Aragonite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium carbonate
Chemical Formula: CaCO3
Method(s) of Verification: Britannic Merthyr Colliery - XRD at the National Museum of Wales (NMW X-1391); Ogof Daren Ciliau - XRD at the National Museum of Wales (NMW X-320).

Chemical Group:

  • Carbonates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
  • Supergene: post-mining oxidation & weathering deposits
Acicular aragonite crystals from Britannic Merthyr Colliery also known as (Scotch Colliery), Gilfach Goch. Field of view 1 cm across. I.E. Jones Collection (no. W94.39). Photo M.P. Cooper, © National Museum of Wales.
Introduction: aragonite is a metastable form of calcium carbonate and tends in time to revert to the more stable polymorph, calcite. Most aragonite occurrences in Wales are thus of relatively recent formation, in environments where calcium carbonate is being dissolved and redeposited. Best known from flowstone formations in limestone caves and karstic springs, aragonite is also widespread as a supergene species in weathered carbonate-bearing hydrothermal mineral deposits.
Occurrence in Wales: limestone caves with attendant flowstone deposits and/or karstic springs are restricted in Wales to two main areas where Carboniferous Limestone outcrops extensively. These comprise the Halkyn-Minera district of NE Wales and the northern outcrop of the limestone around the South Wales Coalfield. These areas contain the chief Welsh aragonite occurrences, in terms of quantity. The scattered occurrences related to supergene alteration of hydrothermal carbonates are characterized by small quantities of aragonite as small to microscopic crystals.

Key Localities:

  • Central Wales Orefield: aragonite occurs at a handful of localities where it has formed during supergene weathering of ferroan vein carbonates. Typically it forms radiating sprays of bladed or prismatic crystals in cavities in the original carbonate. Notable localities are Hafan, Henfwlch, Esgairfraith and Castell mines (Bevins & Mason, 1997); additionally it has been found at Penyclun and at South Nantycar Mine (British Micromount Society Newsletter No. 34). At Dolaucothi Gold Mine stalactitic aragonite occurs.
  • Halkyn-Minera district, NE Wales: aragonite is widespread as stalactitic formations in caves and also as karstic spring deposits in this region. One of the more notable occurrences, temporarily exposed in 1998, was at Hendre Gravel Quarry near Mold. Here, a mass of Recent fluvial sand and gravel, "the size of a house" according to the quarry manager, was found to be cemented by vuggy crystalline aragonite. The aragonite had clearly been deposited from karstic water-flow through the permeable sediments, and the cemented deposit was only removed with some difficulty (Bevins & Mason, 1999).
  • Llantrisant area, South Wales: Alabaster (1990) recorded aragonite replacing alstonite, forming felted overgrowths and acicular radiating sprays, from a small mine dump near Llantrisant.
  • North Wales: aragonite has been reported from a trial level near Bontddu (Saich & Rust, 1987); it also occurs at Hendre Quarry, Glyn Ceiriog, Clwyd (Starkey et al., 1991) and at Ty Gwyn Mine, Great Orme, Llandudno, associated with malachite.
  • South Wales (limestone outcrop): aragonite has been confirmed from Ogof Daren Ciliau, Mynydd Llangattwg, Gwent (Kendall, 1988). It occurs widely across this area in stalactitic formations.
  • South Wales Coalfield: aragonite is probably widespread in the South Wales coalfield. Localities where it has been confirmed include Britannic Merthyr Colliery, Gilfach Goch; Bedwas Colliery, Trethomas; Windsor Colliery, Senghenydd; Marine Colliery; Cwm Colliery; Deep Navigation Colliery; Ferndale Colliery; Gelli Colliery (I.E. Jones, unpublished data).

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. Bevins, R.E. & Mason, J.S., 1999. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Clwyd. National Museums & Galleries of Wales, Cardiff.
  3. Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  4. Kendall, A., 1988. Aragonite in Ogof Daren Cilau. Cave Science, 15, 83-84.
  5. Saich, D.A. & Rust, S.A., 1987. Micro-minerals from a trial level in Wales. U.K. Journal of Mines and Minerals, No. 3, 3-4.
  6. Starkey, R.E., Hubbard, N. & Bayley, M.P., 1991. Mineralization at Hendre Quarry, Glyn Ceiriog, Clwyd, Wales. U.K. Journal of Mines and Minerals, No. 10, 48-51.