Hessite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Silver telluride
Chemical Formula: Ag2Te
Method(s) of Verification: Clogau Mine - EMPA (Naden, 1988).

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
Photomicrograph of hessite in polished section: 3 grains (dark grey, max 80 µm) enclosed by tellurbismuth (pale grey-white). Clogau Mine. © J.S. Mason.
Introduction: hessite is typically found in precious metal-telluride associations with gold occurring with bismuth, lead, silver or gold tellurides in quartz-carbonate lodes and stockworks.
Occurrence in Wales: hessite was first identified in the UK from Clogau Mine by Gilbey (1968) and further occurrences, from different sections of the mine, have since been described by Naden (1988) and Mason et al. (2002). It is one of a number of rare tellurides occurring at the mine which may only be identified using ore petrography techniques backed up with electron microprobe analyses, although it is easier to identify than some of the other tellurides due to its distinctive, sometimes slightly brownish, dull grey colour.

Key Localities:

  • Clogau Mine, Bontddu, Gwynedd: hessite is present in the localized ore-zones where gold and tellurides are present. It is not known where Gilbey's samples were collected within the mine: those described by Naden (1988) were from the 4 Level, Llechfraith section and occurred as 30-40 µm inclusions in galena, while those illustrated by Mason et al. (2002) came from the Tyn Y Cornel section and consisted of hessite associated with tellurobismuth, tetradymite, galena, electrum and chalcopyrite cementing shattered arsenopyrite.

There are no key localities for this specimen.

References:

  1. 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 Unpublished Ph.D thesis, University of London, UK.
  2. 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.
  3. 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 Unpublished Ph.D thesis, University of Aston, UK.

There are no references for this specimen.