An off-white, grayish, brown, yellow, or reddish brown rock composed of amorphous or microcrystalline aluminum oxides and oxyhydroxides, mainly gibbsite Al(OH)₃, bayerite Al(OH)₃, boehmite AlO(OH) , and diaspore AlO(OH) admixed with free silica, 266 silt, iron hydroxides, and esp. clay minerals; a highly aluminous "laterite." It is massive, pisolitic, earthy; occurs as weathered surface deposits after prolonged leaching of silica from aluminous rocks under tropical to subtropical weathering, also transported deposits. Bauxite is the chief ore of aluminum.