Anhydrite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Calcium sulphate
Chemical Formula: CaSO4
Method(s) of Verification: Parys Mountain - optical properties (polarizing microscope).

Chemical Group:

  • Sulphates

Geological Context:

  • Hydrothermal: sedimentary exhalative deposits
Introduction: anhydrite occurs commonly as an evaporite mineral in arid regions; in situations where more moisture is present the hydrated sulphate gypsum is more likely to occur. Another setting in which anhydrite may occur is within syngenetic exhalative (Kuroko-type) sulphide deposits.
Occurrence in Wales: anhydrite is rare in Wales. It occurs on Parys Mountain, which is a modified syngenetic exhalative ore-deposit, while certain horizons in the Triassic of South Wales, such as near Sully in South Glamorgan, show replacement features indicating that evaporites, including anhydrite, were originally present (Tucker, 1977; Waters & Lawrence, 1987).

Key Localities:

  • Parys Mountain, Anglesey: anhydrite has been observed forming cores to voids in siliceous sinter from the Carreg-y-doll Lode zone (Pointon & Ixer, 1980).

References:

  1. Pointon, C.R. & Ixer, R.A., 1980. Parys Mountain mineral deposit, Anglesey, Wales: geology and ore mineralogy. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 89, B143-B155.
  2. Tucker, M.E., 1977. The marginal Triassic deposits of South Wales: continental facies and palaeogeography. Geological Journal, 12, 169-188.
  3. Waters, R.A. & Lawrence, D.J.D., 1987. Geology of the South Wales Coalfield, Part III, the Country around Cardiff. 3rd edition. Memoirs of the Geological Survey of England and Wales. Explanation of Sheet 263.