Kinds Of metamorphic ROCKS

The names of metamorphic rocks are derived from their original rock type, their texture, and mineral assemblages. shales and mudstones have an initial mineral assemblage of quartz, clays, calcite, and feldspar. slate is the low grade metamorphic equivalent of shale and, with recrystallization, is made of quartz and micas. At intermediate grades of metamorphism, the mica grains grow larger so that individual grains become visible to the naked eye and the rock is called a phyllite. At high grades of metamorphism, the rock (shale, for example) now becomes a schist, which is coarse-grained, and the foliation becomes a bit irregular. still higher grades of metamorphism separate the quartz and the mica into different layers; this rock is called a gneiss. For both schists and gneisses, a prefix is commonly added to the names to denote some of the minerals present in the rock. For instance, if garnet grows in a biotite schist, it could be named a garnet-biotite schist.

Fresh basalts contain olivine, pyroxene, and pla-gioclase, none of which contains abundant water. When metamorphosed, however, water typically

Marble (white) and schist (brown) on Charley River in Alaska (USGS)

enters the rock from outside the system. At low grades of metamorphism, the basalt is turned into a greenstone or greenschist, which has a distinctive color because of its mineral assemblage of chlorite (green) + albite (clear) + epidote (green) + calcite (clear).

At higher metamorphic grades, the greenschist mineral assemblage is replaced by one stable at higher temperature and pressure, typically plagioclase and amphibole, and the rock is known as amphibolite. Amphiboles have a chain structure which gives them an elongated shape. When they crystallize in a different stress field like that found in a metamorphic rock, the new minerals tend to align themselves so that their long axes are parallel to the least com-pressive stress, forming a lineation. At even higher metamorphic grades the amphiboles are replaced by pyroxenes and the rock is called a granulite.

Limestone metamorphoses into marble, which consists of a network of coarsely crystalline interlocking calcite grains. Most primary features, such as bedding, are destroyed during metamorphism and a new sugary texture appears.

When sandstone is metamorphosed, the silica remobilizes and fills in the pore spaces between the grains, making a very hard rock called a quartz-ite. Primary sedimentary structures may still be seen through the new mineral grains.

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