Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead antimony sulphide
Chemical Formula: Pb9Sb8S21
Method(s) of Verification: Bwlch Mine - XRD & EMPA (Bevins et al., 1988)

Geological Context:

  • Hydrothermal: epithermal polymetallic veins & pipes
Introduction: semseyite is found, along with other related lead-antimony sulphosalts, in conjunction with stibnite and galena in complex hydrothermal vein and breccia assemblages typical of the epithermal Pb-Sb association. A feature of this group of minerals is that it is often impossible to identify them in hand specimen and even in polished section they can be remarkably difficult to distinguish. Consequently, verification requires detailed analytical techniques in the majority of occurrences.
Occurrence in Wales: semseyite was first recorded from Wales by Russell (1944), from an isolated occurrence of Pb-Sb mineralization worked at the Bwlch Mine near Deganwy on the North Wales coast. Further, detailed investigation of this mineralization (Bevins et al., 1988) provided analytical details of the occurrence and additionally confirmed the presence of a number of related Pb-Sb sulphosalts.

Key Localities:

  • Bwlch Mine, Deganwy, Gwynedd: intergrown with other Pb-Sb sulphosalts and stibnite in small patches to 3 cm in diameter showing well-defined cleavages and as flat prismatic crystals up to 2 mm in length, associated with quartz in a stockwork-like group of veins hosted by altered spherulitic acid ash-flow tuff (Russell, 1944; Bevins et al., 1988).


  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. Russell, A., 1944. Notes on some minerals either new or rare in Britain. Mineralogical Magazine, 27, 1-10.