Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Copper sulphate hydroxide hydrate
Chemical Formula: Cu4(SO4)(OH)6.H2O
Method(s) of Verification: Britannia Mine - XRD; Copa Hill - XRD (NHM, x20730); Crafnant Mine - XRD (National Museum of Wales, NMW X-1164); Penrhiw Mine - XRD (National Museum of Wales, NMW X-884).

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Blue-green feathery aggregates of posnjakite from Ystrad Einion Mine in the Central Wales Orefield. Specimen and photo S.A. Rust. © S.A. Rust.
Introduction: posnjakite is a secondary sulphate mineral typically formed in post-mining environments associated with copper-bearing ore bodies particularly within mine dumps and underground on mine walls.
Occurrence in Wales: posnjakite is rare in Wales and easily confused with the more common mineral, langite. The frequent confused form of crystals makes visual identification difficult and although records are limited, they tend to be accurate as further analytical methods are often employed.

Key Localities:

  • Britannia Mine, Snowdonia, Gwynedd: highly lustrous blue-green translucent plates form stalactitic growths with brochantite and malachite on National Museum of Wales specimen no. NMW 84.7G.M.1 collected from the second lowest accessible level (Bevins et al., 1985).
  • Copa Hill, Cwmystwyth mines, Ceredigion: blue tabular crystals associated with brochantite and galena occur on Natural History Museum specimen no. B.M.1981,461. This is derived from underground workings (Bevins & Mason, 1997).
  • Crafnant Mine, Llanbedr, Harlech, Gwynedd: blue coatings occur underground.
  • Dylife Mine, Penegoes, Powys: tentatively identified by Rust & Rust (1987) forming elongated pseudotrigonal crystals.
  • Penrhiw Mine, Ystumtuen, Ceredigion: two generations are described by Mason & Green (1995). Corroded tabular crystals to 1.5 mm lining cavities in siltstone blocks from which dolomite has partially dissolved and, later less corroded ragged platy crystals overgrowing ramsbeckite and schulenbergite close to the edges of cavities in siltstone blocks left by the dissolution of dolomite veinlets.


  1. Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  2. Bevins, R.E., Rowbotham, G., Stevens, F.S., Turgoose, S., & Williams, P.A., 1985. Lanthanite-(Ce), (Ce,La,Nd)2(CO3)3.8H2O, a new mineral from Wales, U.K. American Mineralogist, 70, 411-413.
  3. Mason, J.S. & Green, D.I., 1995. Supergene minerals including exceptional ramsbeckite from Penrhiw Mine, Ystumtuen, Dyfed. UK Journal of Mines & Minerals, 15, 21-27.
  4. Rust, S. & Rust, D., 1987. Micro-minerals from Dyfngwm Mine. U.K. Journal of Mines and Minerals, No. 2, 28-32.