Altered iron-titanium oxides in alkali olivine basalt

Haematite, oxidized magnetite and chalcopyrite. Derbyshire, Britain

Click hereSkeletal magnetite crystals (cruciform-type, brown-white, centre left) and ilmenite laths (brown-white, right centre) are intergrown with lath-shaped plagioclase (centre left) and equant pyroxene (both light grey) and poorly polished clay minerals (dark grey), some of which have pseudomorphed olivine. Both magnetite and ilmenite have oxidized with a lightening of their surface colour. Fine-grained haematite (white, poorly polished, centre) has partially pseudomorphed olivine (centre) and also forms veinlets cutting across the silicates. Very fine-grained haematite has a characteristic green-brown colour which is partially due to red internal reflections. A small crystal of chalcopyrite (yellow, high reflectance, centre top) is present close to two polishing pits (black, centre top).

Polished block. plane polarized light, x 80, air

Altered magnetite and anatase. Derbyshire, Britain

Click hereMagnetite (light brown) occurs as cruciform skeletal crystals as a result of rapid crystallization and is intergrown with plagioclase (dark grey and featureless). The pale body colour of the magnetite is due to oxidation and incipient alteration to haematite. Anatase (light grey, centre top) is abundant, forming aggregates of small crystals with strong yellow-grey internal reflections. Most of the anatase in this photograph is below the surface of the section and is seen by its internal reflections. It is associated with titanaugite crystals (featureless to pale internal reflections).

Polished block. plane polarized light, x 160, oil

Ilmenite, spinel and haematite. Derbyshire, Britain

Click hereParallel ilmenite laths (pale brown, left) arc intergrown with plagioclase (dark grey and featureless) and pyroxene (bottom left) showing orange-brown internal reflections due to the presence of fine-grained haematite/limonite. A large pseudomorphed olivine (orange internal reflections, right) encloses euhedral small spinel crystals (brown-grey, centre right) and haematite (white, right centre) growing along a fracture. Very fine-grained haematite and limonite form the margin to the olivine and show faint red internal reflections (bottom centre). Fine-grained anatase shows abundant light-coloured internal reflections (centre left), and is associated with titanaugite.

Polished block. plane polarized light, x 160, oil

Ilmenite, haematite and TiO2 minerals. Derbyshire, Britain

Click hereRelict cores of ilmenite (brown, centre) are enclosed in a fine-grained Mixture of TiO2 minerals and haematite. The TiO2 rich minerals have a lower reflectance and lilac-blue colour against the haematite-rich phase (cream-white, top centre). Secondary acicular haematite (white, bottom centre) forms a small aggregate, has a higher reflectance and is whiter than haematite pseudomorphing ilmenite. Euhedral pyroxene (pale grey, top centre, right centre) and plagioclase (dark grey and featureless) are the main silicates.

Polished block, plane polarized light, x 160, oil

Magnetite, ilmenite, TiO2 minerals and haematite. Derbyshire, Britain

Click hereEuhedral equant magnetite (brown, left) enclosing silicate inclusions (grey, left centre) is overgrown by smaller magnetite crystals (centre left). Incipient alteration of magnetite is seen by lighter 'veinlets' cross-cutting it (left centre). An ilmenite lath is almost totally altered to a mixture of TiO2-rich (lilac-blue, centre) and higher reflectance haematite-rich phases (cream-white, centre). Minor relict ilmenite (pale brown) cores remain and unaltered ilmenite forms the lower rim to the lath. Subhedral pyroxene (light grey, internal reflections, top centre) and plagioclase (dark grey, bottom) form the silicate matrix.

Polished block. plane polarized light, x 160, oil

Ilmenite, TiO2-rich minerals and haematite. Derbyshire, Britain

Click hereTwo laths of ilmenite show widespread to complete alteration. The central lath comprises a number of ilmenite crystals (light to darker brown, centre) which show reflection pleochroism and bireflectance (centre). Alteration is extensive and two main 'phases' are present: TiO2-rich areas (light brown-grey, centre) and a higher reflectance 'phase' which is a mixture of haematite plus minor TiO2-rich minerals (cream-white, centre). The more altered ilmenite lath (top left) now consists of a very fine-grained porous mixture of haematite and TiO2-rich minerals. The silicates are subhedral pyroxene (grey, bottom left) and plagioclase (dark grey to black and featureless). The orange area is due to stray internal reflections within the silicates. At higher magnification in oil, the fine-grained alteration products can be more easily resolved.

Polished block. plane polarized light, x 160, oil