31alunite 169amazonite 164amber 41amblygonite 176amethyst 143amphibolite 70amygdaloidal basalt 61analcime 189anatase 121andalusite 205, 208

chiastolite 205andesine 167andesite 58, 59

porphyritic 58andradite 211

demantoid 211melanite 211topazolite 211

anglesite 99anhydrite 174ankerite 129, 152annabergite 93anorthite 166anorthosite 53anthophyllite 200anthracite 40, 41antigorite 202apatite 157, 179

chlorapatite 157hydroxylapatite 157

apophyllite 195aquamarine 179aragonite 151

flos ferri 151arkose 23, 25

grey 25torridonian 25

arsenopyrite 126astrophyllite 184

atacamite 113augen gneiss 73, 74augite 50, 185aurichalcite 106autunite 137axinite 210azurite 99, 113,

215

Bbanded iron formation

36banded ironstone 36banded metatuff 73banded rhyolite 56baryte 155barytocalcite 153basalt 50, 61, 62, 63, 67,

69, 193, 196amygdaloidal 61porphyritic 61vesicular 61

bauxite 132bohmite 132diaspore 132gibbsite 132

benitoite 204gemstone 204

bentonite 169beryl 157, 179

aquamarine 179common 179emerald 179heliodor 179morganite 179

biotite 161bismuthinite 90black shale 31, 40 black slate 79bloodstone 144blue-grey marble 77Blue John 156blue lace agate 145blueschist 71bog iron ore 35bohmite 132borax 174, 175bornite 109bournonite 97braunite 120breadcrust bomb 67breccia 21, 22, 62,

82, 83, 85polygenetic 21

brochantite 114bronzite 162brookite 121brucite 203

nemalite 203bytownite 167

Ccalamine 105calcareous mudstone 29calcite 8, 35, 82, 150, 169,

173dog-tooth spar 150iceland spar 150nail head spar 150

calcrete 39calc-silicate marble 77, 81campylite 100carbonatite 55carbonatite agglomerate 64carlsbad twin 165carnelian 144carnotite 136cassiterite 103, 134

varlamoffite 134cat’s eye 148cavansite 196celestine 154cerussite 98

jack-straw 98chabazite 193chalcedony 36, 37, 143, 144

agate 145bloodstone 144blue lace agate 145carnelian 144chrysoprase 144heliotrope 144jasper 144moss agate 145sardonyx 145

chalcocite 108, 109, 111chalcopyrite 96, 109, 110,

124chalcotrichite 111chalk 33, 37chamosite 170charnockite 75chatoyancy 148chert 36, 37, 144chiastolite 205china clay 29, 168chlorapatite 157chloritoid 207chondrites 84chrome diopside 213chrome-zoisite 206chromite 51, 141chromite-serpentinite 51chrysoberyl 148

alexandrite 148cat’s eye 148

chrysocolla 116chrysoprase 144chrysotile 202cinnabar 88citrine 143

2 2 0 I N D E X

clay 24, 29, 168china 29, 168kaolinite 29

cleavage 16cleavelandite 166clinochlore 198, 207

kammererite 198clinozoisite 205, 206coal 40, 41

peat 40cobalt bloom 91cobaltite 91cog-wheel ore 97colemanite 175columbite-tantalite 138common beryl 179common corundum 147common opal 146conglomerate 21

polygenetic 21copper 108cordierite 209cordierite hornfels 81corundum 147

common 147green sapphire 147padparadscha 147ruby 147white sapphire 147yellow sapphire 147

covellite 109crinoidal limestone 32crocidolite 163crocoite 102cross-bedded muddy

sandstone 24 cross-bedded sandstone 24

cross-bedded muddy 24cryolite 176crystal forms 15crystal systems 15crystal tuff 65cumulate rocks 51

chromite-serpentinite 51norite 51troctolite 51

cuprite 95, 111cyanotrichite 114cyprine 210

Ddacite 48, 59

extrusive 59pyroclastic 59

demantoid 211density, mineral 18descloizite 101desert rose 173desert-varnished laterite 36diamond 54, 186

colourless gemstone 186yellow gemstone 186

diaspore 132

diatomite 38diopside 77, 213

chrome 213star 213violan 213

dioptase 116diorite 44, 48, 49, 50dipyre 216dog-tooth spar 150dolerite 50, 54, 61dolomite (mineral) 34, 152dolomite (rock) 32, 34, 77,

