Unmineralized intermediate rocks

Magnetite and haematite. France

An andesite contains subhedral to euhedral magnetite crystals (light brown, centre) that are partially oxidized to haematite (blue-white, centre left). The alteration proceeds along crystal edges and along curved fractures. A large pyroxene crystal (bottom right) has higher reflectance than the fine-grained, poorly polished, groundmass silicates.

Polished block, plane polarized light, x40, air

Magnetite, ilmenite, TiO₂ minerals and haematite. Guernsey, Channel Islands, Britain

A diorite contains magnetite (light brown-grey, centre right) that has oxidation-exsolution lamellae of ilmenite (light pink-brown, centre) parallel to (111) of the magnetite. The lamellae have altered to a fine-grained intergrowth of TiO₂ minerals and haematite (blue-white to light grey, centre left). Amphibole (bottom left) shows cleavage and biotite (top right) has light brown internal reflections.

Polished thin section, plane polarized light, x 80, air

Ilmenite. Glen Gairn, Scotland

A diorite has lobate crystals of ilmenite that show reflection pleochroism from brown (centre) to grey-brown (left centre). They occur within coarse-grained biotite (black).

Polished thin section, plane polarized light, x 160. oil

Sphene, magnetite and ilmenite. Glen Gairn, Scotland

A diorite has euhedral rhombs of sphene (light grey with brown internal reflections, centre left) showing faint zoning (top left) and associated with oxidized subhedral magnetite (light brown, centre). Anhedral ilmenite (brown, bottom left) occurs within biotite. One crystal of ilmenite has a sphene rim (blue-grey, centre bottom) which has a lower reflectance than euhedral sphene. The silicate matrix is biotite (bottom left) and feldspar (faint internal reflections, bottom right).

Polished thin section, plane polarized light, x 160, oil

Ilmenite, sphene, haematite and epidote. Glen Gairn, Scotland

A diorite contains haemoilmenite (centre) that comprises an ilmenite host (brown) with very fine haematite (white) exsolution discs along (0001) of the host ilmenite. Later haematite (white, centre right) has overgrown the haemoilmenite but lies within sphene (light grey, centre), which rims the primary iron-titanium oxides. A subhedral crystal of pyrite (pale yellow-white, bottom left) is associated with epidote (medium grey, bottom left), which has a slightly lower reflectance than sphene, although the difference is difficult to see. Elsewhere, fine-grained epidote lies along the cleavage of the chloritized biotite host (top right). Epidote is more easily identified in transmitted light.

Polished thin section, plane polarized light. x 160, oil

Ilmenite, sphene, pyrite, limonite and haematite. Glen Gairn, Scotland

A diorite contains haemoilmenite that has coarse-grained haematite (white) exsolution discs in an ilmenite (brown, top centre) host. Sphene (light grey) rims the iron-titanium oxides. Pyrite (yellow-white, high reflectance, centre) has oxidized to limonite (blue-grey, higher reflectance than sphene, centre). Biotite (dark green-black) is intergrown with quartz (bottom left) that carries fluid inclusions (bottom left).

Polished thin section, plane polarized light, x 160. oil