Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Sodium calcium aluminium silicate hydrate
Chemical Formula: NaCa2Al5Si5O20.6H2O
Method(s) of Verification: Marchlyn Mawr - wet chemical analysis (Williams, 1924); Usk – EMPA (D.T. Moffat, unpublished data)

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal: amygdale infill & veins in volcanic rocks
Introduction: thomsonite is a zeolite mineral and almost all occurrences are of hydrothermal origin. It is found commonly in vesicles and veins in altered igneous rock, typically of basaltic composition, and also in some alkali igneous rocks. It may occur with other zeolite minerals such as mesolite and chabazite
Occurrence in Wales: two occurrences of thomsonite are known from Wales, both as vesicular infillings in basaltic rocks which formed by local hydrothermal activity during the cooling of the magma.

Key Localities:

  • Marchlyn Mawr, Snowdonia, Gwynedd: fibrous, radiating aggregates of thomsonite occur as an infilling to vesicles, up to 20 mm in diameter in the Marchlyn Dyke; an olivine dolerite of Palaeogene age (c. 60 million years old), exposed in Central Snowdonia (Williams, 1924, 1930).
  • Usk, Monmouthshire: thomsonite has been reported from amygdales in monchiquitic basanite (alkali basalt) from dyke rocks at Great House and Glen Court, both near Usk (D.T. Moffat, unpublished data quoted in Bevins, 1994).


  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. 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.
  3. Williams, D., 1924. On two olivine-dolerite dykes in Snowdonia. Proceedings of the Liverpool Geological Society, 14, 38-47.