Bornite, chalcopyrite and altered pentlandite. Palabora, Republic of South Africa

Click hereBornite (brown) is intergrown with laths and irregular-shaped areas of chalcopyrite (yellow), much of which is crystallographically oriented along (100) of the bornite. Subhedral altered pentlandite (light yellow, centre) is fractured. Dark grey areas are silicate gangue. Black areas are polishing pits. This type of chalcopyrite-bornite texture can be the result of exsolution or replacement processes.

Polished block, plane polarized light. x40, air

Bornite, chalcopyrite, magnetite and TiO2, mineral. Palabora, Republic of South Africa

Click hereBornite (brown) is intergrown and replaced by chalcopyrite (yellow) along (100), some of which forms irregular spindle-shaped lamellae growing from the margin of bornite towards its centre. Magnetite (light grey, right) is inclusion-free. Small TiO2. crystals (medium grey, top centre) have lower reflectance than magnetite. Silicate minerals (centre left, top left) have lower reflectance than trigonal carbonate (centre), which shows bireflectance associated with polysynthetic twinning. Pseudobrookite, rather than rutile, is the main TiO2, mineral at Palabora.

Polished block, plane polarized light, x80, air

Magnetite, bornite, valleriite and chalcocite. Palabora, Republic of South Africa

Click hereInclusion-free magnetite (light brown, bottom) is rimmed by very poorly polished valleriite (brown-grey, low reflectance, bottom left and top right) which is difficult to see. Bornite (orange) and chalcocite (blue) form a symplectite-like intergrowth (centre top). Slight differences in the surface colour of chalcocite are due to burnishing. Dark grey areas are gangue minerals.

Polished block, plane polarized light x 80, air.

Bornite, magnetite, chalcopyrite, valleriite, TiO2 mineral and ulvöspinel. Palabora, Republic of South Africa

Click hereMagnetite (light grey, top) carries small exsolved, euhedral inclusions of ulvöspinel (dark grey, top left). Bornite (pink brown, high reflectance, bottom) is replaced by chalcopyrite (yellow, bottom left) which forms laths and irregular lamellae. Valleriite (brown and medium grey, low reflectance, centre bottom) forms incomplete rims around the copper-iron sulphides. Small lath-shaped crystals of a TiO2, mineral (light grey, reflectance similar to magnetite, left centre) occur within the gangue. The gangue comprises silicate (hard, faint internal reflections, centre), a phyllosilicate (lowest reflectance, poor polish, centre) and carbonate. Black areas are polishing pits. Two adjacent crystals of valleriite show its extreme reflection pleochroism from yellow-brown (oriented east-west, centre bottom) to charcoal-grey (oriented north-south, centre right).

Polished block, plane polarized light. x 160. air

Bornite, chalcopyrite and valleriite. Palabora, Republic of South Africa

Click hereBornite (pink-brown, top) is replaced by chalcopyrite (yellow, centre right) along (100). Valleriite (golden yellow, centre top and blue-green anisotropy colours, top centre) forms an incomplete rim around the copper-iron sulphides. The gangue is trigonal carbonate and shows curved deformation twins (centre top) and very faint internal reflections. Both valleriite and trigonal carbonates are strongly anisotropic, although the anisotropy of carbonates is often masked by their strong internal reflections.

Polished block, partially crossed polars. x 160, air

Chalcopyrite, pentlandite and altered pentlandite. Palabora, Republic of South Africa

Click hereChalcopyrite (yellow) contains triangular-shaped inclusions of pentlandite (pale yellow, highest reflectance, centre bottom). Pentlandite has extensively altered, from its grain boundaries inwards, to a fine-grained mixture of violarite and bravoite (lower reflectance, pink-brown, centre bottom). At higher magnification the two secondary minerals can be distinguished from each other. Black areas are silicates.