A very fine-grained, foliated, non-layered metamorphic rock, generally produced by metamorphism of shale under relatively low pressure and temperature. It occurs in many varieties, including clay, hornblende, mica, talc slate, and others, all of which have the common property of splitting readily into thin plates.

Source: Art and Architect Thesaurus

http://vocab.getty.edu/page/aat/300011646