In an attempt to explain the anomalous optical properties of certain cubic, tetragonal, and hexagonal (trigonal) mineral species, variations in 2V, composition, and space-group symmetry have been determined for eight different minerals including garnet (And-Gross), rutile, jeremejevite [Al6B5O15(F,OH)3], apatite, kleinite [Hg2N(Cl,SO4xH2O], beryl, quartz, and elbaite. The space-group symmetries of the specimens, determined by single-crystal X-ray techniques, showed no deviations from previously-observed crystallographic symmetry, yet significant deviations from isometric and uniaxial optical character were observed. All specimens except rutile were found to be compositionally zoned, but no systematic correlation was found between elemental zoning and 2V measured along the same traverses. The anomalous optical properties are thought to be due to strain induced by chemical substitutions and/or defects occurring during crystal growth, by rapid temperature or pressure quenches, or by mechanical deformation.

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