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