Lab 2 Geosciences 2014

GEOS 210B – GEOSCIENCE LABORATORY 2 B.A.Sc – PETROLEUM ENGINEERING

INTRODUCTION

Sedimentary rocks are formed from a process called lithification. As particles of sediment are

deposited out of air, ice, or water flows, layers of sediment are built up overtime. Overburden

pressure acting on the sediments causes the sediments to solidify into rock. Sediments undergo

chemical, physical, and biological changes after initial deposition and during and after

lithification. The composition of sediments provides us with clues as to the original rock.

Differences between successive layers indicate changes to the environment which have occurred

over time.

There are three basic types of sedimentary rocks:

clastic sedimentary rocks that are formed from the mechanical weathering of debris.

These rocks are made from clastic sedimentary particles that are classified in terms of

size. (See Table below)

Name ofParticle Size Range Loose

Sediment Consolidated Rock

Boulder >256 mm GravelConglomerate or Breccia (depends on roundingCobble 64 - 256 mm Gravel

Pebble 2 - 64 mm GravelSand 1/16 - 2 mm Sand SandstoneSilt 1/256 - 1/16 mm Silt SiltstoneClay <1/256 mm Clay Claystone, Mudstone and shale

chemical sedimentary rocks that form when dissolved materials precipitate from solution

biochemical sedimentary rocks which form from the accumulation of plant or animal

debris

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The report that follows highlights some types of sedimentary rocks and describes each rock

in terms of their physical and chemical properties where applicable. It also briefly states their

geologic characteristics with respect to formation and their environment.

AIM

To describe the physical, mineralogical and chemical properties and geologic characteristics of

the sedimentary rocks listed below -

Conglomerates

Breccias

Sandstomes

Siltstones

Shale

Coal

Chert

Limestone

Dolomite

Halite

Arenites

Gypsum

Graywackes

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Arkoses

SCOPE

The report seeks to:

1. discuss how sedimentary rocks are formed

2. describe each rock in terms of their physical, mineralogical and chemical properties

3. state their geologic characteristics

It will also show an illustration of each rock so a clearer understanding with respect to texture

and colour could be gained. References will also be cited in relation to the material that is

presented in this lab.

METHODOLOGY

Information about the varieties of sedimentary rock mentioned was accessed via the World

Wide Web. Data was collected via on-line research, analyzed, discussed and presented using

tables and illustrations.

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Conglomerates

Geologic Characteristics

Conglomerates are formed from larger fragments of weathered material. Clasts are generally

gravel sized and rounded in shape. The rounded shape reveals that fragments are transported far

from their point of origin. This consolidated rock is composed of a variety of weathered

material.

Classification Clastic Sedimentary Rock

PhysicalProperties Rock Sample

Porosity Low

Figure 1. Conglomerate

Grain size > 2mmTexture Rounded, Poorly sortedStructure Cross beddingColour Various colours

MatrixMixture of sand, mud and chemical cement

Chemical / MineralogicalProperties

Non uniform Fragments that make up conglomerates

have many different chemical and

mineralogical compositions. They may

be sedimentary, igneous, metamorphic

fragments, all of which have a great

number of different minerals and

chemicals

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Mineralogical and chemical characteristics is mostly non - uniform in conglomerates

Sandstones


Sandstone is a type of sedimentary rock made up of tiny grains of mainly quartz and arkose.

These tiny ‘clasts’ are deposited in layers, over long periods of timeand become buried and

compacted to form sandstone. Sandstone can be found in both marine and non-marine

environments such as oceans, lakes, deltas, estuaries or beaches.



Hardness 6.5 - 7 Figure 2. Sandstone

Density 2.3. - 2.4 kg/cm3Porosity LowGrain size 1/16 mm - 2mmTexture SortedStructure Cross bedding

ColourBrown, tan, yellow, red, grey, white, pink

ChemicalProperties

Silica 95% - 97%Alumina 0.5% - 1.5%Soda <1%Lime <0.5%Magnesium Oxide

<0.5%

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Mineralogical and chemical characteristics is mostly uniform in sandstone

Siltstone


Siltstone, like sandstone is made up of sedimentary particles that are transported, deposited and

lithified. These particles are much smaller in size and are usually formed offshore or in much

quieter environments, like lakes and lagoons.



