Rhodonite

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Manganese iron magnesium calcium silicate
Chemical Formula: (Mn2+,Fe2+,Mg,Ca)SiO3
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, Gwynedd: 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.

There are no key localities for this specimen.

References:

  1. 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.
  2. Andrew, A.R., 1910. The geology of the Dolgelley gold-belt, North Wales. Geological Magazine, 47, 159-171, 201-221, 261-271.
  3. Bennett, M.A., 1987b. Genesis and diagenesis of the Cambrian manganese deposits, Harlech, North Wales. Geological Journal, 22, 7-18.
  4. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  5. Cox, A.H., & Wells, A.K., 1927. The geology of the Dolgelley district, Merionethshire. Proceedings of the Geologists’ Association, 38, 265-318.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. Woodland, A.W., 1956. The manganese deposits of Great Britain. Symposium sobreyacimientos de manganeso, tomo 5, Europa. 20th International Geological Congress, Mexico, 197-218.
  11. Woodland, A.W., 1935. Spessartine in the Cambrian manganese ores of Merionethshire. Geological Magazine, 72, 384.

There are no references for this specimen.