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 key localities for this specimen.


There are no references for this specimen.