Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Magnesium sulphate hydrate
Chemical Formula: MgSO4.6H2O
Method(s) of Verification: Treharris - XRD (National Museum of Wales, NMW X-282).

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: hexahydrite is a member of the hexahydrite group, that forms typically through the dehydration of epsomite.
Occurrence in Wales: a number of specimens from Treharris in the National Museum of Wales Mineral Collection display hexahydrite alteration from epsomite. This is the only record of hexahydrite from Wales.

Key Localities:

  • Treharris, South Wales: hexahydrite is present on a number of specimens labelled ‘epsomite’ in the mineral collection of the National Museum of Wales. It is likely that this formed through the dehydration of epsomite in a museum environment (Bevins, 1994).


  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.