1. Practical Igneous Petrology (for Dummies)

2. 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.

3. Make the Most of the Properties.
Colour
Hardness
Transparency (transparent, translucent, opaque)
4. Lustre (adamantine, dull, metallic, pearly, vitreous etc)

4. Make the Most of the Properties.
5. Cleavage and fracture (number of cleavages, concoidal fracture, irregular fracture).

5. Make the Most of the Properties.
6. Crystal system (cubic, tetragonal, orthorhombic, monoclinic, trigonal, hexagonal)
Beryl
Barytes

6. Make the Most of the Properties.
6. Crystal habit (fibrous, prismatic, tabular)
Acicular
Tabular
fibrous
Prismatic

7. 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

8. 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

9. 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)

10. 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.

11. Know the Properties of Important Minerals.

12. Minerals in Thin-Section
Relief
Colour (in PPL)
Pleochroism (in PPL)
Birefringence (colours in XPL)
Crystal Shape
Twinning (e.g. Plag)
Extinction Angle

13. Pyroxene: An Example in Thin-Section

14. Pyroxene: An Example in Thin-Section

15. Pyroxene: An Example in Thin-Section

16. Pyroxene: An Example in Thin-Section
Extinction Angle

17. Mineral Cheat Tree

18. Textures of Igneous Rocks
Holohyaline
Aphantic
Phaneritic

19. 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)

20. Textures of Igneous Rocks
Granular
Porphyritic

21. Textures of Igneous Rocks

22. Classification of Igneous Rocks

23. Describing Igneous Rocks