Duftite, (after Calcite (?) )

Mineral: Calcite, Duftite
Size: 80 mm
Owner: Mariusz Oleszczuk
Photographer: Mariusz Oleszczuk

A cast of olive-green duftite, epimorphous after a rhombohedral carbonate - probably calcite (?), but possibly dolomite (?) or even smithsonite (?). Photograph 1 shows the convex surface of cast (which formed distal to the carbonate substrate); Photograph 2 shows the concave (or proximal) surface.

The carbonate over which the duftite crystallised has completely dissolved away, leaving a shell-like structure of duftite (1-2 mm thick), with idiomorphic molds that indicate formation over a bed of intergrown carbonate crystals (to c. 9 mm).  

Similar casts of dolomite after calcite are common at Tsumeb, but duftite epimorphs like this one are comparatively rare.

The specimen was originally identified as mottramite, but analysis shows it to be duftite, with a degree of calcium enrichment, placing it somewhere between duftite and conichalcite in composition.