Polymerization of Silica Each Si atom is surrounded by 4 Chapter 4 INTRUSIVE IGNEOUS ROCKS Polymerization of Silica oxygens Polymerization is the degree to which the oxygens are shared with other Si atoms Polymerization is the number of Si O Si bonds Olivine has none isolated SiO4 groups Pyroxene has one third SiO3 chains Mica has has 40 shared Si4O10 sheets Quartz and feldspar have 100 Si Al O2 framework Polymerization of Silica Pyroxene Tetrahedron Olivine MAGMA Magma is the term for any molten silicate material whether below the surface or on top Volcanic rocks are erupted on the surface Volcanic rocks are fine grained 1 mm Intrusive igneous rocks crystallize from magma below the surface Intrusive igneous rocks are coarse grained 1mm Pegmatites are very coarse grained 1cm Mica Sheet Quartz Framework Intrusive Igneous Rocks Composition Mineralogy Geologic Setting Mantle Oceanic Crust Continental Crust Igneous Fractionation How the chemistry evolves 1 Igneous Rock Compositions Rock compositions are described in weight percents of oxides The principal variation in igneous rock compositions is silica SiO2 content The degree of polymerization of silica increases with silica content both in crystals and melt Igneous Rock Names and Compositions Volcanic Composition Intrusive Name Rock Name Rock Name Ultramafic Mafic Intermediate Silicic felsic Peridotite Gabbro Diorite Granite Komatiite Basalt Andesite Rhyolite Igneous Rock Compositions Igneous rocks vary in composition from The mantle is peridotite ultramafic The ocean basins gabbro mafic The continents are granite silicic Igneous Rock Mineralogy Peridotite Ultramafic Gabbro Mafic Olivine Mg Fe 2SiO4 Pyroxene Mg Fe Ca SiO3 Granite Silicic Feldspar CaAl2Si2O8 Feldspar NaAlSi3O8 Pyroxene Mg Fe Ca SiO3 Mica K Mg Fe 3 Si Al 2O10 OH 2 Quartz SiO2 Feldspar NaAlSi3O8 Feldspar KAlSi3O8 Mica biotite K Mg Fe 3 Si Al 4O10 OH 2 Mica muscovite KAl2 Si Al 4O10 OH 2 Feldspar CaAl2Si2O8 Pyroxene Mg Fe Ca SiO3 Olivine Mg Fe 2SiO4 Garnet Mg3Al2Si3O12 or Spinel MgAl2O4 Igneous Rock Mineralogy Diorite Intermediate ultramafic 40 wt peridotite mafic 48 55 gabbro intermediate 55 65 diorite silicic 65 75 wt granite Geologic Settings Compositions Where Ultramafic Mafic Intermediate Silicic felsic Mantle Oceanic Subduction Zones Continental Mass 2 Geologic Settings Mantle Mid ocean ridges Oceanic Islands Island arcs Continental There appear to be 13 major plates that cover the globe Igneous Fractionation Peridotite basalt granite Partial Melting Rocks have a melting interval of several hundred degrees C First melt is rich in silica water and trace elements Fractional Crystallization First formed crystals low in silica settle to the bottom 3 Igneous Fractionation Layered Mafic Intrusion Intrusive forms Dikes Tabular discordant Pluton Any large discordant intrusion at depth Batholith large 100 km2 Stock small 100 km2 Dike Tabular non conformable Sill Tabular conformable Dikes Tabular discordant Sills Tabular conformable 4 Small Pluton Stock Intrusive forms Pluton Any large discordant intrusion at depth Batholith large 100 km2 Stock small 100 km2 Dike Tabular non conformable Sill Tabular conformable Terms Polymerization Magma Pegmatitie Igneous Fractionation Partial melting Fractional Crystallization Ultramafic Mafic Intermediate Silicic Peridotite Gabbro Diorite Granite Pluton Stock Batholith Dike Sill 5
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