F13 quiz 1 condensed notes

Lecture 7 isotopic ages james hutton the scottish

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Unformatted text preview: osed of calcite is called limestone. Some calcite precipitates by inorganic means, but most calcite is secreted by organisms as shell or skeletal material (oysters, clams, certain species of algae, etc.) Chert (also called flint) is a biochemical sedimentary rock made of microcrystalline that forms in a similar manner to limestone, but from the remains or secretions of organism with silica ­rich hard parts. Bioclastic sediments are composed of broken fragments of biologically precipitated particles. LECTURE 6: METAMORPHIC ROCKS “Metamorphism” comes from the Greek meaning “to transform.” Metamorphism typically occurs at high temperature, but not hot enough for the rock to melt; metamorphism is a solid ­ state process. Any original parent rock (protolith)—an igneous, sedimentary, or even a metamorphic rock—can be metamorphosed. Metamorphism generally occurs in a closed chemical system, and metamorphic minerals are created from the starting ingredients of the parent rock. For example, a sedimentary protolith that contains quartz and kaolinite includes the components silicon, oxygen, aluminum, and water. During metamorphism, those materials can reform into crystals of the minerals andalusite, kyanite, or sillimanite. Each of these three aluminosilicate polymorphs (minerals with the same chemical composition, but different crystal frameworks) is a metamorphic index mineral that is stable in a restricted range of metamorphic pressure and temperature. Important factors for metamorphism include: (i) elevated temperature (T), (ii) elevated pressure (P), (iii) shear stress associated with deformation of rocks, and (iv) just a trace of H2O to facilitate chemical reactions. Water tends to be driven out of a rock during metamorphism. Because reactions proceed very slowly when H2O is absent, a new set of minerals does not develop when erosion brings a once deep ­seated metamorphic rock back up toward shallow depths. As rocks recrystallize during metamorphism, a new set of minerals develops that is stable under conditions of elevated T and P, typically without much change of overall chemical composition. Changes of mineral textures also occur, including the growth of larger crystals and fusing of sedimentary grains. Metamorphism of quartz sandstone creates quartzite, the toughest, most resistant to weathering and erosion of all common rocks. Metamorphism of limestone or dolomite creates marble. In these examples, the metamorphic minerals are still quartz, calcite, or dolomite, just as in the original rock, but they have a metamorphic texture. Metamorphism of shale, a chemically more complex rock, develops both new minerals and new texture. Low ­ grade metamorphism creates slate, a fine ­grained rock with mic...
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This document was uploaded on 03/03/2014 for the course GEO 303K at University of Texas.

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