Igneous Rocks

Classification of Igneous Rocks

• Most Abundant Elements: O, Si, Al, Fe, Ca, Mg, K, Na

• Calculate Elements as Oxides (Account for O)

• How Much SiO2? (Account for Si)• What Feldspars are Present? (Account for

Al, Ca, Na, K)• What Else is Present? (Account for Mg,

Fe)

Silica Content

• Oversaturated: Excess of Silica– Quartz Present

• Saturated: Just enough silica to combine with other ions

• Undersaturated: Silica-deficient Minerals Present– Olivine, Nepheline, Corundum, etc.– Can’t coexist with quartz

Feldspars

• Plagioclase vs. K-Spar (Ca and Na vs. K)

• Relative Aluminum Content– Peraluminous: Al left over after Feldspars

form• Sillimanite, garnet, corundum may be present

– Peralkaline: Al insufficient to form Feldspars• Riebeckite, Aegerine, may be present

Other Ingredients

• Ferromagnesian minerals heavily influenced by characteristics like water– The only difference between rocks with

biotite, amphibole or pyroxene may be water content

• Basis for classification of ultramafic rocks.

“Mainstream” Igneous Rocks

• Ultramafic <40% SiO2

– Plutonic: Dunite Volcanic: Komatiite

• Mafic 40-50% SiO2

– Plutonic: Gabbro Volcanic: Basalt

• Intermediate 50-60% SiO2

– Plutonic: Diorite Volcanic: Andesite

• Felsic >60% SiO2

– Plutonic: Granite Volcanic: Rhyolite

The Feldspars

• Potassium Feldspars– T dependent– Microcline, Orthoclase, Sanidine

• Plagioclase– Classic Example of Solid Solution– Ca vs. Na content

• Perthite: exsolution texture

• Anorthoclase: K, Ca, Na mixture

Potassium Feldspars• Microcline

– Lowest Temperature variety– Plutonic rocks– Almost always perthitic

• Orthoclase– Medium Temperatures– Volcanic and Plutonic Rocks

• Sanidine– Highest Temperature– Volcanic Rocks– May Have Appreciable Na

• More a function of cooling rate and pressure than temperature?

Plagioclase Feldspars• Albite (0-10% Ca): Where Na goes in

metamorphic rocks, metasomatism• Oligoclase (10-30% Ca): Granitic rocks• Andesine (30-50% Ca): Intermediate rocks• Labradorite (50-70% Ca): Mafic rocks• Bytownite (70-90% Ca): Rare - too sodic

for marble, too calcic for magmas• Anorthite (90-100% Ca): Impure

metamorphosed limestones

Perthite and Anorthoclase• Ionic Radii (nm)

– K: 0.133– Ca 0.099– Na 0.097

• Ca and Na substitute freely• K can fit in lattice at high T• Na can fit in K-spar lattice but not Ca• Perthite: K-spar and plagioclase separate

during cooling (Exsolution)• Anorthoclase: Na-K mix, 10-40% K-spar

The Feldspars

Overview of the IUGS

classification of igneous

rocks

Silica-Saturated Rocks

Foids (Feldspathoids)

• Fill the “ecological niche” of feldspars when insufficient silica is available

• Major Minerals:– Nepheline (Na,K)AlSiO4

– Leucite KAlSi2O6

Silica-Deficient Rocks

Volcanic and Plutonic Equivalents

• Granite

• Granodiorite

• Tonalite

• Syenite

• Monzonite

• Diorite

• Gabbro

• Foid Syenite

• Foid Monzonite

• Foid Gabbro

• Rhyolite• Dacite• Dacite• Trachyte• Latite• Andesite• Basalt• Phonolite• Tephrite• Basanite

Olivine• Like Plagioclase, a solid solution

– Forsterite (Mg2SiO4) and Fayalite (Fe2SiO4)

• Becomes More Fe-Rich as Magma Cools• Forsterite

– Can be nearly pure in metamorphic rocks– Cannot coexist with quartz

• Fayalite– Rarely found pure– Can coexist with quartz

Ortho- and Clinopyroxene• Orthopyroxene

– Orthorhombic

– Mixture of Enstatite (Mg2Si2O6) and Ferrosilite (Fe2Si2O6). The generic mixture is termed Hypersthene ((Mg,Fe)2Si2O6)

• Clinopyroxene– Monoclinic

– Mixture of Diopside (CaMgSi2O6) and Hedenbergite (CaFeSi2O6) The generic mixture is termed Augite ((Ca,Mg,Fe)2Si2O6)

Ultramafic Rocks

Mode and Norm

• Mode: What is actually present

• Norm: Ideal mineral composition– Ignores water– Assumes minor components used predictably– Assumes major minerals form in predictable

sequence– Purpose is to visualize rock from chemical

data

CIPW Norm

• Cross, Iddings, Pirrson and Washington

• All Cations treated as oxides

• Anions (S, F, Cl) treated as elements

• Convert wt% to molecular proportions (Wt%/Mol Wt)

• Allocate oxides to mineral phases

Allocate minor elements• Ba, Sr Ca; MnO FeO

• CO2 Calcite (with CaO)

• P2O5 Apatite (with CaO)

• S Pyrite (with FeO)

• TiO2 Ilmenite (with FeO)

• F Fluorite (with CaO)

• Cr2O3 Chromite (with FeO)

• Cl Halite (With Na2O)

Start Forming Silicates

• ZrO2 Zircon (with SiO2)

• Form provisional Feldspars– Na2O Albite

– K2O K-Spar

– CaO Anorthite

– With SiO2 and Al2O3

– May need to convert to foids if SiO2 runs out

Allocate FeO, MgO and CaO

• Fe2O3 Acmite (With Na2O and SiO2) and Magnetite (With FeO)

• FeO and MgO Hypersthene (provisional)

• CaO + Hy Diopside

• Excess SiO2 Quartz

If Silica Runs Out

• Hypersthene Olivine

• Albite Nepheline

• K-Spar Leucite

Example

• SiO2 83

• TiO2 2

• Al2O3 16

• Fe2O3 2

• FeO 10

• MgO 17

• CaO 17

• Na2O 5

• K2O 1

Let the Games Begin

• Ilmenite: TiO2 0; FeO 10 - 2 = 8

• K-Spar: K2O 0; Al2O3 16 – 1 = 15; SiO2 83 – 6K2O = 77

• Albite: Na2O 0; Al2O3 15 – 5 = 10; SiO2 77 – 6Na2O = 47

• Anorthite: CaO 0; Al2O3 10 – 17 = -7!

– Excess CaO

– CaO 17-10 = 7; Al2O3 0; SiO2 47 – 2CaO = 27

Final Allocations

• Magnetite: Fe2O3 0; FeO 10-2 = 8

• FeO + MgO = 8 + 17 = 25

• Diopside: CaO 0; FeO + MgO = 25 – 7 = 18; SiO2 SiO2 – 2CaO = 27-14 = 13

• Hypersthene: FeO + MgO 0; SiO2 13 – 18 = -5 (Call this -D)

• Olivine: Ol = D = 5

• Hypersthene: Hy – 2D = 18 – 10 = 8

Final Result

• Ilmenite: 2• K-Spar: 1• Albite: 5• Anorthite: 10

• These are molecular proportions

• Magnetite: 2• Diopside: 7• Olivine: 5• Hypersthene: 8

• Multiply by Mol. Wt. and normalize for Wt%