Amgueddfa Cymru - National Museum Wales

Orpiment

Crystal System: Monoclinic
Status of Occurrence: Unconfirmed Occurrence
Distribution: Rare
Chemical Composition: Arsenic sulphide
Chemical Formula: As2S3
Method(s) of Verification: none.

Chemical Group:

  • Sulphides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
  • Hydrothermal: epithermal polymetallic veins & pipes
Orange powdery realgar from Dolyhir Quarry. Photo D.I. Green. © D.I. Green.
Introduction: orpiment commonly forms as an alteration product of the relatively unstable arsenic sulphide realgar. It can also occur as a primary mineral in near-surface epithermal sulphide deposits and in mineral deposits formed by surface/subsurface fumarolic activity.
Occurrence in Wales: powdery yellow and orange alteration products of realgar, observed on specimens collected in recent years from Dolyhir Quarry, have been referred to as orpiment and pararealgar by some collectors. Both species are known to form through the alteration of realgar upon exposure to daylight. However, analysis of typical specimens has identified the presence of only realgar (Cotterell et al., 2011). Therefore the occurrence of orpiment in Wales awaits confirmation.

Key Localities:

There are no key localities for this specimen.

References:

  1. Cotterell, T.F., Green, D.I., Hubbard, N., Mason, J.S., Starkey, R.E. and Tindle, A.G., 2011. The Mineralogy of Dolyhir Quarry, Old Radnor, Powys, Wales. UK Journal of Mines and Minerals, 32, 5-61.

There are no references for this specimen.