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Earth Sciences
Earth Sciences 1022A/B

Earth Rocks Notes MINERALS Silicates- most abundant minerals, made of the silicon-oxygen tetrahedron Two Types: Ferromagnesian (DARK) silicates- contain Fe and Mg cations and include: Olivine- covalently-bonded silica tetrahedral surrounded by Fe, Mg cations. Fe, Mg substitute for each other to form a group of minerals. Pyroxene- single chains of tetrahedral ionically bonded to metallic cations, nearly right angle cleavage. Amphibole- double chains of tetrahedral ionically bonded to metallic cations, 120 degree cleavage. Biotite- tetrahedral sheets ionically sandwich K, perfect platy cleavage. Nonferromagnesiam (LIGHT) silicates- Muscovite- tetrahedral sheets with perfect platy cleavage Feldspar- strongly bonded 3-D network of silica tetrahedral, K variety is called orthoclase and Ca-Na group is called plagioclase Quartz- made entirely of silica tetrahedral, covalently bonded, hard, has no cleavage Carbonates- metals bonded to CO g3oup to form minerals like calcite (used for lime, cement) and dolomite Halite and Gypsum- (used to make plaster and drywall) are formed by evaporation of ancient shallow seas and salt ponds Ores of metals- hematite, shpalerite, galena, gold & silver IGNEOUS ROCKS - Crystallize from molten silicate material called magma Extrusive- Magma that reaches the surface by volcanoes Intrusive- Magma that cools below the surface How Magmas Evolve- discontinuous (different molecular structures of olivine, pyroxene, amphibole, biotite) and continuous (same plagioclase structure) - repeated crystal settling in a magma chamber results in the melt crystallizing through a series of minerals according to their decreasing melting temperatures—remaining crystals react with the melt that is enriched in lighter elements (magmatic differentiation); dark crystals form early, settle and take heavy elements with them. Light crystals then form the remaining light elements, eventually to quartz; thus granite can ultimately evolve from an original magma having basaltic composition. - Magma composition can also change through assimilation (pieces of host rock fall into the magma and melt) and magma mixing where one body of magma invades another, producing a composition intermediate between the two Rock Composition: Mafic- dark, less silica Felsic- light, more silica Texture: Aphanitic- fine, formed by rapid cooling at or near the ground surface Phaneritic- coarse, formed by the slow cooling below the surface Porphyritic- larger crystals surrounded by smaller ones Glassy- is lava was quenched so fast that crystals had no time to form Pyroclastic- (fragmental) when magma was ejected violently into the air then fell as particles onto the ground Naming Igneous Rocks—use rock textures and mineral compositions (dark vs. light) Felsic (granitic) rocks (70% silica) – granite and rhyolite are common in mountainous areas; granite is widely used for building stone, monuments and headstones Intermediate (andesitic) (60%) – andesite and diorite found near subduction zones Mafic (basaltic) rocks (50%) – basalt is the most common rock, found mainly in upper oceanic crust and volcanic islands; gabbro found mainly in lower oceanic crust Ultramafic (45%) – peridotite is probably the main rock in the upper mantle Intrusive Igneous Bodies—most magma crystallizes at d
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