Braunite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Manganese silicate
Chemical Formula: Mn2+Mn3+6SiO12
Method(s) of Verification: Nant Mine - XRD at the Natural History Museum, London (X-ray numbers x17911 & x18399).

Geological Context:

  • Hydrothermal
  • Hydrothermal: limestone hosted hematite deposits
Introduction: braunite is a brownish black mineral that forms a series with abswurmbachite, (Cu,Mn2+)Mn3+6SiO12. It is formed by metamorphism of manganese silicates and oxides and may also be produced as a weathering product.
Occurrence in Wales: two occurrences of braunite are known from Wales. Criddle & Symes (1977) reported the first UK occurrence of braunite from the manganese deposit at Tŷ Coch in South Wales. Braunite has subsequently been confirmed from specimens from Nant Mine, Llŷn collected, but not analysed, prior to the work of Criddle & Symes (1977).

Key Localities:

  • Nant Mine, Nant-y-Gadwen, Rhiw, Llŷn, Gwynedd: The Natural History Museum, London holds specimens from the Russell Collection from which the presence of braunite has been confirmed by X-ray analysis. No further data is available currently.
  • Tŷ Coch, near Porthcawl, South Wales: braunite occurs in manganese ore from a disused mine at this locality. It forms euhedral to subhedral grains associated with hausmannite, interstitial and vug-filling calcite, manganocalcite, ferroan calcite, dolomite, barite and rarer pyrobelonite and vanadinite (Criddle & Symes, 1977).

References:

  1. Criddle, A.J., & Symes, R.F., 1977. Mineralization at Tŷ Coch, Glamorgan (Mid Glamorgan), Wales: the second occurrence of pyrobelonite. Mineralogical Magazine, 41, 85-90.