Abstract: Detailed observation has been made on the surface structures of the basal plane of hematite crystals from Ponta da Serreta, Terceira (Azores Islands – Portugal), by means of a phase-contrast microscope and multiple-beam interferometry. The results of these observations and measurements are described in terms of growth patterns and natural etch patterns. Mechanism of grouth and etching of these crystals is discussed and explained. It has been found that the grouth of hematite mainly takes place by the spreading and piling up of two-dimensional layers, and that surface imperfections formed by internal and external stresses and screw dislocations play an important role in growth. It is also shown that the etching process is of the same type as growth process but in reverse.From the observations of the surface structures, it is suggested that the hematite crystals from the Azores Islands are formed under fairly low supersaturation conditions, and that they crystallized in lava of submarine volcanoes.

Detailed observations are also made on the characteristics of twin boundaries of the crystals, and a new interpretation of the mechanism of contact twin formation is proposed. Mineralogical description of the crystals is briefly given.