Brochantite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Copper sulphate hydroxide
Chemical Formula: Cu4(SO4)(OH)6
Method(s) of Verification: many localities confirmed by XRD including: Copa Hill (NHM, x20226); Dolwen Mine (NMW X-1025); Dylife Mine - (NHM, 7057F); Eaglebrook Mine - (NHM, x13826); Esgair Hir Mine -(NHM, 6669F); Geufron Mine - (NHM, 7862F).

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Bow-tie brochantite crystals (to 1 mm across) from Eaglebrook Mine. Specimen J.S. Mason Collection (no. JMNC061). Photo D.I. Green, © National Museum of Wales.
Bladed brochantite crystals up to 0.5 mm across, from Lodge Park copper trial, in the Central Wales Orefield. J.S. Mason Collection (no. JMLP539). Photo M.P. Cooper, © National museum of Wales.
Introduction: brochantite is a secondary mineral which occurs in the oxidized zone of copper-bearing ore bodies and within copper-rich dump material, along with other secondary copper sulphates and carbonates.
Occurrence in Wales: brochantite is generally regarded as an uncommon mineral in Wales having, first been reported from the principality in the late 1970s (Jones & Moreton, 1977). In actual fact, brochantite is relatively widespread, particularly in the Central Wales Orefield where it is well-developed as a post-mining phase albeit in microcrystals. The number of sites where brochantite has been recorded in Central Wales bears testimony to the interest shown by micromineral collectors in the abandoned metal mine workings. The majority of discoveries have been made in weathered dumps, where post-mining alteration of sulphide-bearing veinstone has resulted in the ideal conditions for the formation of secondary base metal sulphates. Brochantite is also known to crystallize on mine walls where chalcopyrite-bearing veinstone has been left in situ, and it is under these conditions that the largest crystals have formed (see Lodge Park copper trial entry below). Elsewhere in Wales, brochantite has been collected from underground in Britannia Mine on Snowdon (Bevins et al., 1985) and a small trial level near Bontddu in the Dolgellau Gold-belt (Saich & Rust, 1987). Whilst in South Wales from the Vale of Towy Mine near Carmarthen (Bevins, 1994).

Key Localities:

