Crystal System: Monoclinic
Status of Occurrence: Unconfirmed Occurrence
Distribution: Rare
Chemical Composition: Sodium manganese oxide hydrate
Chemical Formula: Na4Mn14O27.9H2O
Method(s) of Verification: none recorded.

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: birnessite is a secondary manganese mineral which typically forms as a dense to earthy black crust. It develops from the weathering of manganese ores resulting in the breakdown of primary manganese minerals.
Occurrence in Wales: birnessite is believed to occur on a specimen from Benallt Mine in the collection of the Natural History Museum, London, but the method of identification is unknown.

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: a single specimen of birnessite is held by the Natural History Museum, London (B.M. 1944, 28). No further contextual details are known.


There are no references for this specimen.