Leadhillite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead sulphate carbonate hydroxide
Chemical Formula: Pb4SO4(CO3)2(OH)2
Method(s) of Verification: Esgair Hir Mine - XRD (NHM, 6979F); Cwmystwyth Mine - XRD (NHM, 7999F); Penrhiw Mine - XRD (Manchester Museum, MANCH:XRD150).

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
Introduction: leadhillite is a secondary lead mineral named after the locality in Scotland where it was first discovered. Polymorphous with susannite and macphersonite, leadhillite is found typically in the oxidized portion of lead-bearing orebodies and as a post-mining development within mine dumps.
Occurrence in Wales: leadhillite was first reported from Wales in the early 1980s, forming typical pseudohexagonal tabular microcrystals within weathered galena-bearing veinstone from the Central Wales Orefield. See for example Braithwaite (1982) and Jones (1983). As the popularity of micromineral collecting increased during the 1980s, so to the number of reported occurrences increased. By 1994, nine localities had been described (Bevins, 1994), however, the visual similarities between leadhillite and the polymorph susannite have created doubts over the authenticity of all of these accounts, particularly as very few specimens have been analysed. Of those leadhillite-group specimens which have been analysed the majority have proven to be susannite. Consultation of the susannite entry is recommended.

Key Localities:

  • Cwmystwyth Mine, Cwmystwyth, Ceredigion: a specimen from Cwmystwyth has been confirmed by the Natural History Museum, London.
  • Darren Mine, Pen-bont Rhydybeddau, Ceredigion: Bevins (1994) noted that leadhillite had been identified at Darren Mine. The method of identification is not specified, therefore the possibility remains that this is susannite.
  • Dolyhir Quarry, Old Radnor, Powys: minute typical pseudohexagonal crystals were found by M. Rothwell on two specimens (D.I. Green unpublished data).
  • Dylife Mine, Penegoes, Powys: discovered by S. & D. Rust in 1986 (British Micromount Society Newsletter, No. 16) occurring very rarely as colourless to creamy pseudohexagonal crystals associated with other supergene species on old mine dumps between Dylife Mine and Dyfngwm Mine. Crystals on occasion reach 0.7 mm and are late within the secondary assemblage. No formal analysis has been made of these crystals, therefore there remains the possibility that this occurrence represents susannite.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: Braithwaite (1982) described microscopic hexagonal tablets of leadhillite with other supergene oxidation products. Jones (1983) expands on this occurrence, recording that leadhillite is extremely rare, typically clear to yellowish and perched on quartz with associated galena. As this occurrence has not been confirmed it is highly likely that the crystals described are actually susannite.
  • Esgairhir Mine, Tal-y-bont, Ceredigion: Rust and Mason (1988) provide a detailed description of leadhillite and susannite from Esgair Hir Mine. Leadhillite occurs as uncommon tabular, pseudohexagonal crystals up to 1 mm across and very rare prismatic crystals to 0.4 mm long. It has also been confimed as a white replacement of an unknown lath-like mineral.
  • Frongoch Mine, Devil's Bridge, Ceredigion: initially described by S & D. Rust in 1985 (British Micromount Society Newsletter, No. 15) forming platy intergrown pseudohexagonal crystals typically associated with caledonite and on one specimen thicker intergrown crystals. Research into the secondary mineralization at Frongoch by Green et al. (1996) revealed, that all specimens of leadhillite-group minerals analysed, turned out to be the polymorph susannite.
  • Glogfawr Mine, Ysbyty Ystwyth, Ceredigion: thick and thin tabular pseudohexagonal crystals to 1.5 mm visually identified by S. & D. Rust in 1986 (British Micromount Society Newsletter, No. 16).
  • Gorlan Mine, Trefriw, Gwynedd: leadhillite is listed by Bevins & Mason (1998), but no further details are given.
  • Hendre Quarry, Glyn Ceiriog, Clwyd: British Micromount Society Reference Collection specimen no. 953 is labelled as leadhillite from Hendre Quarry. No method of verification is given and the presence of leadhillite at this quarry seems unusual given the geological environment.
  • Llettyhen Mine, Bontgoch, Ceredigion: identified by R.S.W. Braithwaite (Bevins, 1994).
  • Nant-y-creiau Mine, Devil’s Bridge, Ceredigion: blocky pseudohexagonal prismatic microcrystals within heavily weathered sulphidic veinstone found within a dried-up stream bed appear to be a leadhillite-group mineral, but further identification has not been made. Primary sulphides include galena, sphalerite, chalcopyrite and pyrite with intense alteration to serpierite, linarite, anglesite, cerussite, brochantite, langite and the leadhillite-group phase.
  • Penrhiw Mine, Ystumtuen, Ceredigion: typical transparent to translucent colourless blocky pseudohexagonal crystals to 0.4 mm are described by Mason and Green (1995). Crystals are very rare and form intergrown crystalline crusts coated by earthy anglesite.

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. Braithwaite, R.S.W., 1982a. Wroewolfeite in Britain. Mineralogical Record, 13, 167-174.
  4. 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.
  5. Jones, A.D., 1983. Nant-y-Cagl. Mineral Realm, 3, 42-76.
  6. Mason, J.S. & Green, D.I., 1995. Supergene minerals including exceptional ramsbeckite from Penrhiw Mine, Ystumtuen, Dyfed. UK Journal of Mines & Minerals, 15, 21-27.
  7. Rust, S. & Rust, D., 1987. Micro-minerals from Dyfngwm Mine. U.K. Journal of Mines and Minerals, No. 2, 28-32.