Surface structures on the basal plane of hematite crystals from many localities have been observed with a phase contrast microscope. Patterns formed by natural etching are distinguished from the growth patterns, and their characteristics are described. The mechanism of etching is explained on the basis of these observations.
Etching principally takes place two-dimensionally. It preferentially starts at the step of growth layers or at the edges of screw dislocations which terminate on the surface. Etch pits are formed evenly all over the surface and the depth of etching is in the order of two or three unit cell heights. These features are explained on the basis of surface imperfections and an unsaturated zone on the finished surface of a crystal. It is also suggested that the depth of an unsaturated zone is that of two or three unit cells. From the difference in degree of etching, growth conditions of different localities are discussed.