Essential Guide to Igneous Rocks: Types and Classification

1. Igneous Rocks

2. 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)

3. 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

4. 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

5. 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.

6. “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

7. 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

8. 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?

9. 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

10. 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

11. The Feldspars

12. Overview of the IUGS classification of igneous rocks

13. Silica-Saturated Rocks

14. Foids (Feldspathoids) • Fill the “ecological niche” of feldspars when insufficient silica is available • Major Minerals: • Nepheline (Na,K)AlSiO4 • Leucite KAlSi2O6

15. Silica-Deficient Rocks

16. Granite Granodiorite Tonalite Syenite Monzonite Diorite Gabbro Foid Syenite Foid Monzonite Foid Gabbro Rhyolite Dacite Dacite Trachyte Latite Andesite Basalt Phonolite Tephrite Basanite Volcanic and Plutonic Equivalents

17. 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

18. 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)

19. Ultramafic Rocks

20. 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

21. 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

22. 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)

23. 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

24. 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

25. If Silica Runs Out • Hypersthene  Olivine • Albite  Nepheline • K-Spar  Leucite

26. Example • SiO2 83 • TiO2 2 • Al2O3 16 • Fe2O3 2 • FeO 10 • MgO 17 • CaO 17 • Na2O 5 • K2O 1

27. 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

28. 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

29. 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%