slate

The Columbia Encyclopedia, 6th ed.

slate

slate, fine-grained rock formed when sedimentary rocks such as shale are metamorphosed by great pressure. Slate splits into perfectly cleaved, broad thin layers; this characteristically regular and planar cleavage is called slaty cleavage. In the formation of slate, pressure causes the flaky minerals within the sedimentary rock, such as mica, clay, and chlorite, to be reoriented; the flat faces of the minerals lie at right angles to the source of the pressure, and the planes of easy cleavage are also at right angles to the source of the pressure. The rock is not necessarily compressed in the same direction as the sedimentary layers were originally laid down, and because the compression crumples and deforms the original sedimentary layers, the planes of slaty cleavage usually cut through the old bedding planes. Slate is intermediate in hardness between mica schists and shale; the better grades are used for roofing. Its characteristic color is gray-blue. Slate is mined in Maine, Vermont, Pennsylvania, Georgia, Lake Superior, and the Rocky Mts.

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