Boulangerite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead antimony sulphosalt
Chemical Formula: Pb5Sb4S11
Method(s) of Verification: Alltycrib Mine - optical properties; Bwlch Mine - EMPA; Clogau Mine - EMPA; Esgairhir Mine - optical properties.

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: epithermal polymetallic veins & pipes
Boulangerite inclusions in galena, the longest is 0.05 mm (50 ┬Ám). Alltycrib Mine. Polished section in plane polarized light (left) and crossed polars (right). The boulangerite is not very conspicuous in the left image. © J.S. Mason.
Introduction: boulangerite is one of a large group of lead-antimony sulphosalts that commonly occur with stibnite and galena in low-temperature veins and stockworks. It may also occur in polymetallic ore deposits, as part of an inclusion-forming assemblage hosted primarily by galena. Lead-antimony sulphosalts tend to be so similar to one another in appearance that when massive they require petrological and chemical analysis to distinguish them. However, in polished section boulangerite is relatively easy to identify when present as inclusions within galena, as due to its anisotropy it blends completely into the background and suddenly reappears as the stage is rotated under crossed polars.
Occurrence in Wales: the four recorded occurrences of boulangerite in Wales are all of relatively recent discovery. One example is noted from the Dolgellau Gold-belt (Naden, 1988), one from the unusual Pb-Sb deposit at Bwlch near Deganwy (Bevins et al., 1988), and two from the Central Wales Orefield (Mason, 1994; 1998). In all cases these reflect the increasing use of reflected light microscopy in the investigation of Wales' ore deposits.

Key Localities:

  • Alltycrib Mine, Tal-y-bont, Ceredigion: elongated microscopic inclusions of boulangerite have been identified in polished sections of foliated galena which occurs in some abundance on the tips of this mine (Mason, 1994; 1998).
  • Bwlch Mine, Deganwy, Gwynedd: boulangerite has been tentatively identified in association with stibnite and other Pb-Sb sulphosalts (Bevins et al., 1988).
  • Clogau Mine, Bontddu, Gwynedd: Naden (1988) recorded isolated inclusions of boulangerite in quartz adjacent to gold and pyrite, on un-numbered samples from the Kingsbury Collection (Natural History Museum).
  • Esgairhir Mine, Tal-y-bont, Ceredigion: present as microscopic inclusions in galena along with bournonite and ullmannite (Mason, 1994; 1998).

There are no key localities for this specimen.

References:

  1. Bevins, R.E., Alderton, D.H.M. & Horak, J.M., 1988. Lead-antimony mineralisation at Bwlch Mine, Deganwy, Wales. Mineralogical Magazine, 52, 391-394.
  2. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  3. Mason, J.S., 1998. Tucekite, a mineral new to Britain, and other rare ore minerals from the Central Wales Orefield. UK Journal of Mines and Minerals, 19, 30-36.
  4. 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.