any of the illite group of clay minerals, including illite, bramallite (a sodium illite), and glauconite. They are structurally related to the micas; glauconite is also a member of the common-mica group. The hydrous micas predominate in shales and mudstones, but they also occur in other sediments. They are present among the clay minerals in soils; they are essentially nonswelling and impart excellent tilth, but they bind both potassium and aluminum ions in forms that plants cannot feed upon. See also illite.