Freibergite

Crystal System: Cubic
Status of Occurrence: Unconfirmed Occurrence
Distribution: Uncommon
Chemical Composition: Silver copper antimony sulphide with variable amounts of arsenic and other metals
Chemical Formula: (Ag,Cu)10(Zn,Fe)2(Sb,As)4S13
Method(s) of Verification: no published chemical data, all identification by reflected light microscopy.

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: epithermal polymetallic veins & pipes
  • Hydrothermal: volcanogenic massive sulphides
Introduction: freibergite is a silver-rich member of the tetrahedrite group. The most recent definition of freibergite (Johnson et al., 1986) has, in the (Ag,Cu)10 part of the chemical formula, Ag present in equal or greater proportions than Cu. Freibergite tends to occur together with other silver minerals, galena and sulpharsenides in high-grade silver veins of low- to medium temperature of formation.
Occurrence in Wales: several occurrences of silver-bearing tetrahedrite have been described, with analyses, from Wales (Gilbey, 1968; Mason, 1998; Mason & Bevins, 2002). Gilbey (1968) described 'freibergite' from Tyddyn Gwladys mine in the Dolgellau Gold-belt, with 23.7% silver. However, in all of the above-cited analyses, Cu dominates Ag: therefore these are all merely argentian tetrahedrites with near-freibergite compositions, based on the latest definition of the mineral.

Key Localities:

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 Unpublished Ph.D thesis, University of London, UK.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. Johnson, N.E., Craig, J.R. & Rimstidt, J.D., 1986. Compositional trends in tetrahedrite. Canadian Mineralogist, 24, 385-397.
  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. Mason, J.S. & Bevins, R.E., 2002. St Elvis Mine, Solva, Pembrokeshire: Another Elizabethan tetrahedrite occurrence? British Mining 71, 5-12, Northern Mines Research Society.

There are no references for this specimen.