Vermiculite

Vermiculite, foliated clay mineral formed as a change in biotite. Vermiculite is soft, yellow or brown in color, with a monoclinic crystal structure. When heated, it can expand to a lightweight, highly absorbent, fireproof material about 16 times its original volume. Deposits occur in the United States, Australia, the USSR, South Africa, and Brazil. Vermiculite is used in construction as a sound and thermal insulator, as a fireproofing element, and as a potting medium for plants.