Pyrite, carbonaceous matter, TiO2 mineral and marcasite. Lancashire, Britain
An argillaceous limestone in which pyrite (light yellow-white, centre) forms poorly crystalline aggregates which have extensively replaced fossil fragments. The cores of the replaced shell fragment (oriented north-south), which are more crystalline and have slightly higher reflectance (centre), comprise marcasite crystals. Carbonaccous material (brown-grey, centre left) has lower reflectance than authigenic TiO2 (blue-grey, centre) crystals. The matrix is calcite showing faint bireflectance.
Polished block, plane polarized light, x 80, air