Paralstonite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Barium calcium carbonate
Chemical Formula: BaCa(CO3)2
Method(s) of Verification: Dolyhir Quarry - XRD at the National Museum of Wales (NMW X-1336, X-1569 & X-1579).

Chemical Group:

  • Carbonates

Geological Context:

  • Hydrothermal
Spiky paralstonite microcrystals encrusting pyramidal ewaldite and prismatic harmotome from Dolyhir Quarry. Photo D.I. Green. © D.I. Green.
Scanning electron micrograph of minute, doubly-terminated paralstonite crystals partially overgrowing cruciform twinned harmotome crystals from Dolyhir Quarry. National Museum of Wales Collection (NMW 2002.31G.M.1). © National Museum of Wales.
Scanning electron micrograph of a pyramidal alstonite (larger crystal) with smaller, elongated doubly-terminated paralstonite crystals, from Dolyhir Quarry. National Museum of Wales Collection (NMW 2002.31G.M.1). © National Museum of Wales.
Introduction: paralstonite is trimorphous with alstonite and barytocalcite, but is the rarer polymorph, although occuring in similar geological settings, namely hydrothermal veins.
Occurrence in Wales: first noted in Wales during analysis of a white powdery phase coating fractures in Precambrian sediments from Dolyhir Quarry. X-ray diffraction revealed the presence of two phases, alstonite and paralstonite. Visual distinction between the two phases is difficult, given the fine-grained nature of the powder, but paralstonite is generally more steeply terminated. Only a few tens of specimens are recorded, but given the extremely fine-grained nature of this mineral, it may be more common, but is easily overlooked. Only by detailed X-ray analysis will further specimens come to light.

Key Localities:

  • Dolyhir Quarry, Old Radnor, Powys: described in detail by Cotterell and Dean (2007) paralstonite is rare, forming inconspicuous microcrystalline crusts on harmotome, calcite, alstonite and ewaldite within thin fractures in Precambrian Yat Wood Formation sediments and dolerite. Scanning electron microscopy of typical chalky white crusts reveals elongated 'rice grain'-like bipyramidal crystals. The original specimen, collected in June 2001 by A. Dean which was collected from the northern face on the upper level of the quarry is preserved in the mineral collection of the National Museum of Wales (specimen no. NMW 2002.31G.M.1). Further specimens were discovered, again, in the northern part of the quarry, in May 2006, but by far the richest examples in terms of crystal size (up to 0.1 mm long) and coverage were found by N. Hubbard in 2010 (Cotterell et al., 2011).

References:

  1. Cotterell, T.F. & Dean, A.C., 2007. The first British occurrence of paralstonite at Dolyhir Quarry, Old Radnor, Powys, Wales. UK Journal of Mines and Minerals, 28, 31-35.
  2. Cotterell, T.F., Green, D.I., Hubbard, N., Mason, J.S., Starkey, R.E. and Tindle, A.G., 2011. The Mineralogy of Dolyhir Quarry, Old Radnor, Powys, Wales. UK Journal of Mines and Minerals, 32, 5-61.