%alumina%

n. [ L. alumen, aluminis. See Alum. ] (Chem.) One of the earths, consisting of two parts of aluminium and three of oxygen, Al2O3. [ 1913 Webster ]

☞ It is the oxide of the metal aluminium, the base of aluminous salts, a constituent of a large part of the earthy siliceous minerals, as the feldspars, micas, scapolites, etc., and the characterizing ingredient of common clay, in which it exists as an impure silicate with water, resulting from the decomposition of other aluminous minerals. In its natural state, it is the mineral corundum. [ 1913 Webster ]