Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Silver sulphide
Chemical Formula: Ag2S
Method(s) of Verification: not verified analytically, but the black tarnish on silver is diagnostic.
- Supergene: post-mining oxidation & weathering deposits
Introduction: acanthite is a low-temperature mineral that can occur in hydrothermal veins or as a supergene alteration product of native silver. The familiar black tarnish on sterling silver is acanthite, forming as a consequence of the reaction between silver and atmospheric sulphur compounds.
Occurrence in Wales: acanthite was reported from Gwynfynydd Mine by Pryor (1988), associated with sylvanite. However doubts have been raised as to the identifications of these minerals (D.H.M. Alderton, pers. comm.). Two occurrences that have more credibility are both instances in which the acanthite has formed after collection of the specimens i.e. whilst specimens were in storage. These comprise a polished section (J.W.G. Gilbey Collection no. 87, NMW Collection) that includes low-fineness (high-silver) electrum in a sample from Tyddyn Gwladys Mine in the Dolgellau Gold-belt (Mason et al., 2002) and a small specimen of native silver from Frongoch Mine (Green et al., 1996); in both cases a black coating has developed on the silver. Since these coatings did not develop in the original geological setting neither can be regarded as an acanthite locality.
There are no key localities for this specimen.
- Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
- Mason, J.S., Bevins, R.E. & Alderton, D.H.M., 2002. Ore Mineralogy of the mesothermal gold lodes of the Dolgellau Gold Belt, North Wales. Transactions of the Institution of Mining and Metallurgy (Section B, Applied earth science), 111, B203-B214.
- Pryor, M.J., 1988. Geological and fluid inclusion studies at Ogofau and Gwynfynydd gold mines, Wales. South African Journal of Geology, 91, 450-464.