Grain size 1/256 - 1/16 mm

Figure 3. Siltstone

TextureSorted, soft and crumbly due to

absence of clay matrix

Structure Cross bedding

Colour Brownish, reddish, greyish hues

Chemical Properties

Mostly Silt

Mineralogical and chemical characteristics is mostly uniform in sandstone

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Limestone


Limestone is one of the most abundant chemical or biochemical sedimentary rock. Most

limestone is biochemical because organisms are important to its origin. They contain calcite

which is precipitated from solution or from the leaching of calcium enriched matter such as

shells, bones and marine micro-organisms. Most limestones form in shallow, calm, warm marine

waters such as coral reefs or the bottom of oceans.

Classification Chemical / Biochemical Sedimentary Rock


Hardness 3 - 4 Figure 4. Limestone

Density 2.5 - 2.7 kg/cm3Porosity HighGrain size 1/16 mm - 2mm

TextureVaries - crystalline, granular, massive

Structure Fossilsiferous

Colour

Naturally white, but also brown, yellow due to iron oxides, and blue, black, grey due to presence of Carbon

Chemical/MineralogicalProperties

Lime (CaCO3) 38% - 42%Silica 15% - 18%Alumina 3% - 5%Iron Oxides 1 - 1.5%Alkalis 1 - 1.5%

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Magnesium Oxide 0.5% - 3%

Mineralogical and chemical characteristics are varied. They may be found in many forms such

as:

1. Chalk - soft limestone with fine texture. Formed mainly from calcareous shell remains of

microscopic marine organisms

2. Coquina - Poorly cemented limestone composed mainly of broken shell bones

3. Fossiliferous limestone - contains abundant fossils that are skeletal and shell like.

4. Travertine - formed form evaporative precipitation

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Dolomite

Dolomite is a rock forming mineral which is the primary component in the sedimentary rock

dolostone. Rock rich in dolomite would have been deposited as Calcium carbonate and

subsequently altered by magnesium rich water to form dolomite. Dolostone which is similar to

limestone is considered to be formed when magnesium replaces some of the calcium in calcite,

thereby converting calcite to dolomite.

Classification Chemical / Biochemical Sedimentary Rock


Hardness 3 – 5 Figure 5. Dolomite

Fracture ConchoidalTenacity BrittleLuster Vitreous, pearly

TextureVaries - crystalline, granular, massive

Streak White

ColourColourless, pink, green, grey, black, white

Diaphaneity Transparent to translucentSpecific Gravity

2.8 - 3.0

Cleavage 1,3 rhombohedralCrystal structure

Hexagonal

Chemical/MineralogicalProperties

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Calcium Magnesium Magnesium - Ca Mg(CO3)2

Equal amounts of Calcium to Magnesium

Small amounts of Iron, Magnesium, Manganese

Impurities include calcite, quartz and feldspar

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Coal

Geologic characteristics

Sedimentary rock that forms from the accumulation and preservation of plant debris. This

sedimentary rock is usually found in swamp or bog environments whereby lack of oxygen

increases the rate of decay of plant material. The decayed plant remains are buried by mud or

sand and washed into the swamp. Overburden pressure acting on these materials compacts the

plant debris and aids in the transformation to coal. In addition, insects and other organisms that

might consume the plant debris on land do not survive well under water in an oxygen deficient

environment.

Classification Organic Sedimentary Rock


Hardness Average, 3.2 - 3.2 Figure 6. Coal

Fracture Semi - conchoidal to conchoidalLuster Dull to vitreousStreak BlackColour Dark brown to BlackDiaphaneity OpaqueSpecific Gravity 1.2 - 1.5

ChemicalProperties

Coal is composed primarily of carbon along with

variable quantities of other elements, chiefly

hydrogen, sulphur, oxygen, and nitrogen.

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Mineralogical Properties

Although as many as 120 different minerals have been

identified in coal, only about 33 of them commonly

are found in coal, and of these, only about 8 (quartz,

kaolinite, illite, montmorillonite, chlorite, pyrite,

calcite, and siderite) are abundant enough to be

considered major constituents.

As plant debris is exposed to heat and pressure due to overburden stress, it changes in

composition and properties. The rank of a coal is a measure of how much change has occurred.

Rank from lowest to highest

Peat

Mass of accumulated to partially carbonized pant debris Carbon content <60%

in a dry ash free basis

Lignite

Lowest rank of coal Brown black coal formed form the transformation of peat Carbon content 60% to 70% in a dry ash free basis Heating value <8300 BTU

Sub Bituminous

Lignite subjected to increased level of organic metamorphism Carbon content 71% to 77% in a dry ash free basis Heating value 8300 to 13000 BTU

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Bituminous

Sub bituminous is subjected to increased level of organic metamorphism Carbon content 77% to 87% in a dry ash free basis Heating value > 13000 BTU

Anthracite

Highest rank of coal Carbon >87% in a dry ash free basis Heating value > 13000 BTU

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Breccia

Breccia is a term most often used for clastic sedimentary rocks that are composed of large

angular fragments (over 2 mm in diameter). The spaces between the large angular fragments can

be filled with a matrix of smaller particles or mineral cement that binds the rock together.

