Mimetite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Lead chloro-arsenate
Chemical Formula: Pb5(AsO4)3Cl
Method(s) of Verification: Dolyhir Quarry - XRD (NMW X-1312); Dylife Mine - XRD (NHM, 7420F); Gwaith-yr-afon Mine - XRD (NMW X-867 and NHM) & IR; Hendrefelen Mine - XRD (NHM, 8501F).

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Sprays of acicular mimetite crystals from Dolyhir Quarry, Powys. N. Hubbard Collection. Photo D.I. Green, © D.I. Green.
Introduction: mimetite was named in 1835 from the Greek for imitator because of its resemblance to pyromorphite – with which it forms a series. Mimetite is a secondary mineral that forms in the oxidized portion of lead-bearing ore bodies.
Occurrence in Wales: generally uncommon in Wales. Mimetite, the arsenate analogue of pyromorphite - a mineral widely developed in Wales - is unusual due to the scarcity of arsenic in the lead deposits of Wales. An early reference by Andrew (1910), suggests that mimetite occurs in the Dolgellau Gold-belt, however as yet no confirmation has been made. Further south, mimetite has been recorded in small quantities at a number of mines in the Central Wales Orefield, but remains uncommon in comparison with pyromorphite. Recent investigations into mineralization at Dolyhir Quarry in the Welsh Borderlands has shown mimetite to be present in association with oxidized tennantite-bearing veins.

Key Localities:

  • Darren Mine, Pen-bont Rhydybeddau, Ceredigion: reported by S.A. Rust (Bevins, 1994) forming microcrystals (Bevins & Mason, 1997).
  • Dolyhir Quarry, Old Radnor, Powys: prismatic microcrystals and murky-green powdery coatings associated with weathered tennantite-bearing veinstone are mimetite.
  • Dylife Mine, Penegoes, Powys: single crystals (to 0.75 mm in length) and very rare divergent groups of acicular habit, ranging from almost colourless to golden-yellow are reported as new finds by S. Rust (British Micromount Society Newsletter No. 24) in 1988.
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: the richest locality for mimetite in Central Wales with, occasional specimens displaying extensive coatings on joint surfaces in the veinstuff (Rust & Mason, 1994). Crystals are typically prismatic to blocky and up to 1.5 mm in size, ranging in colour from colourless to yellow and occasionally pale green.
  • Gwynfynydd Mine, Ganllwyd, Dolgellau, Gwynedd: Andrew (1910) reported that orpiment, pyromorphite and mimetite are said to occur in the Gwynfynydd Lode, citing a reference by Dr. Ure. This reference is actually Ure’s Dictionary of Arts, Manufactures and Sciences, volume 2, edited by R. Hunt (1878). A small number of specimens from the Morgan Mine (Gwynfynydd), in the National Museum of Wales Mineral Collection show, minute sprays of beige coloured prismatic crystals, the identification of which remains to be confirmed.
  • Hendrefelin Mine, Ysbyty Ystwyth, Ceredigion: Clark & Rust (1993) confirmed mimetite from the mine dumps. Divergent crystal groups to 1 mm across have been collected by S.A. Rust.
  • Llettyhen Mine, Bontgoch, Ceredigion: yellowish globular crystals showing a passing resemblance to the phosphatian variety campylite have been collected. Further analysis is required to establish whether they are mimetite or pyromorphite.
  • Nant-y-mwyn Mine, Rhandirmwyn, Carmarthenshire: extensive powdery yellow coatings associated with jackstraw cerussite on quartz veinstone await analysis.
  • Sigenlas Mine, Llanidloes, Powys: small pale yellow-green crystals, associated with suspected bayldonite are likely to be mimetite, but require analysis (National Museum of Wales, unpublished data).

References:

  1. Andrew, A.R., 1910. The geology of the Dolgelley gold-belt, North Wales. Geological Magazine, 47, 159-171, 201-221, 261-271.
  2. Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  3. Clark, A.M. & Rust, S.A., 1993. Bottinoite, a mineral new to Britain. Mineralogical Magazine. 57, 543-544.
  4. Hunt, R. (ed.), 1878. Ure’s Dictionary of Arts, Manufactures and Sciences 7th ed., vol. ii, p. 692-693.
  5. Rust, S.A. & Mason, J.S., 1994. An unusual occurrence of arsenate minerals at Gwaith-yr-Afon mine, Dyfed, Wales. Journal of the Russell Society, 5(2), 109-113.