Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Copper sulphate hydroxide hydrate
Chemical Formula: Cu4(SO4)(OH)6.2H2O
Method(s) of Verification: many visual identifications, XRD data from: Cwmystwyth - (NHM, 3213F); Eaglebrook Mine - (NHM, 5843F); Frongoch Mine - (NHM, x10099, x10100 & 338F); Llechweddhelyg Mine - (NHM, 8032F & National Museum of Wales, NMW X-1123).
- Supergene: post-mining oxidation & weathering deposits
Langite crystals (up to 1 mm in length) from Lodge Park copper trial, near Tre'r-ddol, in the Central Wales mining district. Specimen J.S. Mason Collection. Photo M.P. Cooper, © National Museum of Wales.
Blue-green, pseudohexagonal langite crystal from Darren Mine in the Central Wales Orefield. Specimen and photo S.A. Rust. © S.A. Rust.
Introduction: langite is a secondary mineral dimorphous with wroewolfeite and typically found within oxidized copper-bearing veinstone within mine dumps and underground as flowstone on mine walls. langite is frequently confused with wroewolfeite and posnjakite, other so-called langite-group minerals because of their similar blue-green colour. Well-crystallized specimens can be identified visually with relative confidence, but more often finely crystallized or amorphous-looking aggregates are encountered, which require additional analysis.
Occurrence in Wales: langite is the commonest of the so-called langite-group of minerals in Wales. The earliest account of langite from Wales is that by Jones & Moreton (1977) recording langite from a couple of sites in the Central Wales Orefield (Frongoch Mine and Esgair Fraith Mine). However, langite was confirmed from Frongoch during the 1960s by the Natural History Museum, London, on a sample collected by R.S.W. Braithwaite in 1962. Many more occurrences have come to light during the past twenty years, but the majority continue to be from the Central Wales Orefield and typically as microcrystals. The richest discovery to date was recorded by (Mason & Green, 1996) from the Lodge Park copper trial in north Ceredigion, where langite crystals coat adit walls.
- Bontddu, Dolgellau, Gwynedd: langite is present underground within a short trial near Bontddu occurring, on joints in veinstone as rare flattened crystals (< 2 mm), branch-like reticulated masses (< 1.3 mm) and as extremely rare euhedral pseudocubic crystals (around 0.5 mm) (Saich & Rust, 1987).
- Bwlchrhennaid Mine, Goginan, Ceredigion: langite is reported as very rare minute crystals with brochantite (Rust, 1986).
- Crafnant Mine, Llanbedr, Harlech, Gwynedd: Bevins & Mason (1998) reported undifferentiated langite-type salts which, on subsequent analysis have proved to include posnjakite.
- Dylife Mine, Penegoes, Powys: Rust & Rust (1987) described langite forming in a number of habits from dumps at Dyfngwm (since shown to be a western working of Dylife Mine). Typical pseudohexagonal crystals to 0.6 mm and pseudocubic crystals to about the same size are noted along with a number of unusual habits.
- Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: mainly post-mining in origin. Langite occurs on a variety of matrices and in a wide range of forms. These include pseudohexagonal to blocky rhombic deep blue to almost purple crystals, and lighter blue to blue green elongated crystals and crystal groups showing superb “Christmas-tree” twinning (Jones, 1983). The polymorph, wroewolfeite also occurs at Eaglebrook and is frequently mistaken for langite by the unwary.
- Esgairhir Mine, Tal-y-bont, Ceredigion: langite is uncommon. Several forms are recorded (Rust & Mason, 1988) including, euhedral pseudohexagonal crystals and rare blocky crystals to 0.5 mm.
- Frongoch Mine, Devil's Bridge, Ceredigion: identified during the 1960s as blue microcrystals associated with cerussite on a specimen collected by R.S.W. Braithwaite in 1962. This specimen now forms part of the National Museum of Wales Mineral Collection (NMW 68.576.GR.6). At Frongoch, langite typically occurs as occasional euhedral pseudohexagonal blue-green crystals to 1 mm, and as etched blocky crystals to 0.5 mm (Green et al., 1996).
- Glogfawr Mine, Ysbyty Ystwyth, Ceredigion: visually identified as elongated boat-shaped crystals (Rust & Rust, 1986).
- Gwaith-yr-Afon Mine, Goginan, Ceredigion: small, blue, tabular post-mining crystals associated with brochantite have been visually identified as belonging to the langite group of minerals (Rust & Mason, 1994).
- Llechweddhelyg Mine, Penrhyncoch, Ceredigion: listed as rare by Bevins & Mason (1997). Langite occurs as deep blue-green blocky pseudohexagonal microcrystals on pale bleached mudstone vein breccia (National Museum of Wales specimens).
- Lodge Park Copper Trial, Tre’r-ddol, Ceredigion: specimens from Lodge Park are amongst the finest collected in Wales. Langite is common as transparent to translucent sky blue to blue-green post-mining growths and flowstone on tunnel walls. A number of forms are present including, euhedral to subhedral pseudohexagonal twins, snowflake twins and occasional blocky crystals. The coarsest crystal growths - the snowflake twins – individually reach 3 mm, and occur as part of the flowstone on the level walls in intimate intergrowths with brochantite and occasionally malachite (Mason & Green, 1996).
- Ystrad Einion Mine, Furnace, Ceredigion: Mason & Rust (1997) describe langite as occurring both above and below ground as typical dark to light blue-green pseudohexagonal crystals up to 0.6 mm in size.
- 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.
- 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.
- 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.
- Jones, A.D., 1983. Nant-y-Cagl. Mineral Realm, 3, 42-76.
- Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
- Mason, J.S. & Green, D.I., 1996. Supergene copper mineralisation in situ at Lodge Park Copper trial, Dyfed. UK Journal of Mines and Minerals. 17. 19-23.
- Mason, J.S. & Rust, S.A., 1997. The Mineralogy of Ystrad Einion Mine, Dyfed, Wales. U.K. Journal of Mines and Minerals, 18, 33-36.
- Rust, S., 1986. British Micro Localities. No. 15: Bwlchrennaid Mine or Level Newydd (SN 706 823), Goginan, Tirymynach, Wales. British Micromount Society Newsletter, 18, 9-10.
- Rust, S. & Rust, D., 1987. Micro-minerals from Dyfngwm Mine. U.K. Journal of Mines and Minerals, No. 2, 28-32.
- Rust, S., & Rust, D., 1986. British Micro Localities. No. 12 (continued): Dyfngwm Mine, Penegoes, Powys (SN 850931). British Micromount Society Newsletter. 16, 15-16.
- Rust, S.A., 1992. Ramsbeckite, the first three British occurrences. U.K. Journal of Mines and Minerals, No. 11, 24-25.
- 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.
- Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.
- 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.