Epsomite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Magnesium sulphate hydrate
Chemical Formula: MgSO4.7H2O
Method(s) of Verification: Cymmer Colliery - XRD at the National Museum of Wales (XRD numbers NMW X-861, X-1932 and X-1941); Deep Navigation - XRD at the National Museum of Wales (NMW X-282, X-1935 and X-1943).

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Sedimentary: chemical deposit (evaporites & carbonates)
Introduction: epsomite forms as secondary efflorescence on the walls of underground mine workings and as evaporites in salt lakes and oceanic salt deposits.
Occurrence in Wales: the only records of epsomite from Wales are from collieries in the South Wales Coalfield where it occurs associated with other sulphate precipitates (Bevins, 1994; Cotterell, 2009).

Key Localities:

  • Cymmer Colliery, Porth, South Wales: a crust of epsomite is associated with pickeringite from the Lower Five-Feet (Jubilee) Seam (National Museum of Wales specimen, no. NMW 27.383.GR.1). Epsomite has also been detected intergrown with pickeringite from the Seven-Feet Seam (National Museum of Wales specimen, no. NMW 31.573.GR.1) as reported by Cotterell (2009).
  • Deep Navigation Colliery, Treharris, South Wales: Bevins (1994) described stalactitic masses with hexahydrite (possibly post-collection in origin) on a specimen in the National Museum of Wales collection (no. NMW 27.128.GR.2). A second specimen was recorded in the R.S.W. Braithwaite collection (no. RSWB 68-347). The museum specimen is derived from the Two-Feet-Nine Seam and displays massive opaque white epsomite with colourless crystallized tschermigite (Cotterell, 2009). Epsomite has been identified by XRD on another museum specimen (NMW 26.151.GR.4) as a yellow surface coating (with probable tamarugite) on fibrous pickeringite (Cotterell, 2009).

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Cotterell, T.F., 2009. A review of halotrichite group minerals in Wales. UK Journal of Mines and Minerals, 30, 43-47.