Caledonite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Lead copper carbonate sulphate hydroxide
Chemical Formula: Pb5Cu2(CO3)(SO4)3(OH)6
Method(s) of Verification: All of the following mines by XRD: Bwlch Glas - (NHM 10077F); Darren -(NHM 7733F); Esgair Hir - (NHM 4512-F, 6802F & 7178F); Esgairmwyn - (NHM 7539F); Frongoch - (NMW X-1155 ); Hendrefelen - (NHM 8174F & 8237F); Llechweddhelyg - (NMW X-1123 & X-1124).

Chemical Group:

  • Carbonates
  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Acicular caledonite crystal spray (2 mm across). Esgair Hir Mine, Central Wales Orefield. S.A. Rust Collection (no. 920). Photo S.A. Rust and © S.A. Rust.
Acicular caledonite from Esgair Hir Mine, Central Wales Orefield. Specimen and photo S.A. Rust. © S.A. Rust.
Sprays of acicular caledonite from Frongoch Mine, Central Wales Orefield. Specimen and photo S.A. Rust. © S.A. Rust.
Introduction: caledonite is a secondary mineral that typically forms in the oxidized zone of lead- and copper-bearing sulphide deposits.
Occurrence in Wales: a relatively new mineral to Wales, caledonite was first recorded from the Central Wales Orefield in the mid 1980s. Numerous subsequent occurrences have come to light, all of which to date have likewise been from Central Wales. Post-mining caledonite tends to form sprays of microscopic acicular crystals, in contrast with the more usual blocky prismatic form generally associated with natural, in-situ, formation. A large proportion of recorded occurrences have been verified by XRD at the Natural History Museum and the National Museum of Wales.

Key Localities:

  • Bwlch-glas Mine, Tal-y-bont, Ceredigion: found in situ in the top adit as blue-green prismatic crystals to 0.4 mm associated with susannite in brecciated galena-sphalerite-quartz veinstone with minor chalcopyrite (Rust, 1996).
  • Darren Mine, Pen-bont Rhydybeddau, Ceredigion: small blades and globular aggregates of minute needles found sparingly in post-mining supergene mineralization (Rust, 1990).
  • Dylife Mine, Penegoes, Powys: described as extremely rare (Rust & Rust, 1987), caledonite forms light greenish blue to bluish green divergent prismatic crystals, and as botryoidal crystalline masses to 1.7 mm associated with anglesite, leadhillite and cerussite from dumps derived from outcrop workings near to the boundary with Dyfngwn Mine. It is of post-mining origin.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: present rarely as light blue aggregated prismatic crystals to 0.75 mm (British Micromount Society Newsletter No. 40, 1995).
  • Esgairhir Mine, Tal-y-bont, Ceredigion: discovered during the mid-1980s, caledonite is present as light blue to bluish-green prismatic crystals up to 0.7 mm in length, as divergent crystal groups up to 2 mm, and rarely as botryoidal aggregates, associated with leadhillite, cerussite, langite, anglesite and rarely mattheddleite, within veinstone that has undergone intense post-mining alteration (Rust & Mason, 1988).
  • Frongoch Mine, Devil's Bridge, Ceredigion: some of the finest examples of post-mining caledonite from the Central Wales Orefield have been collected at this locality. It occurs as groups of pale-blue to greenish-blue crystals to 2 mm, as drusy botryoidal crusts, and as pale green crystal groups to 1.5 mm. It is commonly found with susannite and occasionally linarite on oxidized galena in quartz cavities (Green et al., 1996).
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: caledonite is rare at this locality but the occurrence in situ is of significance. It is present as blocky prismatic crystals (up to 1 mm) associated with linarite within gossan at the end of the shallow adit level (T.F. Cotterell, unpublished data).
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: extremely rare as blocky microcrystals typical of in-situ formation found associated with bladed linarite within malachite and cerussite-rich gossan (J.S. Mason, unpublished data).
  • Nant Melyn Mine, Hafren Forest, Powys: occurs rarely within a post-mining alteration assemblage in association with linarite and brochantite as encrustations on weathered veinstone (J.S. Mason, unpublished data).

References:

  1. 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.
  2. Rust, S., 1996. Susannite and caledonite in-situ in Central Wales. U.K. Journal of Mines and Minerals, 17, 18.
  3. Rust, S. & Rust, D., 1987. Micro-minerals from Dyfngwm Mine. U.K. Journal of Mines and Minerals, No. 2, 28-32.
  4. Rust, S.A., 1990a. Mattheddleite from the Darren Mine, Dyfed, Wales. UK Journal of Mines and Minerals. 8, 47-48.
  5. Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.