Pyrrhotite, marcasite and TiO2 mineral. Wales, Britain
A low grade 'greenstone' in which pyrrhotite (brown, centre) has partially altered to marcasite (white, centre left), showing its characteristic lath-shaped habit of small crystals. Minor amounts of a TiO2 mineral (light grey, bottom left) are present. The main matrix minerals are coarse-grained carbonate (grey, showing bireflectance, top right) and chlorite (less well polished). Black areas are polishing pits.
Polished block, plane polarized light, x 80, air