diatomite (di-at'-o-mite). A light-colored soft friable siliceous sedimentary rock, consisting chiefly of opaline frustules of the diatom, typically white, but also pale yellow, gray, or tan. Its low density enables it to float on water. Some deposits are of lake origin but the largest are marine. Owing to its high surface area, high absorptive capacity, and relative chemical stability, diatomite has a number of uses, esp. as a filter aid and as an extender in paint, rubber, and plastics. The term is generally reserved for deposits of actual or potential commercial value. Syn: diatomaceous earth; kieselguhr. Obsolete syn: tripoli-powder. See also: tripoli; tripolite.