Unmineralized sandstones

Haematite, TiO₂ minerals and zircon. Bridgnorth, Britain

A rounded zircon (light grey, centre top) is zoned. Haematite (blue-white) occurs as very poorly polished fine-grained intergrowths cementing silicate grains (low reflectance, right top and bottom); as rounded martite grains pseudomorphing magnetite (centre right) and as single coarse-grained crystals (top centre, bottom left). TiO₂ minerals took similar to haematite, but their reflectances are slightly lower and they have pink-grey surface colours. The coarse oxide cementing quartz grains (bottom right) is haematite, whereas that cementing quartz (top right) is TiO₂. An equant grain of primary iron-titanium oxide (centre left) has been pseudomorphed by a fine-grained intergrowth of haematite and TiO₂ and is enclosed within a single authigenic crystal of haematite.

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).

Haematite and TiO₂ minerals. St Bees Sandstone. Cumbria, Britain

This is the same area as previous section but in partially crossed polars. Coarse-grained haematite shows anisotropy (blue-white to white, bottom centre, top left). The characteristic orientation of haematite as laths along (1 I 1) of the original magnetite, so forming martite, can be seen (bottom centre). The red internal reflections of haematite are enhanced (right centre) and light-coloured internal reflections of TiO₂ (white to yellow) also are present (top right). The multiple nature of the detrital TiO₂ grain and its overgrowths is clear (top right). The light-coloured internal reflections of TiO₂ minerals allow them to be distinguished easily from haematite in crossed polars.

Haematite and TiO₂ minerals. St Bees Sandstone. Cumbria, Britain

Very fine-grained haematite lies along the fabric of phyllosilicate grains (left). Both the green-white colour and incipient red internal reflections are characteristic of this type of fine-grained haematite. TiO₂ (pink-brown) forms euhedral crystals with faint light-coloured internal reflections (bottom right), but is present as a rounded detrital grain that forms the light brown core (centre) to euhedral lanceolate haematite crystals (white, centre). The original iron-titanium oxide grain is now pseudomorphed by a fine-grained intergrowth of very minor haematite (white) and TiO₂ (pink-white). Other white areas are haematite.

Grain mount, plane polarized light, x 180, oil

Haematite, TiO₂ minerals and zircon. St Bees Sandstone. Cumbria, Britain

Haematite as martite pseudomorphs after magnetite (two crystals, centre right) and as a rounded lath-shaped pseudomorph (bottom centre) is white and shows faint bireflectance (blue-white to white, centre right). Fine-grained haematite (centre left) is green-blue and is accompanied by incipient red internal reflections. TiO₂ (pink-grey, bottom centre) forms an authigenic overgrowth around the detrital haematite pseudomorph, and a discrete triangular detrital grain (top right). Two zoned zircon crystals (centre and top right) are grey and have low reflectance, and show faint red internal reflections due to very fine haematite inclusions. The central zircon has a higher reflectance core and lower reflectance rim.

Grain mount, plane polarized light. x 180, oil

Ilmenohaematite and haematite. Arran, Britain

Two detrital grains of ilmenohaematite (centre, right centre) comprise exsolution bodies of ilmenite (brown) in a haematite host (white). The right-hand ilmenohaematite has an ilmenite-free haematite overgrowth (left margin). Very poorly crystalline haematite (green-white, top centre) is poorly polished and is a pseudomorph after an original ferromagnesium mineral, whereas a single crystal of haematite (top left) is well polished.

Grain mount, plane polarized light, x 180, oil