On the origin of anomalous birefringence in grandite garnets

The origin of anomalous birefringence of grossular-andradite (grandite) gar nets from skarns in Mall and Russia was considered. The crystals had comple x optical patterns which can be induced by superposition of two phenomena: mismatch compositional strain (stress birefringence) and growth ordering of atoms (growth dissymmetrization). Study of the crystals using several expe rimental techniques (optical microscopy, microprobe analysis, X-ray diffrac tion topography and X-ray single crystal diffraction) as well as calculatio ns of anomalous birefringence has confirmed this hypothesis. Depending on t he crystal composition and growth conditions, the relative magnitude of eac h phenomenon controls the various optical effects. As a result one can see two groups of crystals which are found to have fundamentally different anom alous optical properties: crystals with low (<0.001) and high (0.001-0.015) values of birefringence. The spatial distribution of birefringence within each group is different and this fact is related to different mechanisms ca using optical anomalies: stress birefringence and growth dissymmetrization for these two groups, respectively.