Study guide for igneous rocks

Practical

1. Know your optical mineralogy and how to determine all common properties of minerals in thin section. Note that all common properties may not be determinable in any particular mineral grain.
2. Be able to identify the common plutonic and volcanic rocks by eye without a rock classification chart.
3. Be able to make a reasonably accurate identification of large (>1 mm), common minerals in hand samples.
4. Be able to identify in thin section all of the minerals seen in the Igneous Minerals lab. This can be done with a book (like Tröger or DHZ), but it will slow you down enormously without your own honed experience.
5. Be able to estimate mineral modes in thin section and to classify the rock reasonably well according to that mode without a rock classification chart.
6. Be able to interpret mineral inclusions and other common textures in terms of a simple crystallization and reaction sequences under magmatic and/or subsolidus conditions.

Theoretical

1. Understand the tectonic settings in which magmas are commonly found.
2. Know the likely mechanisms by which melting takes place (what causes melting) in different tectonic environments.
3. Understand the effect of source composition and mineralogy on magmas that are produced from them by partial melting.
4. Understand the effect common magmatic processes have on the major and trace element compositions of evolving magmas (assimilation, magma mixing, fractional crystallization, equilibrium crystallization).
5. Know the differences between different types of igneous bodies (e.g., batholith, sill, dike, lava flow, ignimbrite, ash, etc.).
6. Know the phase rule and how to apply it.
7. Know the lever rule and how to apply it to any 2 or 3 component chemical system.
8. Know the workings of these liquidus systems: An-Di, Fo-Sil, An-Ab, An-Di-Fo, An-Fo-Sil, Ab-Or-Sil.
9. Understand and be able to make chondrite-normalized rare earth element diagrams.
10. Understand and be able to make reasonable interpretations of major element chemical variation diagrams with respect to identifying the sequence of crystallization of common phases from the liquid.
11. Know, in general, how compatible and incompatible elements behave during ideal fractional and equilibrium melting and fractional and equilibrium crystallization.
12. Pay attention during field trips, because test questions may include material from them.

Common optical properties you should be able to determine

1. Basic optical indicatrix type: isotropic, uniaxial, biaxial.
2. Optic sign.
3. 2V.
4. Sign of elongation.
5. Extinction angle of elongate crystals (e.g., micas, amphiboles, pyroxenes, apatite, chlorite).
6. Sign of dispersion.
7. Pleochroism.
8. Refractive index relative to adjacent minerals.