Mattheddleite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead silicate sulphate chloride
Chemical Formula: Pb20(SiO4)7(SO4)4Cl4
Method(s) of Verification: most occurrences confirmed by XRD including, Darren Mine (NHM), Esgair Hir Mine (NHM), Frongoch Mine (Leeds University) and Llechweddhelyg Mine. Eaglebrook Mine occurrence based on the distinctive crystal morphology.

Chemical Group:

  • Silicates
  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Scanning electron micrograph of acicular mattheddleite crystals lining a cavity. Photo T.F. Cotterell. © National Museum of Wales.
Aggregates of spiky mattheddleite crystals from Darren Mine in the Central Wales Orefield. Specimen and photo S.A. Rust. © S.A. Rust.
Introduction: mattheddleite is an extremely rare secondary species typically forming microcrystals in cavities in oxidized lead-rich mine spoil.
Occurrence in Wales: records of mattheddleite in Wales are few. Rust & Mason (1988) provided the first account of mattheddleite in Wales and to date only six localities have been reported, all in microscopic amounts, with mattheddleite always forming late in the paragenetic sequence of secondary minerals. In Wales mattheddleite is invariably associated with both caledonite and leadhillite-group minerals.

Key Localities:

  • Darren Mine, Pen-bont Rhydybeddau, Ceredigion: small (0.2-1.5 mm) colourless acicular crystals occur within oxidized ore from the dump at the mouth of Darren Fach adit (Rust, 1990).
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: minute (up to 0.15 mm), colourless, acicular crystals displaying major hexagonal prisms with acute pyramidal terminations were observed encrusting the extremely rare mineral laurionite (Rust et al., 1995). Associated supergene minerals include anglesite, caledonite and probable susannite-leadhillite, within a matrix of highly oxidized remnant of galena, limonite replacing ferroan dolomite, and chalcopyrite (Rust et al., 1995).
  • Esgairhir Mine, Tal-y-bont, Ceredigion: colourless acicular crystals to 0.2 mm overgrow caledonite in areas to 5 mm|2 with associated leadhillite (Rust & Mason, 1988).
  • Frongoch Mine, Devil's Bridge, Ceredigion: colourless acicular crystals to 0.25 mm, associated with susannite and caledonite are described by Green et al. (1996).
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: listed without description by Bevins & Mason (1997).
  • Machen Quarry, Caerphilly, South Wales: described by Plant & Jones (1995) as forming very rare, free-standing, transparent, hexagonal crystals (to 0.5 mm) with pointed terminations in association with supposed scotlandite. However, the same authors point out that identification was based purely on the characteristic crystal morphology and appearance, by a collector who remains anonymous. Until further details are made available, this occurrence is regarded as dubious.

References:

  1. 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.
  2. Bevins, R.E. & Mason, J.S., 2000. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Glamorgan and Gwent. National Museums & Galleries of Wales, Cardiff
  3. 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.
  4. Plant, S.P. & Jones, I.E., 1995. Minerals of Machen Quarry, Mid Glamorgan, Wales. Journal of the Russell Society, 6(1), 31-36.
  5. Rust, S., Burchmore, S. & Foy, E., 1995. Some interesting new finds from the Nant-y-cagle mine. British Micromount Society Newsletter, 40, 8-10.
  6. Rust, S.A., 1990a. Mattheddleite from the Darren Mine, Dyfed, Wales. UK Journal of Mines and Minerals. 8, 47-48.
  7. Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.