magnesia, common name for the chemical compound magnesium oxide, MgO. It occurs as colorless, cubic crystals. It is refractory, melting at about 2,800°C. It is very slightly soluble in pure water but is soluble in acids and solutions of ammonium salts. The magnesia of commerce is a fine white powder used in soaps, cosmetics, pharmaceuticals, and as a filler in rubber goods. Magnesia is used to make crucibles and other ceramic goods. Crude magnesia is prepared by roasting dolomite (calcium magnesium double carbonate) or magnesite (magnesium carbonate). Pure magnesia is prepared by refining the crude product. Magnesia is also extracted from seawater. It occurs in nature as the mineral periclase.