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ROCK CLASSIFICATION <br />(Based on ASTM C-294) <br />Igneous Rocks <br />Igneous rocks are formed by cooling from a molten rock mass (magma). Igneous rocks are divided into <br />two classes (1) plutonic, or intrusive, that have cooled slowly within the earth; and (2) volcanic, or <br />extrusive, that formed from quickly cooled lavas. Plutonic rocks have grain sizes greater than <br />approximately 1 mm, and are classified as coarse- or medium-grained. Volcanic rocks have grain sizes <br />less than approximately 1 mm, and are classified as fine-grained. Volcanic rocks frequently contain <br />glass. Both plutonic and volcanic rocks may consist of porphyries that are characterized by the <br />presence of large mineral grains in afine-grained or glassy groundmass. This is the result of sharp <br />changes in rate of cooling or other physico-chemical conditions during solidification of the melt. <br />Granite Granite is a medium- to coarse-grained light-colored rock characterized by the <br />presence of potassium feldspar with lesser amounts of plagioclase feldspars and <br />quartz. The characteristic potassium feldspars are othoclase or microcline, or <br />both; the common plagioclase feldspars are albite and oligoclase. Feldspars are <br />more abundant than quartz. Dark-colored mica (biotite) is usually present, and <br />light-colored mica (muscovite) is frequently present. Other dark-colored <br />ferromagnesian minerals, especially honblende, may be present in amounts less <br />than those of the light-colored constituents. <br />Quartz-Monzonite Rocks similar to granite but contain more plagioclase feldspar than potassium <br />and Grano-Diorite feldspar. <br />Basalt Fine-grained extrusive equivalent of gabbro and diabase. When basalt contains <br />natural glass,- the glass is generally lower in silica content than that of the lighter- <br />colored extrusive rocks. <br />ire <br />