Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Topaz

Crystal System: Orthorhombic
Formula: Al2SiO4(F,OH)2
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Aluminium silicate fluoride hydroxide
Method(s) of Verification: all occurrences identified by polarizing microscope.
Chemical Group:
  • Silicates
Geological Context:
  • Igneous
  • Metamorphic : skarn mineralization
  • Metamorphic
  • Sedimentary : allogenic (detrital)
Introduction: topaz is a silicate mineral containing fluorine. It occurs most commonly in acid igneous rocks, where it is found in veins and cavities, and is formed by late-stage volatile-rich fluids. Topaz is also found as a heavy mineral in detrital sediments.
Occurrence in Wales: topaz is not a common mineral in Wales with just two localized occurrences reported in the literature.
Key Localities:
  • Bwlch-y-Cywion, Gwynedd: Williams (1930) described the presence of topaz in the Bwlch-y-Cywion Intrusion, Snowdonia, Gwynedd, which he identified in heavy mineral residues separated from a crush whole rock sample. In the same work Williams (1930) also described topaz from the contact aureole of the same intrusion. The topaz forming irregular, colourless grains up to 3 mm across and commonly hosting smaller inclusions of almandine garnet. The occurrence of topaz in both the intrusion and the contact aureole indicates that volatiles concentrated in the magma, migrated from the intrusion into the host rock.
  • Ludchurch, Pembrokeshire: topaz was reported by Griffiths & Stuart (1940) along with diaspore and other detrital minerals in sandy boulder clay from Ludchurch.
References:
  • Griffiths, J.C. & Stuart, A., 1940 An occurrence of detrital diaspore in South Wales.  Geological Magazine, 77, 74-76.
  • Williams, D., 1930 The geology of the country between Nant Peris and Nant Ffrancon (Snowdonia).  Quarterly Journal of the Geological Society of London, 86, 191-232.