CRYSTAL-OPTICAL MODEL OF ANOMALOUS BIAXIALITY IN UNIAXIAL CRYSTALS (A CASE STUDY OF ICELAND SPAR)
By example of Iceland spar, it is shows that, as one of reasons for the anomalous birefringence (biaxiality) of crystals of hexagonal, trigonal, and tetragonal symmetry (together with mechanical deformations of the structure), one can consider phenomenon of the light polarization due to its reflection and refraction at the boundaries between growth microzones with different refractive indices. In studied crystals from the Razlom deposit (Siberian Iceland spar province), the anomalous birefringence has been detected in growth pyramids of the hexagonal prism {1120} faces in areas where microzone of the crystal growth has width 0.01-0.1 mm, and the difference between microzones in value of the refractive indices Ng and Np (n<sub>o</sub>, n<sub>e</sub>), due to changes in content of the Mn isomorphic admixture, is about ± 1 · 10 <sup>4</sup>. Anomalous biaxiality is revealed by the conoscopic figures observed visually with translucence of plates cut perpendicular to [0001] of the crystal. In this case, the shape of conoscopic figure changes in the same way as in biaxial crystals: while rotation of the crystal plate put between polarizer and analyzer, «cross» of the figure becomes replaced by two differently directed arcs (isogyres). Value of the visible 2 V angle in these crystals is usually 1-2°, rarely 5°. Plane of the anomalously arising optical axes is parallel to growth microzones in gtrowth pyramids of all faces of the hexagonal prism. Theoretically, microzones of growth are considered as a sort of planar light guides polarizing apart of the light flux passing through the crystal and thereby distorting the conoscopic figures of uniaxial crystals. It has been experimentally proved that the signs of anomalous biaxiality become most clear in cases where the thickness of plate is sufficient for polarization and change of trajectory of significant part of the light flux as a result of multiple reflections at the boundaries of microzones. According to the developed crystal-optical model, it is more correct to call this phenomenon not «anomalous biaxiality» but«pseudo-biaxiality», referring to the illusory appearance of two optical axes in uniaxial crystals. Microzonal crystals of calcite, corundum, beryl, tourmaline, as well as of diamond and other minerals and synthetic compounds can be considered as optical systems - planar single-crystal light guides.