Siderite, limonite and pyrite. Great Oakley, Britain
A section through the core (right) to rim (left) of a clay ironstone nodule. The core of the nodule comprises fine-grained siderite crystals (light grey, right) which show faint bireflectance. A thin zone of coarse-grained carbonate (light grey, centre) shows bireflectance and lies between siderite and limonite (blue-grey, higher reflectance, centre and left), which has replaced the siderite extensively. The outer surface of the nodule (top left) is very weathered and all carbonate has been removed. A small pyrite framboid (yellow-white, centre left) lies within the limonite.
Polished block, plane polarized light, x 80, air