Calcite, (w. Duftite)

Mineral: Calcite, Duftite
Chemistry: CaCO3
Size: 41 mm
Owner: Geoff Krasnov
Photographer: Geoff Krasnov

A group of prismatic calcite crystals (to 11 mm) with trigonal "nail-head" terminations.  Duftite (?) has formed selectively over the terminations during a late stage of crystallisation, and then calcite continued to form over the duftite (?).  However, closer examination reveals that the prismatic calcites are, in fact, composed of a stack of tiny lenticular rhombohedrons.  This texture suggests that the calcite has recrystallised and that this clearly occurred after, or possibly at the same time as, the formation of calcite over the duftite (?) layer. Note that the duftite (?) has not been verified by analysis, but this is a classic association at Tsumeb, and there is little doubt regarding its identity.

This specimen was from a pocket collected on 19 Level, in 1981, by a miner named Kiewiet Rust.  When the pocket was first encountered the habit of the calcite led Rust to think that this was tarnowitzite, and it was only on closer examination that he realised this was calcite (Willie Steyn, personal communication to Geoff Krasnov, December 2003).

The occurrence of duftite (?) on calcite in a pocket on 19 Level is notable, because 19 Level lies more or less half way between what is normally considered to be the bottom of the first oxidation zone (c. 15 Level) and the top of the second (c. 26 Level).  This specimen reminds us that the reality is more complex, and that odd pockets of oxidation were occasionally encountered at intermediate depths.