Rock ClassificationRock Classification

Chapter 1, pp. 20-24

Basis for ClassificationBasis for Classification• Minerals present in the rock

– Best for coarse-grained rocks– Useful for field work

• Chemical Composition – Works for fine-grained rock– Expensive and takes time

Chemical ClassificationChemical Classification• CIPW norm

– Calculated minerals from chemical analysis

• Saturation concept– Si saturation

• Acid to basic– Al saturation

• Harker-Peacock index– Alkalies vs calcium

Silica SaturationSilica Saturation

Acid SiO2 > 66 %

Intermediate SiO2 52 to 66 %

Basic SiO2 45 to 52 %

Ultrabasic SiO2 < 52 %

Aluminum SaturationAluminum Saturation

Based on the feldspar ratio 1:1:3 (NaAlSi3O8)

Peraluminous Al2O3 > (CaO + Na2O + K2O)

Peralkaline (Na2O + K2O) > Al2O3

Earth CompositionEarth Composition

There are 4 important compositions for petrologic interpretation of the Earth

• General Earth composition

• Mantle composition

• Oceanic crust composition

• Continental crust composition

Bowen’s Reaction SeriesBowen’s Reaction Series

• What is a reaction?

• Olivine to Pyroxene

Mg2SiO4 + SiO2 = 2MgSiO3

• Reaction textures in the rocks

SpinelsSpinels

• General formula is AB2O4

• A is a +2 cation (Mg, Fe+2, etc.)

• B is a +3 cation (Fe+3, Al, Cr)

• Some Ti+4 can substitute in B

Common Spinel Family Minerals

• Magnetite FeFe2O4

• Spinel MgAl2O4

• Hercynite FeAl2O4

• Chromite FeCr2O4

Triangular Diagrams

Feldspar ClassificationFeldspar ClassificationPyroxene ClassificationPyroxene Classification

Basalt Basalt ClassificationClassification

Phases:

Augite (Cpx)Hypersthene (Hy)Olivine (Oliv)Plagioclase (Plag)Nepheline (Ne)

UltramaficUltramaficClassificationClassification

PlutonicPlutonicClassificationClassification

Based on 4 phases

QuartzAlkali FeldsparPlagioclaseFeldspathoids

Volcanic Volcanic ClassificationClassification

Based on 4 phases

QuartzAlkali FeldsparPlagioclaseFeldspathoids

Basalt Basalt DiscriminatorDiscriminator Basalt TrendsBasalt Trends

Si ActivitySi Activity

• αSiO2• Based on s set of

reactions• Limits three main

magma series