On the origin of inhomogeneity of anomalous birefringence in mixed crystals: an example of alums

Optical anomalies (deviations of the symmetry of optical properties from th e ideal symmetry of the crystal) occur in many minerals and synthetic compo unds and have been under investigation since the last century. An important feature of optically anomalous mixed crystals is a high degree of optical inhomogeneity, whereas the optical patterns of mixed crystals without anoma lies are usually rather uniform. This work is devoted to the study of this phenomenon. As a model object we have chosen mixed alum crystals, which wer e known for their anomalous birefringence and which revealed the following types of optical inhomogeneities: (1) sector zoning; (2) concentric zoning; (3) subsector zoning; (4) stripes normal to growth front. The inhomogeneity of anomalous birefringence of mixed crystals of alums can be explained by superposition of several effects: mismatch strain, strain along dislocations and growth ordering of isomorphous components. Optical i nhomogeneities due to the sector zoning of crystals and their dislocation s tructure arise even under stationary growth conditions and stationary micro morphology of the growing face. Both variable growth conditions and the rel ief of the growing face strongly intensify the optical inhomogeneity due to three interrelated factors: (1) a significantly inhomogeneous mismatch str ain; (2) a variable degree of ordering of isomorphous components due to the compositional inhomogeneities; (3) different degrees of ordering of isomor phous atoms caused by different orientations, heights and velocities of gro wth steps. These effects lead to the formation of subsector zoning and zoni ng superimposed on the optical sector zoning. These optical structures are crossed by birefringent stripes arising from dislocations.