Practical Igneous Petrology (for Dummies)

Minerals in Hand Specimen Have the right tools Make the most of the properties Know what to expect where! Know the key properties of the key minerals.

Make the Most of the Properties. Colour Hardness 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) Acicular Tabular fibrous Prismatic

Know Where to Expect What. <1/3 dark minerals (e.g. Granite, rhyolite) Most common light coloured minerals (in order of frequency): Quartz Alkali feldspar Plagioclase feldspar Muscovite Most common dark coloured minerals (in order of frequency): Biotite Amphibole (hornblende most common but also reidbeckite in alkali granites). Pyroxenes Tourmaline (abundant in some granites and pegmatites). Most common opaque minerals: Pyrite Chalcopyrite Azurite Galena Malachite Magnetite Hematite 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). Quartz Feldspathoids (such as nepheline and leucite). Most common dark coloured minerals (in order of frequency): Hornblende Pyroxenes (augite, hypersthene, pigeonite, diopside) Biotite and/or phlogopite Olivine (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 feldspar Quartz (rare) Alkali feldspar (rare) Feldspathoids (rare) Most common dark coloured minerals (in order of frequency): Pyroxene (augite, hypersthene, pigeonite, diopside) Olivine Phlogopite (a mica) Garnet Amphiboles (rare) Biotite (rare) Melanocratic (mafic and ultramafic)

Know the Properties of Important Minerals. Quartz (SiO2) Characteristic properties Colour 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 Aphantic Phaneritic

Textures of Igneous Rocks Coarse-grained – crystals are > 5mm Medium-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