Siderite, pyrite and marcasite. Great Oakley, Britain
A vugh within a siderite nodule is rimmed by a fine-grained intergrowth of pyrite and marcasite which cannot be distinguished from each other at this magnification. Bladed marcasite (centre bottom) grows from this rim into the vugh. The matrix is a mosaic of siderite crystals which show faint bireflectance from grey-blue to grey-brown (top left).
Polished block, plane polarized light, x 80, air