Pyrolusite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Manganese oxide
Chemical Formula: MnO2
Method(s) of Verification: Benallt Mine - XRD (Natural History Museum, x4271); Tŷ Coch - XRD (Natural History Museum, x18701); Snowbrook Mine - XRD (NMW X-1293); Mynydd Nodol - XRD (NMW X-1477); Nant Uchaf Mine - XRD (NMW X-1503).

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: a manganese oxide belonging to the rutile group of minerals, it is trimorphous (has the same composition but a different atomic strucure) with akhtenskite and ramsdellite. Pyrolusite is a secondary mineral which forms by alteration of manganese ore minerals.
Occurrence in Wales: pyrolusite is probably widespread in Wales, although there are few specific records of its occurrence. Bevins (1994) reported pyrolusite from Benallt Mine, on Llŷn and from Tŷ Coch, in South Wales on the basis of X-ray diffraction analyses at the Natural History Museum, London. Research into the characterization of manganese species from across Wales has produced four further localities (T.F. Cotterell, unpublished data).

Key Localities:

  • Beddgelert, Gwynedd: in a similar setting to Mynydd Nodol, blocky crystals of pyrolusite are developed in cavities within a massive to botryoidal cryptomelane group phase (T.F. Cotterell, unpublished data).
  • Mynydd Nodol Mine, Arenig District, Llanycil, Gwynedd: small, grey metallic blocky crystals on botryoidal hollandite on National Museum of Wales specimen no. NMW 27.111.GR.26 (ex G.J. Williams Collection) has been identified as pyrolusite by XRD (T.F. Cotterell, unpublished data).
  • Nant Uchaf Mine, Abergele, Clwyd: rich acicular pyrolusite encrusts stalactitic cryptomelane on National Museum of Wales specimen no. NMW 83.41G.M.3757 (ex R.J. King Collection).
  • Snowbrook Mine, Llanidloes, Powys: a pebble containing black manganese oxides dominated by pyrolusite was found near to Snowbrook Mine, but the rounded nature of this specimen suggests that it has been transported some distance and is not from the mine itself (T.F. Cotterell, unpublished data).

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.