152dolostone 34dravite 214dufrénite 129dunite 51, 52

Eeclogite 71, 206eisenrose 127elbaite 181

bicoloured elbaite gemstone 181

blue indicolite gemstone 181

watermelon tourmaline 181

yellow elbaite gemstone 181

emerald 179emery 147enargite 108, 111endlichite 101enstatite 162

bronzite 162epidote 70, 200, 201epsomite 171erythrite 91euclase 180eudialyte 184evaporites 42

gypsum rock 42potash rock 42rock salt 42, 171

extrusive dacite 59

Ffault breccia 82fayalite 159feldspathic gritstone 22felsite 48fiorite 146fire opal 146flagstone 23, 28flint 37, 144flos ferri 151flowstone 39flow-top breccia 62fluorite 131, 156

Blue John 156folded gneiss 73

forsterite 159fossiliferous shale 31fracture 16franklinite 104freshwater limestone 32fuchsite 160fuller’s earth 169

Ggabbro 50, 70

leucogabbro 50gadolinite 140gahnite 105galena 8, 94, 96garnet peridotite 52garnet schist 78garnierite 93geological environments 13geyserite 38gibbsite 132glasses 57, 83, 85glauberite 172

pseudomorph 172glauconite 26, 170glaucophane 198gneiss 73, 74, 75, 76, 83

folded 73goethite 35, 128gold 87, 110grain, shape 11

grain, size 11granite 9, 44, 45, 46, 47,

69, 73, 75graphic granite 45, 47granite pegmatite 46granitic gneiss 74hornblende 44orbicular granite 45porphyritic 44white 44

granite pegmatite 46granitic gneiss 74granodiorite 48granophyre 47granulite 70graphic granite 45, 47graphite 135, 216gravel 158green porphyry 60greensand 23, 26green sapphire 147greenschist metabasalt 69,

71greenstone 69green tremolite-actinolite

70, 197greisen 47grey arkose 25grey orthoquartzite 25grey pumice 57, 66greywacke 27, 29gritstone 22

feldspathic 22

I N D E X 2 2 1

grossular 212hessonite 212tsavolite 212

gypsum 173, 174, 175desert rose 173satin spar 173

gypsum rock 39, 42

Hhabits, mineral 16halite 171

rock salt 42, 171hardness 18harmotome 194hausmannite 117haüyne 59, 188heliodor 179heliotrope 144hematite 35, 36, 38, 123,

127kidney ore 127specular 127

hemimorphite 107hessonite 212heulandite 192hiddenite 183hornblende 70, 163hornblende granite 44hornfels 81, 82

cordierite 81hubnerite 134hydroxylapatite 157hydrozincite 106hypersthene 162

Iiceland spar 150idocrase 210igneous rocks 9–13,

43–67ignimbrite 66illite 168ilmenite 123impact rocks 85

moldavite 85tektite 85

imperial jade 199indicolite 181iron meteorite 84iron-rich sandstone 23iron-rich spilite 63ironstone 26, 35

oolithic 35iron sulphide nodule 30

Jjack-straw cerussite 98jadeite 199

imperial jade 199jadeite jade 197, 199, 202jamesonite 97jasper 144

jet 41

Kkammererite 198kaolinite 29, 168

lithomarge 168kidney ore 127kimberlite 54

weathered kimberlite 54komatiitic metabasalt 69kunzite 183kyanite 208kyanite paragneiss 76kyanite schist 78

Llabradorite 167lamprophyre 54

weathered rock 54lapis lazuli 215larvikite 49laterite 36, 132

desert-varnished 36laumontite 194lazulite 149, 215lazurite 149, 187, 215

lapis lazuli cabochons 215

polished lapis lazuli 215rough lapis lazuli 215ultramarine 215

leonhardite 194lepidolite 182leucite 59, 189leucogabbro 50libethenite 115lignite 40limestone 32, 33, 34, 35,

