Aragonite

Aragonite


Composition: CaCO3
Crystal System : Orthorhombic
Colours: Colourless, white, cream, pale-yellow, pale-green, blue
Lustre : Vitreous, resinous
Hardness (H) : 3.5-4.0
Specific Gravity (S.G) : 2.947

Distribution

Common. First, second and probably (?) third oxidation zones.

Occurrence

  • Supergene
  • 1st Oxidation Zone
  • 2nd Oxidation Zone
  • 3rd Oxidation Zone

Notable Finds

Fine specimens of the lead-bearing variety "tarnowitzite", and the zinc-bearing "nicholsonite" were recovered from the first oxidation zone. Numerous specimens of pale-blue copper-bearing aragonite, commonly as sub-botryoidal crusts, are of post-mining origin.

Paragenetic and General Notes

Relatively pure aragonite forms slightly greenish to bluish needles and turquoise-coloured masses and crusts; it occurs only rarely as dull, prismatic, pseudo-hexagonal crystals, and is far more scarce than calcite (Pinch and Wilson, 1977).

Much more common than "pure" aragonite is the lead-bearing variety "tarnowitzite", which forms excellent cream-white prismatic crystals to to several centimetres, terminated by pseudohexagonal pyramids. The zinc-bearing variety "nicholsonite" also forms similar excellent crystals, typically buff-cream or very pale blue-green in colour.

There is circumstantial evidence that such aragonite specimens in early Tsumeb collections were designated as "nicholsonite" on the basis of specific gravity determination; a value higher than for pure calcium carbonate was simply attributed to an assumed presence of zinc but without analytical confirmation,   As a result, examples of "nicholsonite" are common in the early collections, while "tarnowitzite" is generally absent.  The occurrence of "tarnowitzite" at Tsumeb was first described by O'Daniel (1930) and it is notable that post-1930 collections are well-endowed with "tarnowitzite" specimens, but notably defficient in "nicholsonite"!

 

The following minerals have been reported to form pseudomorphs after aragonite: malachite (rare); smithsonite (rare).