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 SiO 2 ? (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% SiO 2 –Plutonic: DuniteVolcanic: Komatiite Mafic40-50% SiO 2 –Plutonic: GabbroVolcanic: Basalt Intermediate50-60% SiO 2 –Plutonic: DioriteVolcanic: Andesite Felsic>60% SiO 2 –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 –Ca0.099 –Na0.097 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)AlSiO 4 –Leucite KAlSi 2 O 6

Silica-Deficient Rocks

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

Olivine Like Plagioclase, a solid solution –Forsterite (Mg 2 SiO 4 ) and Fayalite (Fe 2 SiO 4 ) 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 (Mg 2 Si 2 O 6 ) and Ferrosilite (Fe 2 Si 2 O 6 ). The generic mixture is termed Hypersthene ((Mg,Fe) 2 Si 2 O 6 ) Clinopyroxene –Monoclinic –Mixture of Diopside (CaMgSi 2 O 6 ) and Hedenbergite (CaFeSi 2 O 6 ) The generic mixture is termed Augite ((Ca,Mg,Fe)2Si 2 O 6 )

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 CO 2  Calcite (with CaO) P 2 O 5  Apatite (with CaO) S  Pyrite (with FeO) TiO 2  Ilmenite (with FeO) F  Fluorite (with CaO) Cr 2 O 3  Chromite (with FeO) Cl  Halite (With Na 2 O)

Start Forming Silicates ZrO 2  Zircon (with SiO 2 ) Form provisional Feldspars –Na 2 O  Albite –K 2 O  K-Spar –CaO  Anorthite –With SiO 2 and Al 2 O 3 –May need to convert to foids if SiO 2 runs out

Allocate FeO, MgO and CaO Fe 2 O 3  Acmite (With Na 2 O and SiO 2 ) and Magnetite (With FeO) FeO and MgO  Hypersthene (provisional) CaO + Hy  Diopside Excess SiO 2  Quartz

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

Example SiO 2 83 TiO 2 2 Al 2 O 3 16 Fe 2 O 3 2 FeO 10 MgO 17 CaO 17 Na 2 O 5 K 2 O 1

Let the Games Begin Ilmenite: TiO 2  0; FeO  = 8 K-Spar: K 2 O  0; Al 2 O 3  16 – 1 = 15; SiO 2  83 – 6K 2 O = 77 Albite: Na 2 O  0; Al 2 O 3  15 – 5 = 10; SiO 2  77 – 6Na 2 O = 47 Anorthite: CaO  0; Al 2 O 3  10 – 17 = -7! –Excess CaO –CaO  = 7; Al 2 O 3  0; SiO 2  47 – 2CaO = 27

Final Allocations Magnetite: Fe 2 O 3  0; FeO  10-2 = 8 FeO + MgO = = 25 Diopside: CaO  0; FeO + MgO = 25 – 7 = 18; SiO 2  SiO 2 – 2CaO = = 13 Hypersthene: FeO + MgO  0; SiO 2  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%