Siderite, (after Calcite (?) )

Mineral: Calcite, Siderite
Size: 41 mm
Owner: Neal Luppescu
Photographer: Neal Luppescu

This unusual specimen is an epimorph (or "cast") of siderite (verified by Raman spectroscopy), after a rhombohedral carbonate, probably calcite (?).

The concave surface of the cast (Photograph 1) preserves elements of trigonal symmetry which suggests that the siderite formed, initially, over crystals of a rhombohedral carbonate, that have subsequently dissolved away.  The remaining "shell" of siderite appears to have three distinct layers; a pale, tan-coloured, middle layer is sandwiched between two layers of dark brown rhombohedral siderite crystals.

In the interests of full disclosure it is noted that there has been some debate on the origin of this specimen.

Dolomite casts after calcite are common at Tsumeb; siderite, on the other hand, is an uncommon mineral at this locality and we are unaware of other examples of siderite epimorphs from Tsumeb. It is notable that the specimen bears some similarities (but also some differences) with siderite epimorphs after calcite from Aggeneys, in South Africa. 

With this caveat, however, and on the reasonable assumption that the Uli Bahmann label/provenance is reliable, we have chosen to present the specimen here.  We would be delighted to hear from anyone with similar material attributed to Tsumeb.