55, 77crinoidal 32freshwater 32shelly 32

limonite 128pea iron ore 128

linarite 99lithic tuff 65lithomarge 168lizardite 202loess 24, 29L-S tectonite 83L-tectonite 83lustre 17

Mmagnesite 72, 203magnetism 18magnetite 36, 50, 51, 123,

126, 141malachite 106, 112manganite 117marble 42, 53, 72, 77

blue-grey 77calc-silicate 77white 77

marcasite 125marl 34, 42

nodular grey 34red 34

melanite 211mesolite 190, 191metabasalt 69

greenschist 69komatiitic 69

metamorphic rocks 9–13, 68–85

metatuff 73banded 73

meteorites 30, 84iron 84stony 84stony-iron 84

micaceous sandstone 23Millstone Grit 22

mica pegmatite 46mica schist 47, 78microcline 8, 164

amazonite 164microgranite 45, 47

granophyre 47migmatite 75migmatitic orthogneiss 74millerite 92, 93mimetite 100

campylite 100mineral groups 14mineral identification 14–19

acid tests 18cleavage 16colour 17composition 14crystal forms 15crystal systems 15density 18fracture 16geological association 19habits 16hardness 18lustre 17magnetism 18mineral groups 14parting 16radioactivity 18streak 17transparency 17twinned crystals 15

minerals 8, 14–19, 86–216ore 8, 14–19, 86–141rock-forming 8, 14–19,

142–216minette 54moldavite 85molybdenite 135monazite 139montebrasite 176

2 2 2 I N D E X

montmorillonite 169bentonite 169

moonstone 165cabochon 165

morganite 179moss agate 145mudstone 27, 28, 29, 31,

34, 35calcareous 29

muscovite 160muscovite schist 78mylonite 82, 83

Nnail head spar 150natrolite 190nemalite 203nepheline 187nepheline syenite 49, 59nephrite jade 197nodular grey marl 34nodular phosphorite 26norite 51nosean 188

Oobsidian 57, 83

snowflake 57oil shale 31okenite 196oligoclase 167oligoclase sunstone 167olivenite 115olivine 50, 51, 52,

61, 84, 159fayalite 159forsterite 159

olivinite 52onyx 144oolite 33

pisolite 33oolithic ironstone 35opal 38, 146

common 146fiorite 146wood 146fire 146

orbicular diorite 45orbicular granite 45

orbicular diorite 45ore minerals 8, 14–19,

86–141orpiment 89orthite 140orthoclase 165

adularia 165carlsbad twin 165moonstone 165, 166

orthogneiss 69,73, 74 migmatitic 74

orthoquartzite 23, 25, 76grey 25

Ppadparadscha 147paragneiss 73, 74, 76

kyanite 76parting 16pea iron ore 128peat 40pegmatite 46

granite 46mica 46tourmaline 46

Pélé’s hair 67Pélé’s tears 67pentlandite 92, 93peridotite 52, 71

garnet 52peridotite xenolith 55petalite 177pharmacosiderite 131phillipsite 193phlogopite 161phonolite 59phosphorite 26

nodular 26phyllite 78, 79, 80, 207piemontite 201pillow lava 63pisolite 33pitchstone 57

porphyritic 57plagioclase 50, 51,166

albite 166andesine 167anorthite 166bytownite 167labradorite 167oligoclase 167oligoclase sunstone 167

platinum 141polished lapis lazuli 215polygenetic breccia 21polygenetic conglomerate

21porphyritic andesite 58porphyritic basalt 61porphyritic granite 44porphyritic pitchstone 57porphyry 48, 60

green 60quartz-feldspar 60rhomb 60rhyolite 60

potash rock 42prehnite 195proustite 95pseudotachylite 83psilomelane 118pumice 38, 66pyrargyrite 95pyrite 79, 91, 96, 110, 124pyrochlore 138pyroclastic dacite 59pyrolusite 117, 118

pyromorphite 100pyrope 185pyroxenite 51, 53pyrrhotite 125

Qquartz 35, 63, 82, 143,

177, 187agate 145amethyst 143bloodstone 144blue lace agate 145carnelian 144chalcedony 36, 37, 143,

