The role of the incommensurate phase in the opalescence of quartz

Forty five years ago, an intense light scattering was observed at the α-β transition of quartz but the origin of this opalescence has remained mysterious for a long time. Recently Saint-Gregoire et al and Aslanyan et al have explained the origin of the opalescence by introducing new incommensurate (`inc') phases with ferroelastic properties in the transition region. In this paper we recall the main features of the α-β transition, of the inc phase and of the opalescence of quartz, which presents different properties in two regions of the α-inc phase boundary. We also describe the three typical structures observed in the phase boundary regions by electron microscopy. We present briefly the two previous ferroelastic models and we propose our own explanation for the origin of the opalescence. We discuss the relations of these three models with experimental results concerning thermal behaviour, microscopic structures and the origin of the refractive index variations. Most experimental results are in agreement, at least qualitatively, with our model where the two opalescence regions correspond respectively to the presence of inc rotation patches and of irregular Dauphine microtwins, both in a non-equilibrium state.