Cryptomelane

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Potassium manganese oxide
Chemical Formula: K(Mn4+,Mn2+)8O16
Method(s) of Verification: definitive identification requires a combination of XRD and chemical data. Drosgol, Nant Uchaf and Maenclochog - XRD (numbers NMW X-1488, NMW X-1504 and NMW X-2524 respectively) and EDS at the National Museum of Wales.

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
  • Hydrothermal: epithermal polymetallic veins & pipes
Cryptomelane from Moel Llyfnant, Arenig District, Merioneth. National Museum of Wales specimen no. NMW 2006.28G.M.1.
Introduction: a relatively common, but frequently overlooked, manganese oxide which forms a series with hollandite (barium-rich), coronadite (lead-rich) and manjiroite (sodium-rich). Cryptomelane typically forms hard, grey-black, botryoidal masses of the sort previously called 'psilomelane'.
Occurrence in Wales: cryptomelane and cryptomelane group minerals are relatively widespread in Wales, but have been overlooked, or just mis-identified as 'psilomelane' or pyrolusite. No occurrences of cryptomelane were recorded by Bevins (1994), but specimens are now known from the Arenig District in Merionethshire, Drosgol in the Central Wales Orefield and Gyrn Ddu on Pen Llŷn (T.F. Cotterell, unpublished data). Specific site descriptions will be added in due course.

Key Localities:

  • Arenig District, Llanycil, Gwynedd: the ash-flow tuffs in the Arenig District frequently contain veins of manganese oxide dominated by botryoidal cryptomelane group phases (T.F. Cotterell, unpublished data).
  • Drosgol Mine, Drosgol, Ponterwyd, Ceredigion: supergene manganese mineralization consisting of hard, dull, grey, massive zinc-bearing cryptomelane overgrown by rosettes of highly lustrous, metallic, bluish-black, platy chalcophanite crystals occurs in small surface trials (Cotterell, 2009). Occasionally layers of small, prismatic, ramsdellite crystals cut through the cryptomelane.
  • Maenclochog, Pembrokeshire: a specimen in the collections of the National Museum of Wales (no. NMW 58.464.GR.14) labelled as ,'bog manganese' from, 'nr. Maenclochog' consists of a crumbly mass containing, small, discrete layered crusts of cryptomelane (T.F. Cotterell, unpublished data).
  • Nant Uchaf Mine, Abergele, Clwyd: a specimen of stalactitic manganese oxides in the collections of the National Museum of Wales (no. NMW 83.41G.M.3757) contains a core of dull, black, cryptomelane surrounded by grey, metallic, radiating sprays of pyrolusite (T.F. Cotterell, unpublished data).

References:

  1. Cotterell, T.F., 2009. Supergene manganese mineralization associated with the Camdwr Fault in the Central Wales Orefield. Journal of the Russell Society, 12, 15-25.