Proustite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence - 2nd UK recording
Distribution: Rare
Chemical Composition: Silver arsenic sulphide
Chemical Formula: Ag3AsS3
Method(s) of Verification: Dolyhir Quarry - EMPA & XRD (Manchester Museum).

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: epithermal polymetallic veins & pipes
Gemmy red proustite crystal (1 mm tall) from Dolyhir Quarry, Old Radnor, Powys. N. Hubbard Collection. Photo D.I. Green, © D.I. Green.
Introduction: proustite is typically found in low to medium temperature hydrothermal veins, where it may be associated with tetrahedrite, galena, sphalerite and other silver sulphosalts. It also occurs in the rare but sometimes economically important 'five-metals association', where it is accompanied by other Ag minerals plus those of Ni, Co, As and Bi. Along with pyrargyrite, proustite belongs to the group of minerals sometimes referred to as the Ruby Silvers - a group of silver minerals that are red when fresh but which characteristically tarnish black on exposure to bright light. Well crystallized proustite is highly sought-after by serious collectors of mineral specimens.
Occurrence in Wales: only found relatively recently in Wales, and known from just a small number of specimens, proustite is one of the rarer Welsh species.

Key Localities:

  • Dolyhir Quarry, Old Radnor, Powys: proustite was discovered as tarnished elongated prismatic crystals to about 1 mm, showing the typical deep red colour on thin edges on a single rich micromount specimen collected in 1995 by N. Hubbard. The specimen was found adjacent to a rich tennantite + galena + chalcopyrite + baryte + calcite vein that was temporarily exposed in the mid-1990s: it and the surrounding Wenlock Limestone have since been quarried away. A small number of other specimens bearing gemmy, red, microcrystals have been collected by N. Hubbard from veins within the Precambrian basement beneath the limestone. The genesis of the mineralization at Dolyhir is under investigation but it appears to have Mississippi Valley Type (MVT) affinities.

References:

There are no references for this specimen.