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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:

Geological Context:

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:


  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.