Ilmenite and magnetite in basalts and dolerites

Ilmenite, magnetite, maghaemite and TiO2 minerals. Clee Hills, Shropshire, Britain

Click hereA lobate crystal of ilmenite (pink-brown, centre) is surrounded by magnetite (pink-brown, slightly deeper colour, centre bottom, right and centre left). Magnetite has oxidation-exsolution lamellae of ilmenite (centre bottom, oriented east-west) and is extensively altered to a fine-grained intergrowth of maghaemite (white) and TiO2 minerals (blue-grey, low reflectance, centre right). Pyroxene (grey, right) and plagioclase (strong light coloured internal reflections, left) are the main silicates. Lath-shaped apatite crystals (bottom left) occur within the plagioclase.

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Ilmenite, magnetite, TiO2 minerals and maghaemite. Clee Hills, Shropshire, Britain

Click hereCharacteristic lobate crystals of ilmenite (pink-brown, left) are intergrown with plagioclase (featureless) and, to a lesser extent, with pyroxene (grey, top right). Ilmenite (right centre) is intergrown with magnetite (brown-pink, right centre, below ilmenite) which has thin oxidation-exsolution lamellae of ilmenite. Magnetite has altered to TiO2 minerals (blue-grey, centre right) and minor maghaemite (white, centre right) which enclose relict ilmenite lamellae. Acicular crystals of apatite (white, internal reflections) are present within plagioclase (centre bottom and bottom right).

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Magnetite, ilmenite and haematite. Clee Hills, Shropshire, Britain

Click hereA large equant crystal of magnetite (pink-brown, left) is intergrown with, and encloses, plagioclase (dark grey, featureless). Oxidation-exsolution lamellae of ilmenite (pink-brown, lighter coloured than magnetite, top centre) are present. Magnetite has extensively altered to haematite (white-blue) and minor TiO2 phases (light grey) along fractures and crystal boundaries (centre bottom). Lobate ilmenite (right) is unaltered and is intergrown with plagioclase (dark grey and featureless). Pyroxene (grey, cop right) is present.

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Magnetite and ilmenite. Derbyshire, Britain

Click hereEuhedral magnetite (light brown, centre) carries abundant ilmenite lamellae (darker brown) oriented along (111) and the result of oxidation-exsolution. An incomplete magnetite rim around the euhedral crystal also carries exsolved ilmenite (centre right). Very small grains of tarnished bornite (red-brown, centre right) have replaced original chalcopyrite. Euhedral to subhedral pyroxene (light grey, left) and plagioclase (dark grey, light internal reflections, bottom right) are the main silicate phases. Minor amounts of relict carbon-coating are blue-grey (bottom centre).

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Magnetite, ilmenite and chalcopyrite, Derbyshire, Britain

Click hereMagnetite (brown) forms small euhedral (top centre) or subhedral crystals (centre) intergrown with slightly lighter coloured ilmenite (light brown, centre right). Ilmenite displays a symplectite-like intergrowth with the plagioclase matrix. A rounded intergrowth of fine-grained chalcopyrite (yellow, bottom centre) and ilmenite occurs about the margin of ilmenite, and has the shape of a sulphide droplet.

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Magnetite, pyrite, haematite, pyrrhotite, ilmenite and spinel. St Peter Port gabbro, Guernsey, Channel Islands, Britain

Click hereCoarse-grained magnetite (brown-grey, left and right) crystals carry crystallographically oriented, exsolved, thin spinel crystals (grey, bottom right) and rare ilmenite laths (brown-grey, lower reflectance than magnetite, bottom left, top centre left). Pyrite (yellow, high reflectance) encloses relict pyrrhotite (brown-yellow, centre right) and has poorly crystalline rims (centre left). It is intergrown with subhedral to euhedral haematite (blue) and a second generation of magnetite (brown-grey, centre bottom). This generation of magnetite is inclusion-free and has a slightly higher reflectance than the main magnetite. Elsewhere in the section (not shown), pyrrhotite is the main sulphide surrounding the first generation of magnetite and shows alteration to pyrite, marcasite and magnetite. Therefore, it is possible that the pyrite and second generation magnetite have replaced pyrrhotite here, and that subsequently magnetite has oxidized to haematite.

Polished block. plane polarized light, x 160, oil