”︁Four-fold„ refringence in quartz

An attempt is made to interprete the simultaneous existence of four refractive indexes (two ordinary and two extraordinary) near the α—β phase transition in quartz from the optical point of view. The theory of thermodynamical stability predicts the appearance of fluctuation areas of the α- and β-phases in the vicinity of the α—β transition. On the basis of this theory the formation of infinite clusters, consisting of the merged fluctuation ranges, is assumed.