Abstract
Some 250 pyrite samples from 50 localities were examined by ore microscopic methods. Two final polishing procedures were applied during sample preparation: (a) 0.25 μm or 0.1 μm diamond pastes on “Microcloth”, (b) an alkaline silica solution on “Microcloth”. With the exception of (111) sections, procedure (a) always resulted in the observation of optical anisotropy, while procedure (b) led to isotropic behaviour. Electron channeling patterns showed a strongly damaged surface for samples prepared by method (a), whereas for samples prepared by method (b) and for untreated pyrite faces an undisturbed lattice was observed. This strongly indicates that pyrite is optically isotropic, and that the frequently observed anisotropy is caused by surface deformation due to mechanical polishing procedures. Studies of the isotypic mineral sperrylite (PtAs2), as well as similar investigations on cuprite and minerals of the spinel group, confirm the correlation between surface deformation and optical anisotropy.
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