Supergene alteration

Pyrrhotite, violarite and altered pyrrhotite. Kambalda, Australia

Click hereThe primary ore was pyrrhotite and pentlandite but these minerals have suffered extensive supergene alteration. Relict pyrrhotite (brown, well polished, left, centre right) has an alteration rim of zwischen-produkt (light brown-white, highest reflectance, right bottom) and clearly shows that the alteration of pyrrhotite is crystallographically controlled along (0001). Pentlandite has been totally pseudomorphed by violarite (brown-white, finely pitted surface, centre) but its cleavage and crystal boundaries have been preserved. Silicates are black.

Polished block, plane polarized light. x80, air

Pyrite, marcasite, violarite and pentlandite. Bushveld, Republic of South Africa

Click hereA crystal of pyrrhotite formerly carried pentlandite flames and a rim of granular pentlandite but both minerals are now completely altered. Trace amounts of relict pentlandite (brown, centre left) remain but most has been altered to violarite (brown and poorly polished, centre left). Small areas of violarite (disc-shaped, poorly polished, centre top) within pyrite/marcasite mark the former presence of pentlandite flames. Pyrrhotite has altered along (0001) planes to zwischen-produkt (lower reflectance, centre bottom), and to pyrite and marcasite (yellow-white, centre top). Dark grey areas are silicates.

Polished block, plane polarized light, x80, air

Pyrrhotite and marcasite. Nigadoo River, Canada

Click herePyrrhotite (brown, bottom left) has been extensively altered to a fine intergrowth of marcasite. The alteration has followed the basal (0001) planes of pyrrhotite. The intergrowth comprise a very fine mosaic of marcasite (lower reflectance areas, faint reflection pleochroism, top) which are 'cemented' by coarse-grained marcasite (better polished, yellow-white, centre left), which also forms a rim around the aggregate. Faint reflection pleochroism is visible within the coarse-grained marcasite (centre left).

Polished block, plane polarized light, x 160, oil

Smithsonite, altered sphalerite, chalcopyrite, covelline and sphalerite. Oxclose Mine, South Pennines, Britain

Click hereSphalerite is almost totally altered to smithsonite (light greys, centre) which is the main phase and shows strong bireflectance between grains. Very small relict sphalerite (light grey, bottom centre, centre top left) is harder than the enclosing smithsonite. However, most relict sphalerite is lilac-grey to blue (centre left) and resembles blue copper sulphides. Microprobe analyses of this lilac sphalerite show it to have up to 10% by weight of copper. Covelline is deep blue (top centre, right) and comprises much of the very fine-grained (<10śm) sulphide in the smithsonite. Minor amounts of pyrite (white, top right) are present. Fluorite is dark grey (lower reflectance than the resin, bottom left) and a single lath of poorly polished baryte (oriented north-south, bottom right) is present within a larger lath of smithsonite.

Coarse grain mount. plane polarized light, x 80, air

Galena, cerussite and anglesite. South Pennines, Britain

Click hereGalena (white, top) shows well developed plucking along (100) to give characteristic triangular pits (black). It is altered and replaced by rhythmical aggregates of cerussite (light greys) showing faint bireflectance (bottom left) and anglesite (lower reflectance, poorly polished bands, centre right). This is a fine example of a caries texture. Smaller crystals of galena are totally pseudomorphed by cerrussite and anglesite (bottom). Dark grey areas are fluorite, black areas are polishing pits.

Polished block. plane polarized light, x 80, air

Cuprite, malachite and native copper. Mount Isa, Australia

Click hereNative copper (orange-yellow, high reflectance, centre bottom) is present within cuprite (blue, centre) which in turn occurs within fine-grained malachite (light green-grey), which is replacing chalcocite (left).

Polished block, plane polarized light, x 80, air