Tetradymite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Bismuth tellurium sulphide
Chemical Formula: Bi2Te2S
Method(s) of Verification: Clogau Mine - EMPA (Gilbey, 1968)

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
Tetradymite (L), pale grey with a hint of blue, galena (centre), a distinctive blue-grey, and tellurobismuthite, a bright white colour, all part of the "telluride assemblage" at Clogau. Field of view 0.25 mm. Image: J.S. Mason.
Polished section of intergrown tetradymite (top), tellurobismuthite (bottom & R) and galena (blue-grey, bottom left). A small tension-gash has formed during post-depositional deformation. Field of view 1 mm. Image: J.S. Mason.
Introduction: tetradymite occurs with other bismuth tellurides and gold in several types of gold deposits, in particular gold-quartz lodes and auriferous skarns. It is usually impossible to distinguish tetradymite from tellurobismuthite in hand specimen - both form silvery flakes with a high metallic lustre, scattered in quartz. Ore petrology is required in order to distinguish them.
Occurrence in Wales: the occurrence of tetradymite at Clogau Mine was first reported by Smyth (1862) and repeated by Forbes (1867), Readwin (1888), and Andrew (1910). However, its presence was subsequently questioned by Kingsbury (1965), on the basis of an examination of specimens from the Clogau and Vigra mines, in which he only identified tellurobismuthite. Three years later, Gilbey (1968) corrected the situation by showing that both tellurobismuthite and tetradymite, as well as a number of other tellurides, were in fact present in complex intergrowths, further described and illustrated by Mason et al. (2002).

Key Localities:

  • Clogau Mine, Bontddu, Gwynedd: occurs intimately intergrown with tellurobismuthite and galena plus minor hessite, in association with gold and electrum. Aleksite, altaite and pilsenite are sometimes associated (Gilbey, 1968; Naden, 1988; Mason et al., 2002). Due to the association with 'gold-ore', the telluride-bearing material was picked and milled, so that only traces now occur on the tips. A suite of hand specimens rich in visible tellurides, recovered during previous episodes of mining, is preserved along with several polished sections in the collection of the National Museum of Wales.

References:

  1. Andrew, A.R., 1910. The geology of the Dolgelley gold-belt, North Wales. Geological Magazine, 47, 159-171, 201-221, 261-271.
  2. Forbes, D., 1868. Researches in British Mineralogy. The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 35, 171-184.
  3. Gilbey, J.W., 1968. The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration. Unpublished Ph.D thesis, University of London, UK.
  4. Kingsbury, A.W.G., 1965. Tellurbismuth and meneghinite, two minerals new to Britain. Mineralogical Magazine, 35, 424-426.
  5. Mason, J.S., Bevins, R.E. & Alderton, D.H.M., 2002. Ore Mineralogy of the mesothermal gold lodes of the Dolgellau Gold Belt, North Wales. Transactions of the Institution of Mining and Metallurgy (Section B, Applied earth science), 111, B203-B214.
  6. Naden, J., 1988. Gold mineralisation in the Caledonides of the British Isles with reference to the Dolgellau Gold Belt and the Southern Uplands of Scotland. Unpublished Ph.D thesis, University of Aston, UK.
  7. Readwin, T.A., 1888. Gold in Wales. London, 12pp.
  8. Smyth, W.W., 1862. Gold mining at Clogau, North Wales. Mining and Smelting Magazine, 1, 359-366.