Cymrite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - Type Locality In Wales
Distribution: Rare
Chemical Composition: Barium aluminium silicate hydrate
Chemical Formula: BaAl2Si2O8H2O
Method(s) of Verification: Benallt Mine - XRD (Natural History Museum ref. 1944,36), wet chemistry (ref. 1944,48) (Campbell et al., 1949).

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Metamorphic: low-grade
Scanning electron micrograph of sheaf-like aggregate of prismatic cymrite from Benallt Mine. National Museum of Wales specimen (NMW 2006.15G.M.2), © National Museum of Wales.
Cream coloured pseudohexagonal cymrite crystals to 1 mm across from Benallt Mine. D.I. Green photo, © D.I. Green.
Introduction: cymrite is roughly equivalent to hydrated celsian and appears to form in barium-rich rocks under conditions of low grade metamorphism. Experimental work suggests that it forms from baryte under alkaline conditions and breaks down to celsian and water at around 300°C and 3 kb (or lower temperatures at low pressure) (Hsu, 1994).
Occurrence in Wales: cymrite was first described from Benallt Mine, near Rhiw, Llŷn, North Wales (Campbell Smith et al., 1949) and this locality remains the only known Welsh occurrence of the mineral. Campell Smith et al. (1949) examined material collected by A.W. Groves, who had recently surveyed the mines, but only minute amount of cymrite were present, such that two specimens, from different locations in the mine, were required to characterize fully the new mineral. This type material is held in the Natural History Museum (B.M. 1944, 36 and B.M.1944, 48).

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: dark compact manganese ore from the 60-ft level of the No. 1 ore-body contains colourless plates up to 7 mm across and about 0.5 mm thick (Natural History Museum specimen B.M. 1944,48). In contrast the No. 5 ore-body contains white fibrous, satiny crystals up to 1 mm in length, closely associated with ganophyllite.

References:

  1. Campbell Smith W., Bannister, F.A. & Hey, M.H.,, 1949. Cymrite, a new barium mineral from the Benallt manganese mine, Rhiw, Carnarvonshire. Mineralogical Magazine, 28, 676-681.
  2. Groves, A.W., 1952. Wartime investigation into the hematite and manganese ore resources of Great Britain and Northern Ireland. Ministry of Supply, Permanent Records of Research and Development. Monograph No. 20-703.
  3. Hsu, L.C., 1994. Cymrite: new occurrence and stability. Contributions to Mineralogy & Petrology, 118(3), 314–320