  • Bontddu, Dolgellau, Gwynedd: brochantite is common as crystalline crusts on the level walls of a small trial near Bontddu. Rounded thin tabular crystals to 0.15 mm are uncommon and on extremely rare occasions form star-like crystal clusters to 0.6 mm (Saich & Rust, 1987).
  • Britannia Mine, Snowdonia, Gwynedd: small stalactites collected from underground workings consist of an intimate association of brochantite, posnjakite and malachite (Bevins et al., 1985).
  • Bwlchrhennaid Mine, Goginan, Ceredigion: brochantite occurs rarely as minutely crystalline drusy mats with linarite and malachite (Rust, 1986) from the surface dumps.
  • Dolwen Mine, Devil’s Bridge, Ceredigion: finely crystalline deep green brochantite encrusts weathered chalcopyrite from a small section of dump in the north bank of the Nant Rhuddnant. Linarite is a frequent associate. Malachite is also present in this dump, but tends to replace highly altered chalcopyrite, while brochantite coats slightly fresher chalcopyrite.
  • Dylife Mine, Penegoes, Powys: listed by Jones & Moreton, (1977). Brochantite is not uncommon as a dump-formed secondary species, most notably from outcrop workings on Pen Dylife where, it occurs as rounded tabular microcrystals encrusting weathered chalcopyrite-bearing veinstone (Rust & Rust, 1987).
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: excellent small divergent groups of acicular transparent crystals and small single crystals, associated with covelline and linarite are reported by Jones & Moreton (1977). Jones (1983) regarded brochantite as forming some of the most beautiful microcrystals found at Eaglebrook, typically occurring as small, dark green lath-like to blocky crystals and confused groups of laths with fin-like terminations. Brochantite is recorded on a variety of matrices most notably limonitic gossan and in cuprite vughs. Alteration to langite is also noted.
  • Esgair Fraith Mine, Tal-y-bont, Ceredigion: noted by Jones & Moreton (1977), but far more common at Esgairhir Mine.
  • Esgairhir Mine, Tal-y-bont, Ceredigion: brochantite is a very common post-mining species at Esgairhir. Rust (1985) recorded emerald-green crusts and occasional tabular microcrystals from Penybwlch Shaft dumps. Drusy microcrystalline crusts of brochantite frequently coat altered masses of chalcopyrite (Rust & Mason, 1988). Brochantite occurs in association with most of the other secondary species at Esgairhir, particularly langite and linarite which it is known to partially replace.
  • Geufron Mine, Llanidloes, Powys: abundant brochantite cements rock fragments in a compacted horizon within the mine dumps. Deep emerald green platy microcrystals encrust the muddy rock clasts (National Museum of Wales specimens).
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: not uncommon, occurring as thin emerald-green platy crystals covering areas of chalcopyrite to several centimetres across (Rust & Mason, 1994) collected in situ underground.
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: rich, deep emerald-green coatings have been collected from the walls of the main adit level in association with chalcopyrite-bearing veinstone (T.F. Cotterell, unpublished data). Similar in appearance to material from Gwaithyrafon Mine.
  • Lodge Park Copper Trial, Tre’r-ddol, Ceredigion: a common mineral at this important post-mining mineral locality. Superb platy microcrystals coat mudstone on the walls of the adit particularly close to old shotholes where cracks have developed. Brochantite typically occurs associated with malachite, as lustrous dark green equant to tabular crystals, generally from 0.5 to 1 mm, but occasionally up to 2 mm across (Mason & Green, 1996).
  • Penrhiw Mine, Ystumtuen, Ceredigion: a very common secondary species typically found as dark green microcrystalline coatings on weathered sulphide in the mine dumps (Mason & Green, 1995). At Penrhiw brochantite, to the untrained eye can be confused with ramsbeckite, which is unusually rich within dump material. Brochantite occurs in voids left by the dissolution of carbonate veinlets as rare dark green curved tabular crystals (to 0.5 mm) or more commonly as poorly formed or slightly corroded crystals showing alteration to posnjakite.
  • South Nant-y-car Mine, Rhyader, Powys: reported by Rust (1993) as thin drusy emerald-green crusts with linarite.
  • Vale of Towy Mine, Llandeilo, Carmarthenshire: brochantite occurs in association with linarite and cerussite in dump material from Bonville’s shaft (Bevins, 1994).
  • Ystrad Einion Mine, Furnace, Ceredigion: brochantite is common on the mine dumps as crusts of small emerald-green rounded, bladed crystals and underground encrusting hemimorphite, as crystals exceptionally to 1 mm (Mason & Rust, 1997).

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Bevins, R.E., Rowbotham, G., Stevens, F.S., Turgoose, S., & Williams, P.A., 1985. Lanthanite-(Ce), (Ce,La,Nd)2(CO3)3.8H2O, a new mineral from Wales, U.K. American Mineralogist, 70, 411-413.
  3. Jones, A.D., 1983. Nant-y-Cagl. Mineral Realm, 3, 42-76.
  4. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  5. 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.
  6. 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.
  7. 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.
  8. Rust, S., 1985. British Micro Localities. No. 12: Esgair Hir Mine (SN 734913), Ceulanymaesmawr, Dyfed, Wales British Micromount Society Newsletter. 13. 13-14.
  9. 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.
  10. Rust, S., 1993. British Micro Localities. No. 20: South Nantycar, Rhayader, Powys, Wales. British Micromount Society Newsletter, 34, 5-6.
  11. Rust, S. & Rust, D., 1987. Micro-minerals from Dyfngwm Mine. U.K. Journal of Mines and Minerals, No. 2, 28-32.
  12. 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.
  13. Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.
  14. 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.

There are no references for this specimen.