Pyrite, Lancashire, Britain
An argillaceous limestone in which pyrite has extensively replaced the original carbonate of fossil fragments but has preserved internal morphological features (centre top). Other pyrites form euhedral cubes often with porous cores and well crystalline margins (top left). The differences in pyrite reflectance are due to the presence of very fine carbonate inclusions and the degree of crystallinity of the sulphide. The calcite matrix shows bireflectance from light to dark grey.
Polished block, plane polarized light, x 40, air