The splendent luster and dark sheets of this mineral from the mica group distinguish it from the others in the group. It is one of the rock-forming minerals and is widely distributed in igneous rocks. Vermiculite, which forms as an alteration of biotite, has water molecules interlayered in the sheets. On heating, it loses water and expands into wormlike forms. The mineral was named in honor of the French physicist, J.B. Biot.