Rhodochrosite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Manganese carbonate
Chemical Formula: MnCO3
Method(s) of Verification: Harlech area - EMPA (Bennett, 1987b); Llŷn Peninsula - EMPA (Bennett, 1987b).

Geological Context:

  • Hydrothermal: sedimentary exhalative deposits
Banded sedimentary manganese ore from the Middle Cambrian of the Harlech area. The creamy yellow bands are rich in micritic calcian rhodochrosite. The cut and polished sample, 25 cm across, is from the Llyn Dywarchen mines in the N Harlech Dome area.
Pink rhodochrosite from Nant Mine, nr Rhiw, Pen Ll?n. Specimen approximately 5 cm across. National Museum of Wales specimen no. NMW 27.111.GR.426, ex G.J. Williams collection. © National Museum of Wales.
Introduction: rhodochrosite is perhaps best known as a semi-precious gemstone, forming attractive banded pink masses which are cut and polished. To mineralogists, the large, ruby-red crystals of rhodochrosite on quartz-sulphide matrix, which are mined purely for the specimen market in Colorado, rank, at their best, among the finest mineral specimens known. Rhodochrosite may occur in a variety of settings: it is an important component of bedded manganese deposits but may also occur in a wide variety of low to medium temperature hydrothermal mineral veins, where it may be associated with quartz, fluorite, other carbonates and a wide range of sulphides.
Occurrence in Wales: rhodochrosite was first reported from Wales by Readwin (1888), from the Dolgellau Gold-belt: however, it is not known whether he was referring to the pink-banded bedded manganese ore of the area (which has a complex mineralogy) or making a claim of its occurrence in the gold-bearing veins of the area (in which case it must be treated with the same caution as a number of other unsubstantiated Readwin claims). A subsequent account of rhodochrosite in Wales is provided by Campbell Smith (1913), referring to a specimen said to be from Llanymynech Hill near Oswestry. The sample, in the collection of Thomas Pennant, contained 'dark greyish copper ore, mixed with a pale red...' - the latter was said by Arthur Russell to be rhodochrosite. Nothing more is known of this specimen and no further examples are known. A further report of rhodochrosite-bearing veins was made by Hall (1971), with reference to the isolated Talachddu Mine near Brecon in S Central Wales. Fieldwork and XRD follow-up at the National Museum of Wales correctly identified the pinkish carbonate as dolomite and Hall included this identification in the second edition of the work, published in 1993. The chief occurrences of rhodochrosite in Wales are in the sedimentary-exhalative manganese ores of Cambrian and Ordovician age, occurring in the Harlech and Rhiw areas of N and NW Wales respectively.The most up-to-date work on the Harlech deposits is that by Bennett (1987a, 1987b): his petrological and analytical investigations revealed that, in the banded pink and creamy-yellow ore, the yellowish layers are rich in calcian rhodochrosite (with <20 mol% Ca). Bennett (1987b) also recorded calcian rhodochrosite as a cement to greywackes immediately adjacent to the ore-bed, and noted the presence of the mineral on thin joints and in minute veinlets. On the Llŷn Peninsula, further occurrences of manganiferous Cambrian sediments are known from the St Tudwal's Peninsula, and again calcian rhodochrosite is present (Bennett, 1987b). Rhodochrosite is also present as a component of the major manganese ore-deposits at Benallt and Nant mines (Woodland, 1939, 1956). However, again it is fine-grained, unlike some of the other more coarsely crystalline phases present at these mines. The fine grainsize of rhodochrosite from the confirmed localities in Wales precludes confident identification in hand specimen.

Key Localities:

There are no key localities for this specimen.

References:

  1. Bennett, M.A., 1987a. The Cambrian manganese deposits of North Wales. Unpublished Ph.D. thesis, University of Leeds.
  2. Bennett, M.A., 1987b. Genesis and diagenesis of the Cambrian manganese deposits, Harlech, North Wales. Geological Journal, 22, 7-18.
  3. Campbell Smith, W., 1913. The mineral collection of Thomas Pennant (1726-1798). Mineralogical Magazine, 16, 331-342.
  4. Hall, G.W., 1993. Metal Mines of Southern Wales. 2nd Ed. Griffin Pubs., Herefordshire.
  5. Hall, G.W., 1971. Metal Mines of Southern Wales. Privately Published.
  6. Readwin, T.A., 1888. Gold in Wales. London, 12pp.
  7. 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.
  8. Woodland, A.W., 1956. The manganese deposits of Great Britain. Symposium sobreyacimientos de manganeso, tomo 5, Europa. 20th International Geological Congress, Mexico, 197-218.

There are no references for this specimen.