The Probert Encyclopaedia of Rocks & Minerals

ALUMINA

Alumina (Al2O3) is the single oxide of the metal aluminium. As found native it is called corundum, when crystallized ruby or sapphire, when amorphous emery. It is next to the diamond in hardness. In combination with silica it is one of the most widely distributed of substances, as it enters in large quantity into the composition of granite, traps, slates, schists, clays, loams, and other rocks. The porcelain clays and kaolins contain about half their weight of this earth, to which they owe their most valuable properties. It forms compounds with certain colouring matters, which causes it to be employed in the preparation of the colours called lakes in dyeing and calico-printing. It combines with the acids and forms numerous salts, the most important of which are the sulphate (Alum) and acetate, the latter of extensive use as a mordant.
Research Alumina