Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence - 2nd UK recording
Chemical Composition: Bismuth telluride
Chemical Formula: Bi2Te3
Method(s) of Verification: Clogau Mine - visual; Ffridd-Goch Mine - EMPA Gilbey (1968)
- Hydrothermal: mesothermal polymetallic veins
Tellurobismuthite with gold, Clogau Mine, Dolgellau Gold-belt. Specimen 3 cm across. National Museum of Wales Collection. © National Museum of Wales.
Silvery tellurobismuthite on shale, Clogau Mine, Dolgellau Gold-belt. National Museum of Wales Collection (NMW 85.70G.M.1). © National Museum of Wales.
Tellurobismuthite, Clogau Mine, polished section showing a complex symplectite-like intergrowth consisting of tellurobismuthite (white), galena (blue-grey) and tetradymite (pale grey) is revealed. Field of view 0.2 mm. © J.S. Mason.
Introduction: tellurobismuthite occurs in association with other telluride minerals, native bismuth, bismuthinite, gold and galena in many gold deposits, particular those of the gold-quartz and skarn types. In paragenetic terms gold is often contemporaneous with the bismuth tellurides. Tellurobismuthite is distinctive in hand specimen, forming glittering bright silvery metallic flakes scattered through quartz; however, examination in polished section reveals a more complicated situation in which tetradymite and galena often occur in intimate intergrowth.
Occurrence in Wales: The presence of 'telluric-bismuth' in the Dolgellau Gold-belt has been known of for nearly 150 years (Readwin, 1860). Kingsbury (1965) first reported the presence of tellurobismuthite from the Clogau mine and he considered previous identifications of tetradymite (e.g. Smyth, 1862) to have been erroneous observations of this species. However, the occurrence of both species was demonstrated by Gilbey (1968). Kingsbury (1965) and Embrey (1978) cited this as the first UK occurrence of tellurobismuthite: however, this is not the case as it had previously been reported from Gwavas Quarry in Cornwall by Siddiqui (1964).
- Clogau Mine, Bontddu, Gwynedd: tellurobismuthite is locally common in gold-rich ore-shoots within the mesothermal lodes worked at this mine: outside of such shoots it is very rare. It is commonly intergrown in a complex, symplectite-like texture with galena and tetradymite (Mason et al., 2002); associated minerals are gold, electrum, hessite, altaite and minor aleksite, pilsenite, native bismuth and bismuthinite. Because it is a component of the 'gold ore', very little occurs on any of the tips, the ore having been hand-picked and milled to recover the gold. However, the National Museum of Wales' collection includes a suite of hand specimens recovered during mining operations, some of which are very rich.
- Ffridd-Goch Mine, Llanfachreth, Gwynedd: tellurobismuthite was recorded by Gilbey (1968) and noted in polished sections cut from his samples which were donated by him to the National Museum of Wales in 2000. In contrast to Clogau, the tellurobismuthite is fine-grained and is only visible under high-powered magnification. Electron microprobe studies of this material revealed a number of other bismuth tellurides to be present, which remain to be identified (J.S. Mason, unpublished data). Very minor occurrences of tellurobismuthite were also noted without description by Gilbey (1968) from Caegwian, Vigra and Cefn Coch mines.
- Embrey, P.G., 1978. Fourth supplementary list of British minerals. Mineralogical Magazine, 42, 169-177.
- 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.
- Kingsbury, A.W.G., 1965. Tellurbismuth and meneghinite, two minerals new to Britain. Mineralogical Magazine, 35, 424-426.
- 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.
- Readwin, T.A., 1860. The gold discoveries in Merionethshire. Transactions of the Manchester Geological and Mining Society, 2, 97-101.
- Siddiqui, S.F.A., 1964. The geology and mineralisation of the area between Newlyn and Mousehole, Cornwall. Unpublished Ph.D. thesis, University of London.
- Smyth, W.W., 1862. Gold mining at Clogau, North Wales. Mining and Smelting Magazine, 1, 359-366.