A monoclinic mineral, 8(Al(OH)₃) ; pisolitic; in micalike crystals, or stalactitic and spheroidal forms; a constituent of bauxite associated with boehmite and/or diaspore; formed by weathering of igneous rocks, esp. nepheline syenite; also in veins; a source of aluminum and synthetic abrasives. In emery deposits formed by thermal or regional metamorphism of bauxites, gibbsite occurs as an alteration crust on corundum.