Igneous Rocks. Classification of Igneous Rocks Most Abundant Elements: O, Si, Al, Fe, Ca, Mg, K, Na...
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Transcript of Igneous Rocks. Classification of Igneous Rocks Most Abundant Elements: O, Si, Al, Fe, Ca, Mg, K, Na...
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
Foids (Feldspathoids)
• Fill the “ecological niche” of feldspars when insufficient silica is available
• Major Minerals:– Nepheline (Na,K)AlSiO4
– Leucite KAlSi2O6
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)
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
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