144chrysoprase 144citrine 143hawk’s eye 17heliotrope 144jasper 144moss agate 145rock crystal 143rose 143sardonyx 145smoky 143tiger eye 143

quartz-feldspar porphyry 60quartzite 76quartzo-feldspathic gneiss

74

Rradioactivity 18realgar 89red marl 34rhodochrosite 119rhodonite 120rhomb porphyry 60rhyolite 48, 56, 59

banded 56spherulitic 56

rhyolite porphyry 60riebeckite 163

crocidolite 163rock crystal 143rock cycle 9rock-forming minerals 8,

14–19, 142–216rock identification 10–13

colour 12geological environments

13grain shape 11grain size 11mineral content 11structure 12texture 12types 10where to find rocks 13

rocks 9–13, 20–85igneous 9–13, 43–67metamorphic 9–13, 68–85sedimentary 9–13, 20–42

I N D E X 2 2 3

rock salt 42, 171rock types 10rose quartz 143rubellite 181ruby 147ruby blende 103rutile 22, 122

Ssandstone 22, 23, 24, 25,

28, 35cross-bedded 24cross-bedded muddy 24iron-rich 23micaceous 23slump-bedded 23

sanidine 164sapphire 147

green 147white 147yellow 147

sardonyx 145 satin spar 173scapolite 216scheelite 135schist 78

garnet 78kyanite 78muscovite 78

schorl 180scolecite 190, 191scoria 62scorodite 131sedimentary rocks 9–13,

20–42selenite 173septarian nodule 30serpentine 202

antigorite 202chrysotile 202lizardite 202

serpentinite 52, 53, 72shale 31, 79

alum 31 black 31fossiliferous 31oil 31

shelly limestone 32siderite 35, 129, 152siliceous sinter 38sillimanite 208, 209siltstone 27, 28silver 94skarn 77, 81slate 79, 80

black 79slump-bedded sandstone 23smithsonite 105, 107smoky quartz 143snowflake obsidian 57soapstone 72, 201sodalite 59, 187

cabochon 187

specular haematite 127spessartine 177sphalerite 96, 103

ruby blende 103spherulitic rhyolite 56spilite 62, 63

iron-rich 63spindle bomb 67spinel 105, 149spodumene 183

hiddenite 183kunzite 183

spotted slate 80,81stalactite 39stalagmite 39star diopside 213staurolite 208S-tectonite 83stibnite 90stilbite 192stony meteorite 84stony-iron meteorite 84streak 17strontianite 153sulphur 88syenite 49, 58

larvikite 49, 166sylvite 42, 172

Ttalc 201, 203

soapstone 201tanzanite 206tectonite 74, 83tektite 85tephroite 159tetrahedrite 110thomsonite 191thulite cabochon 206tiger eye 143tillite 22titanite 123topaz 159, 178, 180topazolite 211torbernite 137torridonian arkose 25tourmaline 180, 181

watermelon 181tourmaline pegmatite 46trachyte 58transparency, mineral 17travertine 38, 39, 42

stalactite 39stalagmite 39

tremolite 77, 197troctolite 51tsavolite 212tufa 39

calcrete 39yellow 39

tuff 56, 65, 66, 72crystal 65lithic 65

turbidite 27, 65, 73turquoise 133twinned crystals 15

Uulexite 175ultramarine 215uraninite 136uvarovite 204

Vvanadinite 101

endlichite 101varlamoffite 134vent agglomerate 64vesicular basalt 61vesuvianite 158, 210

cyprine 210violan 213vivianite 130volcanic bomb 67

spindle bomb 67volcanic breccia 55, 62

flow-top 62

Wwad 118watermelon tourmaline 181wavellite 133wernerite 216white granite 44white marble 77white sapphire 147witherite 154wolframite 134, 135

