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ROCK CLASSIFICATION <br />(Based on ASTM C-294) <br />Metamorphic Rocks <br />Metamorphic rocks form from igneous, sedimentary, or pre-existing metamorphic rocks in response to <br />changes in chemical and physical conditions occurring within the earth's crust after formation of the <br />original rock. The changes may be textural, structural, or mineralogic and may be accompanied by <br />changes in chemical composition. The rocks are dense and may be massive but are more frequently <br />foliated (laminated or layered) and tend to break into platy particles. The mineral composition is very <br />variable depending in part on the degree of metamorphism and in part on the composition of the original <br />rock. <br />Marble A recrystallized medium- to coarse-grained carbonate rock composed of calcite or <br />dolomite, or calcite and dolomite. The original impurities are present in the form <br />of new minerals, such as micas, amphiboles, pyroxenes, and graphite. <br />Metaquartzite A granular rock consisting essentially of recrystallized quartz. Its strength and <br />resistance to weathering derive from the interlocking of the quartz grains. <br />Slate Afine-grained metamorphic rock that is distinctly laminated and tends to split into <br />thin parallel layers. The mineral composition usually cannot be determined with <br />the unaided eye. <br />Schist A highly layered rock tending to split into nearly parallel planes (schistose) in <br />which the grain is coarse enough to permit identification of the principal minerals. <br />Schists are subdivided into varieties on the basis of the most prominent mineral <br />present in addition to quartz or to quartz and feldspars; for instance, mica schist. <br />Greenschist is a green schistose rock whose color is due to abundance of one or <br />more of the green minerals, chlorite or amphibole, and is commonly derived from <br />altered volcanic rock. <br />Gneiss One of the most common metamorphic rocks, usually formed from igneous or <br />sedimentary rocks by a higher degree of metamorphism than the Schists. It is <br />characterized by a layered or foliated structure resulting from approximately <br />parallel lenses and bands of platy minerals, usually micas or prisms, usually <br />amphiboles, and of granular minerals, usually quartz and feldspars. All <br />intermediate varieties between gneiss and schist and between gneiss and granite <br />are often found in the same areas in which well-defined gneisses occur. <br />ire <br />