Unmineralized limestones

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 x80, air

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, x80, air

Pyrite and TiO₂ mineral, Lancashire, Britain

An argillaceous limestone in which pyrite is poorly crystalline and porous (light yellow, centre), or well crystalline and euhedral (centre top) or replaces fossil fragments (left). Poorly crystalline TiO₂ (blue-grey, centre) is probably anatase. Calcite (dark grey, well polished) and clay minerals (darker grey, poorly polished) are the main matrix phases. The sulphides and TiO₂ mineral are associated with coarse-grained calcite, rather than the fine-grained matrix carbonate.

Polished block, plane polarized light, x80, air

Pyrite, Lancashire, Britain

An argillaceous limestone in which pyrite has extensively replaced the original carbonate of fossil fragments but has preserved internal morphological features (centre top). Other pyrites form euhedral cubes often with porous cores and well crystalline margins (top left). The differences in pyrite reflectance are due to the presence of very fine carbonate inclusions and the degree of crystallinity of the sulphide. The calcite matrix shows bireflectance from light to dark grey.

Polished block. plane polarized light, x40, air

Pyrite, galena, sphalerite, and marcasite. Freihung Mine, Oberpfalz, FGR

A fragment of fossil wood has been replaced by sulphides. Pyrite (light yellow, bottom) and minor marcasite (light green-yellow, higher reflectance than pyrite, bottom right) have replaced the cell walls and cells of the wood. Within the cells, marcasite is surrounded by pyrite (centre bottom). Surrounding the iron sulphides, the wood has been replaced by galena (light blue-white) and sphalerite (light grey, top left), with sphalerite enclosed within galena. Dark grey areas are gangue.

Polished block, plane polarized light. x80, air