Practical Igneous Petrology (for Dummies)

Minerals in Hand Specimen

A. Have the right tools

B. Make the most of the properties

C. Know what to expect where!

D. Know the key properties of the key minerals.

Make the Most of the Properties.

1. Colour

2. Hardness

3. Transparency (transparent, translucent, opaque)

4. Lustre (adamantine, dull, metallic, pearly, vitreous etc)

Make the Most of the Properties.

5. Cleavage and fracture (number of cleavages, concoidal fracture, irregular fracture).

Make the Most of the Properties.

6. Crystal system (cubic, tetragonal, orthorhombic, monoclinic, trigonal, hexagonal)

Beryl

Barytes

Make the Most of the Properties.

6. Crystal habit (fibrous, prismatic, tabular)

fibrous

Acicular

Prismatic

Tabular

Know Where to Expect What.

<1/3 dark minerals(e.g. Granite, rhyolite)

Most common light coloured minerals (in order of frequency):QuartzAlkali feldsparPlagioclase feldsparMuscoviteMost common dark coloured minerals (in order of frequency):BiotiteAmphibole (hornblende most common but also reidbeckite in alkali granites). PyroxenesTourmaline (abundant in some granites and pegmatites).Most common opaque minerals:PyriteChalcopyriteAzuriteGalenaMalachiteMagnetiteHematite

Leucocratic

Know Where to Expect What.

~50% dark minerals(e.g. Diorite, andesite)

Most common light coloured minerals (in order of frequency):Feldspars (either alkali or plag can dominate).QuartzFeldspathoids (such as nepheline and leucite).

Most common dark coloured minerals (in order of frequency):HornblendePyroxenes (augite, hypersthene, pigeonite, diopside)Biotite and/or phlogopiteOlivine (rare)

Mesocratic

Know Where to Expect What.

>1/3 dark minerals(e.g. basalt, gabbro)

Most common light coloured minerals (in order of frequency):Plagioclase feldsparQuartz (rare)Alkali feldspar (rare)Feldspathoids (rare)

Most common dark coloured minerals (in order of frequency):Pyroxene (augite, hypersthene, pigeonite, diopside)OlivinePhlogopite (a mica)GarnetAmphiboles (rare)Biotite (rare)

Melanocratic(mafic and ultramafic)

Know the Properties of Important Minerals.

Quartz (SiO2)

Characteristic propertiesColour is as variable but clear quartz is by far the most common color followed by white or cloudy (milky quartz). Purple (Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are also common. Cryptocrystalline varieties (agate, chalcedony etc) can be multicolored. Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous.Crystal System: trigonal.Cleavage and Fracture: No cleavage, concoidal fracture.Crystal Habit: variable but often anhedral. Hexagonal prisms or three sided prismatic crystals are common in veins and pegmatites.Hardness: 7 (less in agate and chalcedony).Occurrence: any leucocratic rock is likely to contain quartz, melanocratic rocks are unlikely to contain quatz.Best Indicators are first the fact that it is very common (always assume transparent clear crystals may be quartz), crystal habit, hardness, striations on the surfaces of crystals, good conchoidal fracture and lack of good cleavage.

Know the Properties of Important Minerals.

Minerals in Thin-Section

• Relief• Colour (in PPL)• Pleochroism (in PPL)• Birefringence (colours in XPL)• Crystal Shape• Twinning (e.g. Plag)• Extinction Angle

Pyroxene: An Example in Thin-Section

Pyroxene: An Example in Thin-Section

Pyroxene: An Example in Thin-Section

Pyroxene: An Example in Thin-Section

Extinction Angle

Mineral Cheat Tree

Textures of Igneous Rocks

Holohyaline

PhaneriticAphantic

Textures of Igneous Rocks

Coarse-grained – crystals are > 5mmMedium-grained – crystals are 1-5 mm

Fine-grained – crystals are <1 mm (not including phenocrysts)

Textures of Igneous Rocks

Granular Porphyritic

Textures of Igneous Rocks

Classification of Igneous Rocks

Describing Igneous Rocks