Breccia is very similar to conglomerate rock, in that clasts are generally gravel sized. Breccias

are formed from larger fragments of weathered material making the consolidated rock a network

of a variety of weathered material. Unlike conglomerates the large particles in breccia are

angular in shape unlike conglomerate the particles which are rounded. This reveals that

fragments were not transported too far from their point of origin and near the outcrop where the

fragments were produced by mechanical weathering, the shape is angular.



Porosity Low

Figure 7. Breccia

Grain size > 2mmTexture Angular, Poorly sortedStructure Cross bedding

ColourThe colour of the matrix or cement along with the colour of the angular rock fragments determines its colour.

Hardness Variable soft to hard

Streak Based on mineral composition

Lustre Dull

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S.G. 2.5

Fracture Along fragment lines

MatrixVariable - Mixture of sand, mud and

chemical cement

Chemical

Properties

The cement that binds the clasts in a breccia is generally

one of either calcite, silica or iron oxide

Mineralogical

Properties

Breccia has many compositions. Its composition is

mainly determined by the rock and mineral material that

the angular fragments were produced from. The climate

of the source area can also influence composition. Most

breccias are a mix of rock fragments and mineral grains.

The type of rock that the fragments were produced from

is often used as an adjective when referring to the rock.

Some examples: sandstone breccia, limestone breccia,

granite breccia, chert breccia, basalt breccia and others.

Often a breccia will contain many types of angular rock

fragments. These are known as polymict breccias or

polymictic breccias.

Mineralogical and chemical characteristics is mostly non - uniform in Breccias

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Arenite

Arenites are mainly formed by erosion of other rocks or turbiditic re-deposition of sands. Some

arenites contain a varying amount of carbonatic components and thus belong to the rock-

category of carbonatic sandstones or silicatic limestones. Arenites often appear as massive or

bedded medium-grained rocks with a middling-to wide-spaced preferred foliation and often

develop a pronounced cleavage

Classification - Chemical Sedimentary Rock


Hardness 6.5 – 7 Figure 8. Arenite

Fracture Conchoidal

Luster Dull

TextureVaries - crystalline, granular,

massive

Streak Variable colour streak

ColourTan, brown, yellow, red, pink, grey,

black, white

Diaphaneity Transparent to translucent

Specific

Gravity2.3 – 2.4

Chemical / Mineralogical Properties

Arenite is a sedimentary clastic rock with sand grain

size between 0.0625 mm and 2 mm and contain less

than 15% matrix. Since it refers to grain size rather

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than chemical composition, the term is used for

example in the classification of clastic carbonatic

limestones, as the granulometrically equivalent term

sandstone is not appropriate for limestone. Other

arenites include sandstones, arkoses, greensands, and

greywackes.

Gypsum

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Gypsum is a common mineral, with thick and extensive evaporite beds in association with

sedimentary rocks. Deposits are known to occur in strata from as far back as the Archaean eon.

Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapours,

and sulphate solutions in veins. Hydrothermal anhydrite in veins is commonly hydrated to

gypsum by groundwater in near-surface exposures. It is often associated with the minerals halite

and sulphur



Hardness 2 Figure 9. Gypsum

Fracture Splintery, ConchoidalLuster Vitreous, silky, sugaryStreak Variable colour streak

ColourPure gypsum is white, impurities gives various colours, gray, yellow, red brown


2.3

Crystal structure Monoclinic

ChemicalProperties

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Hydrous Calcium Sulfate – CaSO4·2H2O

Gypsum is a very soft sulphate mineral composed of

calcium sulphate dehydrate, with the chemical

formula CaSO4·2H2O. It can be used as a fertilizer, is

the main constituent in many forms of plaster and is

widely mined.It forms as an evaporite mineral and as

a hydration product of anhydrite.

Gypsum is also formed as a by-product of sulphide

oxidation, amongst others by pyrite oxidation, when

the sulphuric acid generated reacts with calcium

carbonate. Its presence indicates oxidizing conditions.

Under reducing conditions, the sulphates it contains

can be reduced back to sulphide by sulphate reducing

bacteria.

Mineralogical

Properties

Gypsum rocks are sedimentary rocks made up of

sulphate mineral and formed as the result of

evaporating sea water in massive prehistoric basins.

