Cosalite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead bismuth sulphide
Chemical Formula: Pb2Bi2S5
Method(s) of Verification: Braich-yr-oen Mine - EMPA & XRD (National Museum of Wales, NMW X-1160).

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
Prismatic cosalite crystal (1 mm long) embedded in galena from Braich-yr-oen Mine, Snowdon. National Museum of Wales specimen. Photo T.F. Cotterell, © National Museum of Wales.
SEM (backscatter) photomicrograph of ore from Braich-yr-oen Mine, Snowdon showing a 0.1 mm long needle of cosalite embedded in quartz. The bright phase is galena, with a small area of slightly darker chalcopyrite. © National Museum of Wales.
Introduction: cosalite occurs in medium-temperature polymetallic ore deposits with bismuthinite and other lead-bismuth (Pb-Bi) bearing phases. It is difficult to distinguish from other visually very similar Pb-Bi minerals and analytical techniques are required for confident identification.
Occurrence in Wales: Reedman et al. (1985) recorded the presence of small quantities of unidentified Pb-Bi minerals in ore samples from copper mines within the Snowdon Caldera. The presence of cosalite was confirmed during the Gwynedd phase of MINESCAN (Bevins & Mason, 1998), when specimens showing an unusual metallic lath-like mineral were collected from the tips of Braich-yr-oen Mine in Cwm Llan on the south side of Snowdon. It is likely that further field and laboratory work may bring further localities to light in the same area.

Key Localities:

  • Braich-yr-oen Mine, Snowdon, Gwynedd: cosalite occurs in a quartz matrix in which spots of galena, sphalerite and chalcopyrite are abundant and reach several millimetres in diameter. Specimens reveal scattered small (rarely >2 mm long) laths and lamellar masses of cosalite, both embedded in galena and protruding into the enclosing quartz. Electron microscope examination revealed the additional presence of minute (1 micron) grains of native bismuth and a bismuth sulphide occurring along the interface between cosalite and galena (Bevins & Mason, 1998; Mason et al., in press).

References:

  1. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  2. Mason, J.S., Cotterell, T.F. & Bevins, R.E., (in prep.). Cosalite, the first Welsh Occurrence. UK Journal of Mines & Minerals.
  3. Reedman, A.J., Colman, T.B., Campbell, S.D.G. & Howells, M.F., 1985. Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales. Journal of the Geological Society, London, 142, 875-888.