Pyrite and carbonaceous matter. Lancashire, Britain
An argillaceous limestone in which pyrite (light yellow) has replaced fossil fragments (bottom left) or forms euhedral crystals (top centre). Carbonaccous matter (brown, centre top) has carbonate in its core but also shows variation in its surface colour and reflectance, from lower reflectance poorly polished inner areas to higher reflectance rims. Calcite is the main matrix mineral and shows distinct bireflectance differences between crystals.
Polished block, plane polarized light, x 80, air