Talc

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Magnesium silicate hydroxide
Chemical Formula: Mg3Si4O10(OH)2
Method(s) of Verification: St. David's Head - EMPA (R.E. Bevins, unpublished data)

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
Introduction: talc is a metamorphic mineral that commonly occurs in low-grade metamorphic and hydrothermally altered ultrabasic rocks but is also found in metamorphosed siliceous dolomites. Talc only forms where there is insufficient aluminium, calcium or potassium to combine with magnesium to form other minerals, such as chlorite, tremolite or phlogopite.
Occurrence in Wales: all recorded occurrences of talc in Wales are derived from the metamorphism and alteration of basic or ultrabasic igneous rocks (Readwin, 1888, Greenly, 1919, Sargent, 1925, Roach, 1969, Maltman, 1977, Metcalfe, 1990).

Key Localities:

  • Llanelwedd Quarry, Builth Wells, Powys: Metcalfe (1990) recorded talc associated with saponite in quartz-calcite veins cutting altered basaltic lavas. Interpreted as being produced by post-magmatic, hydrothermal, alteration.
  • Penmaenmawr, Conwy: Sargent (1925) recorded talc associated with calcite and quartz in thin veins (< 1.5 cm) cutting the Penmaenmawr Intrusion.
  • St. David's Head, Pembrokeshire: Bevins et al. (1994) report talc replacing olivine (mg-rich) in xenolitic gabbros of the St. David’s Head Intrusion.

References:

  1. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  2. Maltman, A.J., 1977. Serpentinites and related rocks of Anglesey. Geological Journal, 12, 113-128.
  3. Metcalfe, R., 1990. Fluid-rock interaction and metadomain formation during low-grade metamorphism in the Welsh marginal basin. Unpublished Ph.D. thesis, University of Bristol.
  4. Readwin, T.A., 1888. Gold in Wales. London, 12pp.
  5. Roach, R.A., 1969. The composite nature of the St. David's Head and Carn Llidi intrusions of North Pembrokeshire. In: Wood, A. (ed.) The Pre-Cambrian and Lower Palaeozoic rocks of Wales, University of Wales Press, Cardiff, 409-433.
  6. Sargent, H.C., 1924. Notes on the petrology of Penmaenmawr Mountain (Part 1). Proceedings of the Liverpool Geological Society, 14, 82-89.