Non-mineralized metamorphic rocks

Pyrite and sphalerite. Cornwall, Britain

In these low grade metamorphosed shales (the 'killas') framboidal pyrite (yellow-white, centre) has not extensively recrystallized. The variation in reflectance between framboids is a measure of the pyrite to non-pyrite ratio within an individual framboidal aggregate which is partly a measure of the amount of recrystallization. Sphalerite (light grey, bottom right) is present. The matrix is fine-grained phyllosilicate.

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

TiO₂ minerals and sphene. Central Wales, Britain

A metadolerite in which a trellis-like intergrowth of a TiO₂ mineral (light grey), often called 'leucoxene', has replaced ilmenite lamellae within a titanomagnetite which has been completely removed and is represented by iron-stained non-opaque minerals showing brown internal reflections. The original titanomagnetite aggregate can be seen to have comprised two crystals. Sphene (light grey, centre right) is present. The matrix is silicate.

Polished block, plane polarized light. x 160, oil

TiO₂ minerals, chalcocite and chalcopyrite. Wales, Britain

A metadolerite where an original titanomagnetite with oxidation-exsolution ilmenite lamellae has been altered to TiO₂ minerals (light grey, right), which have replaced the ilmenite, accompanied by the complete removal of the magnetite component (faint orange-stained areas). Later chalcopyrite (yellow, centre right) and chalcocite (blue, centre left) have overgrown TiO₂. The TiO₂ minerals show faint reflection pleochroism (left) between adjacent crystals.

Polished block, plane polarized light, x 160, oil

Pyrrhotite, marcasite and TiO₂ mineral. Wales, Britain

A low grade 'greenstone' in which pyrrhotite (brown, centre) has partially altered to marcasite (white, centre left), showing its characteristic lath-shaped habit of small crystals. Minor amounts of a TiO₂ mineral (light grey, bottom left) are present. The main matrix minerals are coarse-grained carbonate (grey, showing bireflectance, top right) and chlorite (less well polished). Black areas are polishing pits.

Small discoid areas of sulphides are common in unmineralized greenstones and comprise pyrite, pyrrhotite and chalcopyrite.

Polished block, plane polarized light, x80, air

Pyrite, graphite and marcasite. Strontian, Scotland

Subhedral crystals of pyrite (yellow-white, centre) are intergrown with minor lath-shaped aggregates of marcasite (white, centre top) which are pseudomorphs after pyrrhotite. Graphite laths (brown, low reflectance) show faint bireflectance. Garnet (light grey, top right), biotite (lith-shaped, bottom left), plagioclase and quartz (featureless) are the main silicates.

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

Graphite. Strontian, Scotland

A metapelite with flakes of graphite that show strong reflection pleochroism and bireflectance, from brown-grey (when oriented east-west, top right) to grey (when oriented north-south, centre) where it is almost indistinguishable from the mica. The silicates are quartz, plagioclase (featureless, hard) and biotite, which shows reflection pleochroism from grey (centre left) to brown (bottom left).

Polished thin section. plane polarized light. x80, air