Mineral Database

Mineral Database
Mineralogy of Wales

Rhodonite

Crystal System: Triclinic
Formula: (Mn2+,Fe2+,Mg,Ca)SiO3
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Manganese iron magnesium calcium silicate
Method(s) of Verification: not known.
Chemical Group:
  • Silicates
Geological Context:
  • Sedimentary
  • Metamorphic
Introduction:

rhodonite occurs in manganese-bearing deposits formed by hydrothermal, contact and regional metamorphic, and sedimentary processes.

Occurrence in Wales:

rhodonite is an uncommon mineral in Wales largely restricted to the manganese deposits of North Wales where it occurs as a minor constituent. Rhodonite was at one time thought to be the main silicate mineral present within the bedded manganese deposits within the Hafotty Formation of the Llanbedr District in the Harlech Dome (Dewey & Bromehead, 1915). This identification appears to have been carried forward by Dewey & Dines (1923) and Cox & Wells (1927). However, Woodland (1935) reported that the major silicate phase in the ore beds is, in fact, the manganese garnet spessartine. Rhodonite is still believed to be present, but only as a minor phase (Woodland, 1939; Woodland, 1956; Allen & Jackson, 1985; Bennett, 1987). To the south, rhodonite has been reported in some parts of the Vigra Lode within the Dolgellau gold-belt (Andrew, 1910). Further north, rhodonite is reported from the Benallt and Nant manganese mines, on Llŷn (Russell, 1946).

Key Localities:
  • Benallt Mine, Llŷn, Gwyneddŷ: found by Dr. W. Campbell Smith (Russell, 1946).
  • Harlech Dome area, Gwynedd: rhodonite occurs as a minor phase within the silicate portion of bedded manganese ore beds within the Hafotty Formation, particularly in the Llanbedr District (Woodland, 1939; Woodland, 1956; Allen & Jackson, 1985; Bennett, 1987).
  • Nant Mine, Nant-y-Gadwen, Rhiw, Llŷn, Gwynedd: found associated with ganophyllite by Arthur Russell in 1911 (Russell, 1946).
  • Vigra Mine, Dolgellau gold-belt, Gwynedd: Andrew (1910) noted that in some parts the Vigra Lode contains rhodonite.
References:
  • Allen, P.M. & Jackson, A.A., 1985 Geology of the country around Harlech.  Memoirs of the British Geological Survey. Explanation of sheet 135, with part of 149, 112pp.
  • Andrew, A.R., 1910 The geology of the Dolgelley gold-belt, North Wales.  Geological Magazine, 47, 159-171, 201-221, 261-271.
  • Bennett, M.A., 1987b Genesis and diagenesis of the Cambrian manganese deposits, Harlech, North Wales.  Geological Journal, 22, 7-18.
  • Bevins, R.E., 1994 A Mineralogy of Wales  National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  • Cox, A.H., & Wells, A.K., 1927 The geology of the Dolgelley district, Merionethshire.  Proceedings of the Geologists’ Association, 38, 265-318.
  • Dewey, H. & Bromehead, C.E.N., 1915 Tungsten and manganese ores.  Memoirs of the Geological Survey. Special Reports on the Mineral Resources of Great Britain, 1.
  • Dewey, H., & Dines, H.G., 1923 Tungsten and manganese ores. Third Edition.  Memoirs of the Geological Survey. Special Reports on the Mineral Resources of Great Britain, 1.
  • Russell, A., 1946 On rhodonite and tephroite from Treburland manganese mine, Altarnun, Cornwall; and rhodonite from other localities in Cornwall, and Devonshire.  Mineralogical Magazine, 27, 221-235.
  • Woodland, A.W., 1939a The petrography and petrology of the Lower Cambrian manganese ore of western Merionethshire.  Quarterly Journal of the Geological Society of London, 95, 1-35.
  • Woodland, A.W., 1956 The manganese deposits of Great Britain.  Symposium sobreyacimientos de manganeso, tomo 5, Europa. 20th International Geological Congress, Mexico, 197-218.
  • Woodland, A.W., 1935 Spessartine in the Cambrian manganese ores of Merionethshire.  Geological Magazine, 72, 384.