Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium silicate carbonate sulphate hydroxide hydrate
Chemical Formula: Ca3Si(OH)6(CO3)(SO4).12H2O
Method(s) of Verification: Merthyr Tydfil – XRD, Differential Thermal Analysis, IR (Wilson, 1978)

Chemical Group:

  • Silicates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: minerals on an artificial substrate
Introduction: thaumasite is a white to yellow mineral which belongs to the etteringite group. It may form as a late-stage mineral in sulphide ore deposits or by the hydrothermal alteration of basalt and tuffs by seawaters where it is typically associated with zeolite minerals. However, the most common occurrence of thaumasite is as a supergene mineral, either occurring on mine tips or as a reaction product of sulphate-attack on concrete. As thaumasite is not a binding mineral in concrete but forms from such minerals it is responsible for significant damage to concrete. Data from the concrete industry indicates that only very low sulphate concentrations are required to form thaumasite.
Occurrence in Wales: the two reported occurrences of thaumasite from Wales both have a supergene origin, one occurring on man-made substrate and the other forming a supergene mineral on a copper mine tip

Key Localities:

  • Glasdir Mine, Dolgellau Gold-belt, Gwynedd: the late George Ryback located thaumasite as white, powdery, acicular crystals in a mineralized breccia, or possibly a post-mining cemented aggregate at this locality (as reported in Bevins, 1994).
  • Merthyr Tydfil, South Wales: thaumasite has been identified in ‘abundant quantities’ on weathered furnace slag from the now disused tips of the Dowlais and Cyfartha Iron Works in the vicinity of Merthyr Tydfil (Wilson, 1978). The Dowlais slag heap, know as the ‘White Tip’, has now been landscaped and removed. At both localities thaumasite is described as ‘ white, vesicular infillings or encrustations with a fluffy or felted texture’ (Wilson, 1978). Individual acicular fibres range in width from 5-10μm, up to 500 μm long and are associated with calcite.


  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Wilson, M.J., 1978. Occurrence of thaumasite in weathered furnace slag, Merthyr Tydfil. Mineralogical Magazine, 42, 290-291.