hubnerite 134wollastonite 214wood opal 146wulfenite 102

Xxenolith 55

peridotite xenolith 55xenotime 139

Yyellow sapphire 147yellow tufa 39

Zzincite 104zinnwaldite 182zircon 158

gravel 158zoisite 206, 205

chrome-zoisite 206gemstone 206tanzanite 206thulite cabochon 206

2 2 4 A C K N O W L E D G M E N T S

DORLING KINDERSLEY would like to thankDavid Summers, Jude Garlick, and Miezanvan Zyl for editorial support; LouiseThomas, Neil Fletcher, Georgina Garner,Kevin Walsh, and Monica Price for pictureresearch; Bob Gordon for design work;John Dinsdale for jacket design; AdamPowley for jacket copyediting; MarizaO'Keeffe for jacket editing; and ErinRichards for additional administrative help.

PICTURE CREDITS

Picture librarians: Richard Dabb, Claire BowersAbbreviations key: a = above, b = bottom, c =centre, f = far, l = left, t = top, r = right.

Key:t=top; b=bottom; l=left, r=right; c=centre

alamy: 204 cl. Ben Hoare: 194 cl. ChrisGibson: 20 ca; 22 cl; 30 tl; 32 tl; 33 cr; 34 cl;37 cr. Colin MacFadyen: 28 tl. DaveWaters: 13 cb, bc; 21 tr; 25 tr, cr; 26 cla; 27cla, tr; 29 tr; 31 tr; 34 cla; 35 cla; 37 cra; 46tl; 48 cl, crb; 49 cr; 50 tl, cl; 52 cl; 53 tr; 54cl; 55 tr; 56 tl, cra; 58 crb; 60 tl; 65 tr; 70 tl;73 cr; 74 tl; 75 tr; 76 tl; 77 tr; 78 tl; 81 cr; 82tl; 110 tl; 140 tl; 141 tr, cr; 161 tr; 162 cl;185 tr; 186 tl; 197 tr; 199 tr; 200 tl; 209 tr.Dreamstime: Daryl Faust 23tr. Earl andMaureen Verbeek: 104 tl, cl; 126 cl. Frankde Wit: 89 cr; 92 cl; 106 cl; 107 cr; 115 tr, cr;126 tl; 152 tl, cl; 157 tr; 188 tl, cl; 197 cr;198 tl. Jim Stuby: 55 cr; 69 tr; 135 cr; 182tl. Kevin Walsh: 29 cr; 33 tr; 36 tl; 45 tr; 47tr, cr; 48 tl; 49 tr; 51 tr; 53 cr; 54 tl; 57 tr; 62cl; 64 tl; 72 tl; cl; 80 tl; 82 cl; 83 cr; 128 tl;135 tr; 138 cl; 148 tl; 155 tr; 161 cr; 187 tr;192 tl; 205 tr; 216 tl. Kim Cofman: 117 cr;120 cl. Monica Price: 101 cr; 105 cr; 116 cl;119 cr. National Museum of NaturalHistory, © 2004 Smithsonian Institution,Photographs by Chip Clark: 92 bl; 100 cr;102 tr; 119 c; 119 br; 120 tr, 148 bc; 148 br.National Trust: 43 c. Neil Fletcher: 13 ca;21 cbr; 24 cl, cra; 25 cb; 26 cra, bl, crb; 27br; 28 ca; 31 cl, cl; 35 cb; 36 bl, cb; 38 cl, bl;45 tcr, ca, cb; 47 br, bl; 48 ca; 49 br; 51 bl; 52cb; 54 ca, crb; 55 ca, cb; 59 br; 62 br; 63 ca,car, br; 64 ca, cal, bl; 67 cal, car; 69 car, clb,crb; 71 ca, cb; 72 bl; 73 cal, car; 74 cr; 75 cb,bl; 76 cal, car; 82 cal, cbl, br; 83 ca, cbl, cbr;84 cbl; 89 car, cbl; 95 br; 98 cbr; 101 cal; 103cl; 104 cr, br; 110 cb; 113 cbl; 117 car; 120cbr; 121 crb; 122 cl; 128 cbr; 130 bc; 132crb; 135 cbr; 141 cal; 143 cbr; 147 cbr; 153cal; 160 br; 170 br; 171 cal; 172 bl; 182 cal;184 car, cbr; 189 br; 190 cal, car; 191 clb;197 cb; 200 bl; 202 br; 206 cr; 211 cb; 213bc. Oxford University Museum of NaturalHistory: Joseph Barrett: 124 tl; 154 cl. HelenCowdy: 110 cl. Monica Price: 87 tr; 88 tl; 90tl, cl; 91 tr; 93 br; 96 tl; 98 tl; 100 tl; 102 cl;

