ROCKGR~1.DOC - GUIDE TO DISTINGUISHING BETWEEN IGNEOUS...

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GUIDE TO DISTINGUISHING BETWEEN IGNEOUS, SEDIMENTARY AND METAMORPHIC ROCKS Igneous Rocks 1. Only contain minerals in Bowen's Reaction Series (olivine, pyroxene, biotite, amphibole, muscovite, K-feldspar, plagioclase and quartz). Furthermore, minerals at the same level in Bowen's Reaction Series tend to occur together (e.g. olivine tends to occur with pyroxene and plagioclase, and quartz is generally associated with K-feldspar and biotite or amphibole. 2. Maximum amount of quartz in an igneous rock is 40% whereas sedimentary and metamorphic rocks may contain up to 100% quartz (e.g. quartz sandstone and quartzite). 3. Crystalline texture, possibly with phenocrysts, vesicles or pyroclastic fragments; may be glassy. 4. If pyroclastic, the volcanic rock fragments may contain phenocrysts, and mat be angular or flattened (i.e. welded). Sedimentary Rocks 1. Clastic rocks characterized by rounded grains of quartz (up to 100%), feldspar and minor dark mica with clay minerals, hematite and/or carbonaceous matter in matrix. Chemical rocks are typically
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Unformatted text preview: crystalline, aphanitic, monomineralic, and characterized by calcite, dolomite, silica, halite or gypsum. 2. Diagnostic minerals are clays, organic matter, halite and gypsum. 3. Sedimentary rocks (except for the quartz-rich ones) are generally softer than igneous or metamorphic rocks. 4. Possess either clastic or chemical texture, with possible presence of bedding, fossils, ripple marks and rounded rock fragments. 5. Clastic sedimentary rocks commonly have detrital muscovite flakes lying parallel to bedding planes. Metamorphic Rocks 1. Diagnostic minerals are garnet, chlorite, graphite and talc. Members of Bowen's Reaction Series and calcite and dolomite are also common. Mica may be very abundant (up to 100% in a mica schist). 2. Characterized by aphanitic or phaneritic crystalline texture and possible presence of porphyroblasts. 3. Metamorphic foliation (slaty cleavage, phyllitic cleavage, schistosity, gneissosity) is diagnostic....
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