Gypsum belongs to a group of minerals called the

sulphates, and is the most common of the

approximately 150 sulphate minerals. Sulphates are

compounds of one or more metals with oxygen and

sulphur. The oxygen and sulphur join together to form

the sulphate ion, SO4-2. Technically, gypsum is

hydrous calcium sulphate because it has water in its

crystal structure, CaSO4.2H2O

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Halite


Halite is mainly a sedimentary mineral that usually forms in arid climates where ocean water

evaporates. They may exist as sedimentary evaporate deposits in saline lakes and water courses,

as bedded sedimentary deposits or as salt domes.



Hardness 2.5 Figure 10. Halite

Fracture ConchoidalLuster Vitreous, glassyTexture CoarseStreak White

ColourWhite, clear, light blue, dark blue, pink, grey due to impurities


2

CleavagePerfect, cubic, 3 directions at right angles

Crystal structure

isometric


Sodium (Na) - 39.34 % Chloride (Cl) - 60.66%

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Chert


Chert forms layers and nodules as well as large beds in sedimentary and metamorphic rocks. It

occurs in epithermal veins as a result of direct deposition around submarine hydrothermal vents

and may form as a product of silicification of biogenic or volcanogenic sediments

Classification - Biochemical Sedimentary Rock


Hardness 6.5 - 7.5 Figure 11. Chert

Fracture ConchoidalLuster Waxy, dullTexture Non-clasticStreak White or lightly coloured

ColourWhite, grey, black, brown and all other colours dependent on impurities


2

Cleavage None observed

Crystal structure

cryptocrystalline


Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of SiO2

Chert is mostly composed of quartz along with minor minerals

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Graywackes


Its formation is due to submarine avalanches or strong turbidity currents. These cause sediments

to be churned and also the mixed-sediment slurries, in which the rocks may exhibit a variety of

sedimentary features. Supporting the turbidity current origin theory is that deposits of greywacke

which are found on the edges of the continental shelves, at the bottoms of oceanic trenches and at

the bases of mountain formational areas. They also occur in association with black shales of deep

sea origin. They are extremely deformed, fractured and veined, due to due to significant amounts

of tectonic movement.


Hardness 5-8 Figure 12. Graywackes

TextureClastic – fine to coarse, gritty, veined non-vesicular, poorly sorted

ColourGrey to black often with white quartz veins

Cleavage Good


Graywackes are generally consisted of quartz, orthoclase and plagioclase feldspars, calcite, iron oxides and graphitic, carbonaceous matters. Biotite, chlorite, tourmaline, epidote, apatite, garnet, hornblende, augite, sphene and pyrites

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Shale


Shale is a fine-grained sedimentary rock that forms from the compaction of silt and clay-size

mineral particles. An accumulation of mud begins with the chemical weathering of rocks.

Weathering breaks the rocks down into clay minerals and other small particles which often

become part of the local soil. Water washing tiny particles of soil from the land and into streams,

gives the streams a "muddy" appearance. When the stream slows down or enters a standing body

of water such as a lake, swamp or ocean the mud particles settle to the bottom. If undisturbed and

buried this accumulation of mud, experiencing overburden pressure is often transformed into

"mudstone" or shale.

Classification - Organic Sedimentary Rock


Grain size <1/256 Figure 13. Shale

Porosity HighHardness 3Texture Clastic, rounded, well sortedStructure Fissile, Laminated

Colour Black, red, brown, yellow, green

Cleavage Good


Shale is a rock composed mainly of clay-size mineral

grains. These tiny grains are usually clay minerals such

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as illite, kaolinite and smectite. Shale usually contain

other clay-size mineral particles such as quartz, chert

and feldspar. Other constituents might include organic

particles, carbonate minerals, iron oxide minerals,

sulfide minerals and heavy mineral grains. These "other

constituents" in the rock are often determined by the

shale's environment of deposition and often determine

the color of the rock. Shales and mudrocks contain

roughly 95 percent of the organic matter.

Arkoses


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Arkoses is generally formed from the weathering of feldspar-rich igneous or metamorphic rocks

consisting of granitic rocks. These are generally composed of quartz and feldspar. These

sediments must be deposited rapidly and or in a cold or dry environment so the feldspar will not

undergo chemical weathering and decomposition


Hardness Relatively hard Figure 14. Arkose

Texture Fine to very coarse

Colour Grey to reddish

Cleavage Good


Arkose is a detrital sedimentary rock which is known as sandstone which contains more than 25% changeable constituents with feldspar forming more than half of this 25%.Arkoses is composed of quartz and feldspar grains together with small amounts of mica.

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Documents

Lab 2 Geosciences 2014