Pyrrhotite, pyrite and galena. Nigadoo River, New Brunswick, Canada

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A small galena crystal (blue-white, centre) lies at the junction of a number of pyrrhotite (brown, centre) grains, which show faint differences in reflectance and surface colour due to the presence of monoclinic (darker brown) and hexagonal (lighter brown) pyrrhotite (centre). Euhedral pyrite crystals (pale yellow, high reflectance, right) carry pyrrhotite inclusions (top right). Pyrrhotite is extensively altered to fine-grained pyrite and marcasite which are not distinguishable from each other at this magnification. The alteration is to lath-shaped pyrite, marcasite aggregates (bottom centre) oriented along the (0001) plane of the pyrrhotite and to areas of very fine-grained pyrite-marcasite (top left); these appear brown due to the fine grain size of the sulphides. Probably all the pyrite in this section was originally pyrrhotite. The gangue minerals are dark grey.

Polished block, plane polarized light, x 80, air

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