Haematite and TiO2 minerals. St Bees Sandstone. Cumbria, Britain
Haematite occurs as very fine crystals (right) that lie along the basal cleavage of phyllosilicates; as martite (blue-white, bottom centre); as a medium grained mosaic (top left) and as authigenic lanceolate euhedral crystals (top left) overgrowing detrital haematite. Haematite shows faint bireflectance and reflection pleochroism from blue-white to white (top left). A detrital TiO2 mineral (light grey-brown, top right) is porous and is enclosed within two well crystalline TiO2 overgrowths. The junction between the two rims is seen as a thin line of trapped silicates (dark).
Grain mount, partially crossed polars, x 160, oil