Calcite, nailhead.

Calcite, nailhead.

Calcite, Calcium carbonate , CaCO3  Calcite is a very common mineral, found in nearly all geologic environments. This specimen is a druse of "nailhead" calcite crystals, on a rhodochrosite base. The crystals have a dusting of a black mineral, probably goethite. The rhodochrosite shows just a hint of pink colouration in places. It was collected from South mine, Broken hill, New South Wales, Australia.

Beryl.

Beryl.

Beryl is a beryllium aluminium cyclosilicate with a formula of Be3Al2Si6O18  The best known varieties of beryl are emerald, aquamarine, and morganite ( pink beryl ) It occurs in many colours, yellow, black, green, pink, and colourless.  Unfortunately, this specimen does not have any visible whole crystals, although there are some still completely buried in the feldspar matrix. The visible broken crystals do have some interest, in that the hexagonal end terminations are black as are the outer surfaces,  while the exposed interiors are pink. which would not have been obvious, but for the damage. The specimen was collected by our club co-founder, Jim Nicholls, from the Triple Chance mine, in Olary, Australia.  

Actinolite with sphalerite.

Actinolite with sphalerite.

Actinolite is an amphibole silicate mineral with the formula Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2   As the formula states, Mg and Fe are variables.  Mg and Fe ions are easily replaced in the crystal structure, with actinolite as an intermediate member in a series between magnesium rich tremolite, and iron rich ferro-actinolite. Fibrous actinolite is classed as an asbestos mineral. This specimen from South Wales, Australia is composed of actinolite and sphalerite with actinolite as the main component. The fibrous nature of actinolite can be seen in the inset, which is an enlargement of part of the main face, which has been cut and semi-polished. The sphalerite is "rubbly" in nature with no good crystals apparent.

Rhodochrosite.

Rhodochrosite.

Rhodochrosite,  MnCO3 is a manganese carbonate mineral, sometimes called mangano calcite. The substitution of calcite for manganese in the mineral leads to a pink coloration. Crystals of rhodochrosite are scalenahedral or rhombohedral, and are usually a deep pink colour. This specimen, from Broken Hill in Australia. is massive, with no crystals present.  

Bornite.

Bornite.

Bornite often has an iridescent surface, and for this reason is popularly known as "Peacock ore" It is a sulphide mineral, composed of copper and iron with the formula  Cu5FeS4   It is an important copper ore, widely occurring in porphyry copper deposits. There can be a wide variance in the amounts of copper and iron, and also a wide variance in coloration. This particular specimen, collected from Broken Hill in South Australia, is probably below average in brilliant coloration, but nevertheless is an interesting specimen.