Tyrolite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Calcium copper arsenate carbonate hydroxide hydrate
Chemical Formula: CaCu5(AsO4)2(CO3)(OH)4.6H2O
Method(s) of Verification: Capel Hermon - XRD (National Museum of Wales); Gwaith-yr-afon Mine - IR (Rust & Mason, 1994).

Chemical Group:

  • Arsenates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Radial tyrolite sprays (to 2 mm across) with azurite in limestone from Dolyhir Quarry. Specimen and photo D.I. Green. © D.I. Green.
Tyrolite-like mineral forming flat-lying, radiating aggregates coating microtonalite. Capel Hermon. Specimen J.S. Mason Collection, photo D.I. Green; © National Museum of Wales.
Radiating tyrolite sprays (to 2 mm in diameter) from Gwaith-yr-Afon Mine, Central Wales Orefield. Photo J.S. Mason, © National Museum of Wales.
Introduction: Tyrolite is a copper-arsenic mineral found in supergene assemblages including post-mining environments.
Occurrence in Wales: tyrolite is a rare mineral in the British Isles. Wales has produced some of the more notable occurrences, particularly over recent years. Saich & Rust (1987) describe a 'tyrolite'-like mineral from a trial level near Bontddu, Dolgellau, Gwynedd. Although still unconfirmed, further discoveries have resulted in some of the best British tyrolite being collected. Fine radiating sprays of a sulphatian tyrolite, coating fracture surfaces in veinstone at Gwaith-yr-afon Mine in Central Wales were collected in the late 1980s (Rust & Mason, 1994) and more recently superb specimens of platy tyrolite rosettes have been found at Dolyhir Quarry in Powys (Cotterell et al., 2011).

Key Localities:

  • Bontddu, Dolgellau, Gwynedd: Saich & Rust (1987) report a 'tyrolite'-like mineral, typically forming exceedingly rare bright blue scattered flattened radial sprays and hemispherical masses to 0.2 mm, and as botryoidal masses on joints in veinstone from a trial level near Bontddu. Bevins (1994) describes the botryoidal masses as reaching 2 mm across.
  • Capel Hermon, Coed y Brenin, Gwynedd: Armstrong et al. (2003) describe tyrolite and malachite altering from small patches of tennantite within intensely sericitised microtonalite from a forest road-cut at NGR SH 748 256. Analysis of material collected by the National Museum of Wales shows this mineral to have an X-ray diffraction pattern similar to tyrolite, although not identical. This tyrolite-type mineral forms flat-lying radiating aggregates (to several mm across) of platy pearly bluish green crystals coating the microtonalite in association with tennantite.
  • Dolyhir Quarry, Old Radnor, Powys: superb crystallized samples to large hand specimen size collected, from a chalcocite vein exposed in Dolyhir Limestone at the northern end of the quarry, display rich, pearly, bluish green tyrolite in radial sprays to 8 mm across and dark blue azurite. Tyrolite has also been found as radiating sprays on fracture planes within gritty sandstone near to the quarry entrance.
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: described by Rust & Mason (1994) as low carbonate, sulphatian tyrolite. Tyrolite, occurs abundantly underground as bright blue, green or whitish flat-lying radial groups of sectile lath-like crystals in fractures in altered quartz-chalcopyrite veinstone. Occasional spherical aggregates of compact platy crystals occur within quartz cavities. Crystal sprays reach 4 mm in diameter, but are typically 1-2 m across. A number of fine small display specimens richly covered with radiating sprays have been collected. In comparison with other arsenate minerals tyrolite is locally very abundant at Gwaith-yr-Afon.

References:

  1. Armstrong, R., Herrington, R.J. & Savage, M.A., 2003. Tennantite and tyrolite from the Coed y Brenin forest, North Wales. Journal of the Russell Society, 8(1), 18.
  2. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  3. 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.
  4. 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.
  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.
  6. Saich, D.A. & Rust, S.A., 1987. Micro-minerals from a trial level in Wales. U.K. Journal of Mines and Minerals, No. 3, 3-4.