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RM3154   Chalcostibite, Bournonite, Tetrahedrite-(Zn)

Julcani Mine, Julcani, Angaraes, Huancavelica, Perú

This specimen was purchased at the Expominer fair (Barcelona) in 1985 from a dealer who brought Peruvian material. It was labeled as "enargite" from Huancavelica, a Peruvian mining region well known among mineral collectors. The specimen shows numerous crystals with a metallic luster, isolated or in groups, on a sulfide matrix (mainly pyrite). By observing them carefully and using a magnifying glass, we were able to see different crystalline forms that indicated the presence of various species. We proceeded to extract some crystals to study them.
SEM-EDS analysis indicated that elongated prismatic crystals with polysynthetic growths on the faces and hopper shapes (perimorphosis?) were a copper sulfoantimonide, with some bismuth and arsenic. This composition and its percentages correspond to chalcostibite, and the Bi would indicate the series with emplectite. Chalcostibite was confirmed by X-ray diffraction (XRD). An unusual species and less so for this Peruvian mine.
Other crystals showed prisms with faces striated and truncated by the pinacoid. Their analysis indicated that it was a copper-lead sulfoantimonide, with traces of Bi, consistent with twinned bournonite, a species also confirmed by XRD.
On the matrix we observe crystals with tetrahedron shapes, sometimes flattened. XRD indicated the presence of tetrahedrite. SEM-EDS analysis of small crystals associated with chalcostibite indicated that it could be tetrahedrite-(Zn), with possible argentotetrahedrite-(Zn). We will send labels and all the analysis reports to the buyer.

Size: 12 x 8.2 x 3 cm.
Found in ±1985.

Excol. Juan M. Casanova.
Analyzed specimen with report