Argillaceous rocks

Argillaceous rocks: A group of detrital sedimentary rocks, commonly clays, shales, mudstones, siltstones, and marls. Two grades of particle size are recognised, silt grade, in which the particles range in size from 1/16 to 1/256 mm, and clay grade, with particles of less than 1/256 mm. In addition to the clay minerals, argillaceous rocks may contain colloidal material, very finely divided quartz, carbonate dust, finely divided carbon and iron pyrite. Argillaceous rocks are almost always laid down in water and their mineralogy is to some extent controlled by their environment of deposition.