Manganite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Manganese oxide hydroxide
Chemical Formula: MnO(OH)
Method(s) of Verification: Rhiw, Llŷn - XRD (Natural History Museum, x4265); all other occurrences appear to be based on visual idenfications which can be unreliable.

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Hydrothermal
Introduction: manganite is a black, to dark steely-grey, manganese mineral with two polymorphs feitknechtite and groutite. It is usually found with other manganese minerals in low-temperature hydrothermal deposits, but is less common than psilomelane or pyrolusite.
Occurrence in Wales: manganite has been noted from Wales on several occasions, but care should be taken with early references as some can confuse pyrolusite for manganite and vice versa. Hall (1868) listed manganite from the Gower, South Wales and Clynnog, Caernarvonshire, but provided almost no details of either occurrence. A specimen from this latter locality is present in the mineral collection of the Natural History Museum, London along with another from Benallt Mine also on Pen Llŷn. Williams (1927) described the presence of manganite in Ordovician-aged tuffs in the vicinity of Lliwedd, in Snowdonia. Specimens of crystallized manganese ore from near Beddgelert, not far from Lliwedd, have all proved to be pyrolusite (T.F. Cotterell, unpublished data).

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: represented by a specimen in the Natural History Museum, London. It is unclear whether this specimen relates to the manganite described as from Rhiw, Carnarvonshire, confirmed by XRD analysis at the NHM.
  • Gyrn Du, near Clynnog, Llŷn, Gwynedd: a specimen of manganite labelled as from Gyrndu, near Clynog is in the Natural History Museum Collection. This is likely to be the same locality listed as Clynnog by Hall (1868).

There are no key localities for this specimen.

References:

  1. Hall, T.M., 1868. The Mineralogists' Directory. London, 168pp.
  2. Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.

There are no references for this specimen.