108 tl; 112 tl; 114 tl; 117 tr; 120 tl; 130 tl;131 cr; 133 cr; 137 tr; 140 cl; 149 cr; 151 tr;153 cr; 160 tl; 162 tl; 163 cr; 165 tr; 168 tl,cl; 176 tl; 177 tr; 180 cl; 184 cl; 194 tl; 196cl; 200 cl; 202 tl; 203 cr; 212 tl; 213 tr; 214cl. Peter Rigg: 73 tr. R Prout: 93 cr. RonBonewitz: 94 cl; 102 tl; 111 cr; 134 cl; 136cl; 147 tr; 169 tr; 183 tr. Roy Starkey: 44 tl;57 cr; 58 tl, cl; 61 tr; 76 cl; 86 ca; 100 cl; 163 tr. Sandesh Bhandare: 196 tl. StephenKline: 13 bl; 24 tl; 37 tr; 42 tl; 63 tr; 67 cr;68 ca; 81 tr; 123 tr; 172 tl. Stock.XCHNG:178 tl; Joerg Burkhardt 40cl; Oscar Dahl105tr; Steve Dorrington 203tr; Hans-Günther Dreyer 121tr; Dynamite 118cl;Torsten Eismann 159tr; Alejandro GonzálezG. 211tr; Tom Haynes 22tl; Craig Johnson38cl; Cerys Jones 27cr; Stephan Joos 45cr;Aneta Kowalski 13 crb; 41cr; Gregor Künzli64cl; Stephan Langdon 175cr; LL 146tl; M Nota 208cl; Ville Pehkonen 177cr; JimRobinson 35cr; Marcelo da Mota Silva 180tl;Moritz Speckamp 169cr; Deon Staffelbach30cl; Stephanie Syjuco 36cl; Claire Talbot207tr; Dennis Taufenbach 205cr; JayThompson 13 cra; 26tl; Tim & Annette 24cl;Tuomo Tormulainen 204tl; Rob Waterhouse134tl; Emmanuel Wuyts 21cr; BinphonYang 122tl. © United States GeologicalSurvey: 63cr; Connie Hoong 59tr; R.G.McGimsey 190tl; C. Nye, Alaska Division ofGeological and Geophysical Surveys 66tl;U.S. Department of the Interior, U.S.Geological Survey 84tl; U.S. Department ofthe Interior, U.S. Geological Survey, Coastaland Marine Geology Program 26cl. © United States Geological Survey; Imagecourtesy Earth Science World ImageBankhttp://www.earthscienceworld.org/imagebank: © ASARCO 92tl, 99cr, 103tr, 109tr; © Anne Dorr, American Geological Institute113cr; © Larry Fellows, Arizona GeologicalInstitute 169tr; © Chris Keane, AmericanGeological Institute 88cl; © Louis Maher59cr, 138tl; © Cindy Martinez, AmericanGeological Institute 52tl, 174tl; © ThomasMcGuire 127tr; © Martin Miller, Universityof Oregon 175tr; © Marcus Milling,American Geological Institute 109cr; © Bruce Molnia, Terra Photographics 108cl,176cl; © National Park Service 125cr, 209cl;© Oklahoma University 150tl, 173tr, 192cl;© United States Geological Survey 13 tr;28cl; © USGS Hawaiian VolcanoObservatory 67tr.

Every attempt has been made to identify andcontact copyright holders. The publishers will beglad to rectify, in future editions, any omissions orcorrections brought to their notice.

All other images © Dorling Kindersley

Acknowledgments

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Rocks and Minerals_1405305940