Lanarkite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead oxysulphate
Chemical Formula: Pb2(SO4)O
Method(s) of Verification: Dolyhir Quarry - XRD; Eaglebrook Mine - visual identification; Frongoch Mine - XRD (Oxford University Museum) & EMPA.

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
Lanarkite (white) within oxidized tennantite-galena-bearing veinstone. Dolyhir Quarry, Old Radnor, Powys. Photo D.I. Green, © D.I. Green.
A divergent spray of bladed lanarkite crystals from Frongoch Mine. Individual crystals to 2.5 mm in length. Steve Rust specimen and photo. © Steve Rust.
Introduction: lanarkite is a rare secondary lead mineral formed through the alteration of primary lead minerals both within the oxidized upper levels of hydrothermal veins and within mine spoil.
Occurrence in Wales: lanarkite was first recorded in Wales by Mason (1994), as part of an extensive partly dump-formed supergene assemblage at Frongoch Mine in the Central Wales Orefield. Microcrystals have since been described from Eaglebrook Mine also within the Central Wales Orefield and a discovery of in-situ lanarkite has been made at Dolyhir Quarry in the Welsh borderland, but lanarkite remains an extremely rare Welsh species represented by only a handful of specimens.

Key Localities:

  • Dolyhir Quarry, Old Radnor, Powys: three specimens containing lanarkite were collected from a galena-rich block discovered in 1996. Lanarkite occurs as masses of white, intergrown, bladed crystals up to about 1 mm in length associated with elyite and probable leadhillite. This occurrence is attributed to natural in-situ alteration, the first from Wales (Cotterell et al., 2011).
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: listed without further description by Rust et al. (1995). Lanarkite is likely to form part of the extensive dump-formed supergene assemblage within the eastern engine-shaft dump.
  • Frongoch Mine, Devil's Bridge, Ceredigion: briefly recorded by Mason (1994) forming part of an extensive partly dump-formed supergene assemblage. Green et al. (1996) provide a detailed description, recording that two specimens of lanarkite have been found, and, appear to be the first reported from Wales. One specimen displays radiating groups of bladed crystals to 2.5 mm and the other, subparallel groups of prismatic crystals to 2 mm. The lanarkite is crystallized on a fine covelline crust, and is associated with minute crystals of susannite and caledonite (Green et al., 1996).

References:

  1. Cotterell, T.F., Green, D.I., Hubbard, N., Mason, J.S., Starkey, R.E. and Tindle, A.G., 2011. The Mineralogy of Dolyhir Quarry, Old Radnor, Powys, Wales. UK Journal of Mines and Minerals, 32, 5-61.
  2. Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
  3. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  4. Rust, S., Burchmore, S. & Foy, E., 1995. Some interesting new finds from the Nant-y-cagle mine. British Micromount Society Newsletter, 40, 8-10.