Barytocalcite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Barium calcium carbonate
Chemical Formula: BaCa(CO3)2
Method(s) of Verification: Llantrisant - XRD at the National Museum of Wales (NMW X-397); Dolyhir - XRD (Manchester Museum).

Chemical Group:

  • Carbonates

Geological Context:

  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: epithermal polymetallic veins & pipes
Sharply terminated prismatic barytocalcite crystals to several millimetres from one of several veins exposed at Dolyhir Quarry. Photo D.I. Green, © D.I. Green.
Massive barytocalcite veining Precambrian sediments from Dolyhir Quarry, Old Radnor, Powys. National Museum of Wales Collection (NMW 2005.2G.M.1). Photo T.F. Cotterell, © National Museum of Wales.
Barytocalcite from Mwyndy Mine, Llantrisant, Mid Glamorgan. National Museum of Wales Collection (NMW 25.554.GR.7). Photo M.P. Cooper, © National Museum of Wales.
Introduction: barytocalcite typically occurs in low-temperature 'Mississippi Valley Type' hydrothermal veins. It tends to be associated with other barium minerals and with base-metal sulphides, particularly galena and sphalerite.
Occurrence in Wales: barytocalcite was first recorded in Wales from Glamorgan (North, 1916), with reference to a specimen presented by Col. Rimington to the British Museum in 1886. Although the exact locality was not recorded, a description by Dr. G. T. Prior led North to suggest that the specimen was probably derived from one of the iron mines at Mwyndy. This occurrence was supported by a donation, in 1925, of a specimen of barytocalcite from Llantrisant, to the National Museum of Wales by the Cymmer Colliery Workmans Library. No other discoveries were made until the mid-1990s when small pseudomorphs after barytocalcite were found at Dolyhir Quarry, near Old Radnor in Powys. Subsequent fieldwork confirmed the presence of primary barytocalcite in significant quantity (for a rare mineral) at this site.

Key Localities:

  • Dolyhir Quarry, Old Radnor, Powys: barite pseudomorphs after barytocalcite covering hand-sized specimens were first collected in 1995 from a limestone-hosted tennantite-chalcopyrite-galena dominated sulphide vein. Subsequently, specimens of unaltered primary barytocalcite, dominantly massive, but with occasional small cavities were discovered in 2000. Further finds have, revealed barytocalcite to be relatively common in veins cutting Precambrian sediments in the north-east section of the quarry. Veins of solid barytocalcite up to 5 cm wide (e.g. National Museum of Wales specimen NMW 2002.51G.M.18) occasionally contain cavities lined with sharply terminated prismatic crystals up to 7 mm in length (NMW 2002.51G.M.17). Many other crystal habits have been observed including unusual cream coloured curved platy crystals, overgrowing quartz in a narrow vein, exposed in May 2003 in the north-west corner of the middle level.
  • Llantrisant area, South Wales: a single specimen in the NMW Mineral Collection (NMW 25.554.GR.7) displays colourless to creamy, transparent bladed crystals on massive barytocalcite, with minor pyrite lining a cavity in siliceous iron ore (Alabaster, 1990). This historic piece was donated to the National Museum of Wales by the Cymmer Colliery Workmans Library in 1925.

References:

  1. Alabaster, C., 1990. Alstonite and barytocalcite from Llantrisant, South Wales and barytocalcite from Holwell, Mendip Hills, England. Journal of the Russell Society, 3, 1-6.
  2. North, F.J., 1916. The minerals of Glamorgan. Transactions of the Cardiff Naturalists' Society, 49, 16-51.