group silica system tetragonal formula SiO2 optic uniaxial pos Too-Vee 0¡ relief.1 moderate pos min. index 1.482 color colorless pleochroism no birefringence.1 0.002 cleavage no elongation yes extinction parallel angle 0¡ alteration quartz, tridymite habit.1 granular, tabular occurence igneous (felsic, volcanic) TwoV relief pleochroism.2 birefringence cleavage.2