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Concept
Mudstone
Résumé
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from shale by its lack of fissility (parallel layering).
The term mudstone is also used to describe carbonate rocks (limestone or dolomite) that are composed predominantly of carbonate mud. However, in most contexts, the term refers to siliciclastic mudstone, composed mostly of silicate minerals.
The NASA Curiosity rover has found deposits of mudstone on Mars that contain organic substances such as propane, benzene and toluene.
There is not a single definition of mudstone that has gained general acceptance, though there is wide agreement that mudstones are fine-grained sedimentary rocks, composed mostly of silicate grains with a grain size less than . Individual grains this size are too small to be distinguished without a microscope, which means that most classifications emphasize texture rather than mineral composition, and mudstones have historically received less attention from petrologists than have sandstones. The simplest definition is that a mudstone is a fine-grained clastic sedimentary rock that is not laminated or fissile. Most definitions also include a requirement that the rock contain significant amounts of both silt- and clay-sized grains. One common requirement is that a mudstone is a mudrock (a rock containing more than 50% silt- to clay-sized particles) in which between a third and two-thirds of the mud (silt and clay) fraction is clay particles. Another definition is that mudstone is a sedimentary rock in which neither silt, clay, nor coarser grains is predominant. Rock of this composition that does show laminations or fissility is sometimes described as mudshale rather than mudstone.
The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.
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