1. GeologyMudstone, 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. 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. == Definition == 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 0.063 millimetres (0.0025 in). 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... from wikipedia.org