Mineral Database

Mineral Database
Mineralogy of Wales

Alstonite

Crystal System: Triclinic
Formula: BaCa(CO3)2
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Barium calcium carbonate
Method(s) of Verification: Llantrisant - XRD (NMW & Alabaster, 1990); Dolyhir Quarry - XRD at the National Museum of Wales (NMW X-1418).

Bipyramidal alstonite crystal, 2 mm long, from Dolyhir Quarry, Old Radnor, Powys. Image, D.I. Green.

Bipyramidal alstonite crystals up to 2 mm in length from a roadcutting on the A4119 south of Llantrisant, Mid Glamorgan. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.

Chemical Group:
  • Carbonates
Geological Context:
  • Hydrothermal : mesothermal polymetallic veins
  • Hydrothermal : Mississippi Valley Type veins
Introduction: alstonite tends to occur in low-temperature hydrothermal mineral veins where it is accompanied by calcite, barite, witherite and sulphides. Its distinctive crystal morphology and Ba-rich associations assists identification, once the mineralogist has gained some experience in such assemblages.
Occurrence in Wales: alstonite is a rare mineral in Wales, only having been verified from three localities. A fourth supposed locality - the Pennant Mine in Clwyd, is labelled as the source for two alstonite specimens in the National Museum of Wales Collection (NMW 83.41G.M.5489 and NMW 83.41G.M.5493). However these so resemble the type locality material from the North Pennines that their provenance has been questioned (B. Young, pers. comm.).
Key Localities:
  • Dolyhir Quarry, Old Radnor, Powys: alstonite was identified at Dolyhir Quarry on specimens collected in 2001 from veins cutting Precambrian strata. Drusy crusts composed of white millimetre size pyramidal crystals have been found, in association with paralstonite, barytocalcite and witherite (NMW Collection). In 2004 a further find of larger crystals (c. 5 mm) was made.
  • Llantrisant area, South Wales: vein and cavity-fill deposits at Cefn-Parc Mine and exposed nearby in a rock-cutting along the Llantrisant Bypass carry alstonite as part of a low-temperature Mississippi Valley-Type (MVT) mineral assemblage (Alabaster, 1990). The host rock in both cases is the Triassic Dolomitic Conglomerate. Good specimens of alstonite, with colourless to pale pink bipyramidal crystals uo to 6 mm in size, were collected during and just after the making of the cutting. Associated minerals are witherite, barite, calcite, dolomite and various Cu, Pb and Zn sulphides. Many of the alstonite crystals are pseudomorphously replaced by barite, or sometimes by aragonite.
References:
  • 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.