The α-β transformation of quartz

Abstract

As is well known, the ordinary form of quartz which has trigonal symmetry changes over reversibly to another form which has hexagonal symmetry at a temperature of 575° C. Though the transformation does not involve any radical reorganization of the internal architecture of the crystal and takes place at a sharply defined temperature, it is nevertheless preceded over a considerable range of temperature (200° -575° ) by a progressive change in the physical properties of 'low' quartz which prepares the way for a further sudden change, when the transition to 'high' quartz actually takes place. The thermal expansion coefficients, for example, gradually increase over this range of temperature, becoming practically infinite at the transition point and then suddenly dropping to small negative values. Young's moduli in the same temperature range fall to rather low values at the transition point and then rise sharply to high figures. The piezo-electric activity also undergoes notable changes.