Slate is mainly composed of the minerals quartz, illite, and chlorite, which account for up to 95% of its composition. Organic carbon in the rock is converted to graphite. Slaty cleavage is fully developed as the clay minerals begin to be converted to chlorite and mica. Grains of platy minerals, such as clay minerals, are rotated to form parallel layers perpendicular to the direction of compaction, which begin to impart cleavage to the rock. The process of conversion of mudrock to slate involves a loss of up to 50% of the volume of the mudrock as it is compacted. These are typically less than 100 μm (micron) thick. Under a microscope, the slate is found to consist of very thin lenses of quartz and feldspar (QF-domains) separated by layers of mica (M-domains). The texture of the slate is totally dominated by these pervasive cleavage planes. Slaty cleavage is continuous, meaning that the individual cleavage planes are too closely spaced to be discernible in hand samples. The direction of cleavage is independent of any sedimentary structures in the original mudrock, reflecting instead the direction of regional compression. This is in contrast to the silky cleaved surfaces of phyllite, which is the next-higher grade of metamorphic rock derived from mudstone. This mild degree of metamorphism produces a rock in which the individual mineral crystals remain microscopic in size, producing a characteristic slaty cleavage in which fresh cleavage surfaces appear dull. It is usually formed by low-grade regional metamorphism of mudrock. Slate is a fine-grained, metamorphic rock that shows no obvious compositional layering, but can easily be split into thin slabs and plates. The phrases "clean slate" and " blank slate" come from this usage. It may mean a single roofing tile made of slate, or a writing slate, which was traditionally a small, smooth piece of the rock, often framed in wood, used with chalk as a notepad or notice board, and especially for recording charges in pubs and inns. The word "slate" is also used for certain types of object made from slate rock. Slate is not to be confused with shale, from which it may be formed, or schist. However, slate occurs in a variety of colors even from a single locality for example, slate from North Wales can be found in many shades of grey, from pale to dark, and may also be purple, green, or cyan. Slate is frequently grey in color, especially when seen en masse covering roofs. When expertly "cut" by striking parallel to the foliation, with a specialized tool in the quarry, many slates display a property called fissility, forming smooth, flat sheets of stone, which have long been used for roofing, floor tiles, and other purposes. The foliation in slate, called " slaty cleavage", is caused by strong compression in which fine-grained clay forms flakes to regrow in planes perpendicular to the compression. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. It is the finest-grained foliated metamorphic rock. Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. Shale is laminated, meaning the rock consists of many thin layers that are bound together.A piece of slate (about 6 cm × 4 cm ( 2 + 1⁄ 2 in × 1 + 1⁄ 2 in)) In general, shale is fissile and readily splits into layers parallel to the bedding plane, which is the plane of clay flake deposition. The grain size and composition of minerals in shale determine its permeability, hardness, and plasticity. Shale containing a lot of calcite tends to be pale gray or yellow. Ferrous iron yields black, blue, and green shale. The presence of ferric iron compounds yields red, brown, or purple shale. Shale with a higher organic (carbon) content tends to be darker in color and may be black or gray. The color of shale depends on its composition. There is siliceous shale (silica), calcareous shale (calcite or dolomite), limonitic or hematitic shale (iron minerals), carbonaceous or bituminous shale (carbon compounds), and phospatic shale (phosphate). Shale is classified based on its mineral content. Oil shale or bituminous also contains kerogen, a mixture of hydrocarbons from deceased plants and animals. Shale consists of at least 30 percent clay, with varying amounts of quartz, feldspar, carbonates, iron oxides, and organic matter. The clay comes from decomposition of feldspar. The clay clasts or particles in shale are less than 0.004 millimeters in diameter, so the structure of the rock only becomes visible under magnification.
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