Melanterite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron sulphate hydrate
Chemical Formula: Fe2+SO4.7H2O
Method(s) of Verification: Parys Mountain - variety pisanite, chemical analysis (Bor, 1950); Great Opencast, Parys Mountain - confirmed as melanterite, XRD (NMW X-1090); Deep Navigation Colliery - XRD (National Museum of Wales, specimen no. NMW 26.151.GR.1).

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Sulphate of iron (melanterite) from Sowerby (1811). The top left figure is a stalactite from North Wales, demonstrating surface alteration.
Sealed glass tube (7.7 cm long) containing fragments of pale green melanterite. Cae Coch Mine, Trefriw, Gwynedd. National Museum of Wales Collection (NMW 84.24G.M.1). Photo T.F. Cotterell, © National Museum of Wales.
Introduction: melanterite commonly forms as a post-mining phase in old coal and metalliferous mine workings as a result of the oxidation of pyrite.
Occurrence in Wales: melanterite has been known from Wales for a very long time (see for example the figured specimen in Sowerby, 1811). Unfortunately as with many early mineralogical references locality details are vague. However, in 1858 Greg & Lettsom described melanterite from Parys Mountain on Anglesey. Melanterite, is not widespread, but has been shown to occur in some abundance given the right geological conditions. Some early mineral texts refer to iron vitriol (melanterite) occurring within coal mines. Although not specifically Welsh coal mines, melanterite was, discovered forming stalactitic growths in collieries within the Coal Measures of South Wales during the early part of the 20th Century. Invariably melanterite is intimately associated with decaying pyrite and as such melanterite is likely to occur underground within pyrite dominated workings as seen at Cae Coch Mine where massive pyrite was worked for its sulphur content.

Key Localities:

  • Cae Coch Mine, Trefriw, Gwynedd: seams of green melanterite occur within underground workings at this disused pyrite mine. Examples include National Museum of Wales specimen no. NMW 84.24G.M.1.
  • Central Wales Orefield: minute crystals of a pale blue mineral resembling the cuprian variety, pisanite have been observed in weathered ore specimens from Brynarian, Frongoch and Cwmystwyth mines. The context of this occurrence suggests a post-collection origin.
  • Llanharry Iron Mine, Llantrisant, South Wales: pale blue melanterite on pyrite was collected during the 1970s.
  • Parys Mountain, Anglesey: Greg & Lettsom (1858) describe stalactitic melanterite, ‘of a fine green colour, formerly, at Pary’s mine, Anglesey’. The copper-bearing variety, pisanite is described by Bor (1950) with specimens collected by R.J. King during the 1950s from the 30-fathom level on Careg-y-ddol Lode in the collection of the National Museum of Wales. Further specimens of pisanite from the Hillside opencast form part of the Harwood Collection subsequently incorporated into the R.J. King Collection and now held by the National Museum of Wales.
  • Red Dragon Mine, Dinas Mawddwy, Gwynedd: massive bottle green melanterite formed through alteration of pyrite occurs underground (Bevins & Mason, 1998). Specimens are attractive but difficult to conserve.
  • South Wales Coalfield: melanterite is well known from the Coal Measures of South Wales (Bevins, 1994; National Museum of Wales & NHM collections). Stalactitic melanterite has been recovered from Deep Navigation Colliery, Treharris (NMW 26.151.GR.1), while melanterite occurs in association with alunogen from the Havod Vein at Cymmer Colliery, Porth (NMW 32.236.GR.1). A specimen of melanterite from Abertysswg Colliery, Rhymney is housed in the mineral collection of the Natural History Museum, London (Bevins, 1994).

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  3. Bor, L., 1950. Pisanite from Parys Mountain, Anglesey. Mineralogical Magazine, 29, 63-67.
  4. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  5. Greg, R.P. & Lettsom, W.G., 1858. Manual of the Mineralogy of Great Britain and Ireland. John van Voorst, London, 483pp.
  6. Hall, T.M., 1868. The Mineralogists' Directory. London, 168pp.
  7. Jenkins, D. A., Johnson, D. B. & Freeman, C., 2000. Mynydd Parys Cu-Pb-Zn mines: mineralogy, microbiology and acid mine drainage. pp. 161-179. In: Environmental Mineralogy: Microbial Interactions, Anthropogenic Influences, Contaminated Land and Waste Management (Cotter-Howells, J. D., Campbell, L. S., Valasami-Jones, E. & Batchelder, M., eds.). The Mineralogical Society of Great Britain & Ireland, London.
  8. Johnson, D.B., Kelso, W.I., & Jenkins, D.A., 1979. Bacterial streamer growth in a disused pyrite mine. Environmental Pollution, 18, 107-118.
  9. Sowerby, J., 1811. British Mineralogy. Volume IV, Richard Taylor, London.

There are no references for this specimen.