Pyrrhotite and marcasite. Nigadoo River, Canada
Pyrrhotite (brown, bottom left) has been extensively altered to a fine intergrowth of marcasite. The alteration has followed the basal (0001) planes of pyrrhotite. The intergrowth comprise a very fine mosaic of marcasite (lower reflectance areas, faint reflection pleochroism, top) which are 'cemented' by coarse-grained marcasite (better polished, yellow-white, centre left), which also forms a rim around the aggregate. Faint reflection pleochroism is visible within the coarse-grained marcasite (centre left).
Polished block, plane polarized light, x 160, oil