J.S. Mason - Central Wales Orefield: Tucekite (3rd world occurrence)

Tucekite (Ni9Sb2S8) is an extremely rare mineral in global terms. It is a nickel antimony sulphide, and belongs to the hauchecornite group of minerals. The only other occurrences of tucekite so far described are the type ones, namely Kanowa, Western Australia (from a mineralised Archaean chlorite schist) and the Witwatersrand, South Africa, where it has been identified in sulphide-bearing concentrates from two mines. And here it is, in Central Wales, in hydrothermal veins!

I first noticed this mineral in the mid 1980s in several polished sections from mines to the east of Talybont: Eaglebrook, Henfwlch (these shots), Esgairhir, Esgairfraith and Hyddgen. It was widespread and unidentified, all the more perplexing because of its tendency to form highly distinctive euhedral tetragonal crystals. A combination of microprobe and XRD analysis with the assistance of Dr David Bland and Mr Peter Nancarrow at BGS finally sorted it out two years later. Why such a globally rare mineral is present in this tight cluster of localities, in quantity, remains to be answered. Tucekite is not that impressive in plane polarised light, but just cross your polars and have another look - it is then one of the most colourful ore minerals in existence as you will see by clicking
HERE. Crystals range in size up to about 0.5mm or so. Associated species are galena, chalcopyrite, ullmannite, pyrite, electrum, siegenite and millerite.