Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Malachite

Crystal System: Monoclinic
Formula: Cu2(CO3)(OH)2
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Copper carbonate hydroxide
Method(s) of Verification: Central Wales mines - XRD, see individual mine entries; also by XRD at NMW: Graig Fawr (NMW X-1182); Llanfair Talhairan (NMW X-950); Machen Quarry (NMW X-777); Penarth (NMW X-67); Ramsey Head (NMW X-1101); Wedding Cave Mine (NMW X-1015).
Chemical Group:
  • Carbonates
Geological Context:
  • Supergene : post-mining oxidation & weathering deposits
  • Supergene : in situ natural oxidation & weathering deposits
Introduction: malachite is a common secondary mineral found in the oxidized zone of copper-bearing orebodies.
Occurrence in Wales: as would be expected for such a common secondary mineral malachite, has, been recorded in most parts of Wales. However, notable occurrences are limited to a small number of sites. Malachite is well-known from Bronze Age copper workings at Great Orme's Head in North Wales. Phillips (1823) noted malachite at Llandidno (Llandudno), while Greg & Lettsom (1858) recorded green crystals of calcite, stained by malachite at Great Orme’s Head. Specimens in the mineral collection of the National Museum of Wales (e.g. NMW 27.111.GR.453) display, malachite on dolomite - a typical association from the Great Orme. Malachite, was in actual fact probably one of the ores mined by the early miners judging by the green stained bone tools discovered in the ancient workings. Of particular note at the Great Orme are rich examples of malachite pseudomorphs after chalcopyrite crystals, scattered across the surface of dolomite. Malachite crystals are, however, rare. The finest crystallized specimens of Welsh malachite have been collected from the 1970s onwards from the small Llechweddhelyg Mine in the Central Wales Orefield. At Llechweddhelyg an unusually deep gossan zone was tried for lead-ore, but copper in the form of malachite was much more prevalent. Specimens of goethite-rich gossan contain sprays of malachite crystals to several centimetres in length which form attractive display pieces. Jones & Moreton (1977) noted that malachite is present on all of the mine dumps in Mid-Wales where copper sulphides are present, listing ten particularly noteworthy sites. Indeed across Wales malachite should be expected in small quantities at most sites where chalcopyrite is present near surface. In recent years, rich masses of velvety malachite coating chalcocite, occasionally with azurite, have, been collected at Dolyhir Quarry in the Welsh Borderlands. Similar, but smaller examples had been collected during the 1970s from a locality called Old Radnor Quarry, which is more than likely part of the now much larger Dolyhir Quarry.
Key Localities:
  • Britannia Mine, Snowdonia, Gwynedd: small tufts of malachite occur on lanthanite-(Ce) from Sneyd’s Level (Bevins et al., 1985).
  • Camdwrbach Mine, Nant-y-moch Reservoir, Ponterwyd, Ceredigion: malachite is present as powdery, poorly formed coatings on weathered, dressed copper ore from small dumps at the northern edge of the reservoir. Chalcocite and cuprite are commonly associated.
  • Cwrt-yr-ala Park Quarry, Penarth, South Wales: malachite is present on a number of specimens in the mineral collection of the National Museum of Wales (e.g. NMW 49.166.GR.1) forming crystalline coatings on altered chalcopyrite in calcite.
  • Dolwen Mine, Devil’s Bridge, Ceredigion: malachite was formerly common on a dump in the north bank of the Nant Rhuddnant, but specimens are increasingly harder to find. Occasional rich specimens display radiating aggregates (up to 10 mm) of green velvety malachite within highly altered iron stained veinstone and weathered masses coat the surface of veinstone. Cuprite is occasionally observed as an associated mineral within the core of malachite masses. Chalcopyrite is the primary copper mineral at Dolwen, but weathering has resulted in gradual alteration to chalcocite, cuprite, malachite, and goethite.
  • Dolyhir Quarry, Old Radnor, Powys: in recent years impressive masses of velvety acicular malachite completely coating chalcocite have been collected from an exposure of Dolyhir Limestone in the northern section of the quarry. Specimens exceptionally reach 22 x 14 x 14 cm, as displayed by National Museum of Wales specimen no. NMW 2002.33G.M.1.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: good microcrystals are reported by Jones & Moreton (1977). Verified by X-ray diffraction at the Natural History Museum (6350F).
  • East Castell Mine, Dyffryn Castell, Ponterwyd, Ceredigion: this small trial to the east of Castell Mine worked copper-ore (Jones, 1922) presumably in gossan, which is present on the overgrown dumps. Malachite occurs both within goethitic gossan and as small (1-2 mm) pale green sprays within cavities in vuggy chalcopyrite-bearing quartz veinstone (T.F. Cotterell, unpublished data).
  • Esgair Fraith Mine, Tal-y-bont, Ceredigion: malachite occurs to a limited extent at Esgairfraith. Weathered chalcopyrite-bearing veinstone typically shows some degree of alteration to malachite with cavities often containing dark-green, spherical masses up to 5 mm across.
  • Geufron Mine, Llanidloes, Powys: Jones & Moreton (1977) noted botryoidal crusts, up to half an inch thick, associated with chalcopyrite and cuprite from the mine dumps. Malachite is quite common at Geufron, particularly from the dumps near the lode outcrop.
  • Great Orme Copper Mines, Llandudno, Gwynedd: a classic Welsh locality for malachite, particularly in the form of pseudomorphs after euhedral chalcopyrite crystals on dolomite. Many fine specimens reside in the collections of the National Museum of Wales including the much figured specimen no. NMW 83.41G.M.4713, formerly in the R.J. King Mineral Collection. Also present at Great Orme is malachite flowstone, which is common in the Bronze Age workings. Well defined crystals are in general rare. However, one specimen (NMW 83.41G.M.4705) collected from dumps above the Lorreto Convent shows a small spray of crystals to 1 mm.
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: malachite is common underground, forming thin botryoidal crusts in association with the supergene arsenate assemblage (Rust & Mason, 1994). Verified by X-ray diffraction at the Natural History Museum (7156F).
  • Hendy Quarry, Miskin, South Wales: occasionally malachite is found as deep green, microcrystalline coatings in rusty goethite/limonite replacement of massive chalcopyrite, with barite (National Museum of Wales specimen NMW 95.25G.M.1).
  • Llanfair Talhaiarn, Denbighshire: malachite occurs within rusty limonite/goethite gossan derived from intensely weathered chalcopyrite (National Museum of Wales specimens).
  • Llantrisant by-pass, South Wales: minor malachite is seen to coat goethite replacing altered chalcopyrite on National Museum of Wales specimens (e.g. NMW 87.73G.M.268).
  • Llanymynech Hill, Powys: Greg & Lettsom (1858) described plumose specimens from Llanymynich Hill to the SW of Oswestry. A specimen in the G.J. Williams Collection, collected by Arthur Russell in 1913, and now part of the National Museum of Wales Collection, consists of small spherical crystals on weathered chalcopyrite-bearing veinstone, but the malachite is fairly inconspicuous.
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: malachite is common on the Eastern Engine-shaft dump and is generally of excellent quality. Jones (1987) reported a number of different habits including: large bright green fibrous; banded sprays (up to 4.5 cm in diameter); deep green masses (up to 2 cm) within galena/goethite matrix; fibrous masses (up to 6 cm); rare botryoidal coatings. Specimens from Llechweddhelyg are without question the most attractive to have been collected in Wales particularly, those that display coarse velvety sprays within gossan. This occurrence has been verified by X-ray diffraction at the National Museum of Wales (NMW X-1141).
  • Lletty Evan-Hen Mine, Bont-goch, Ceredigion: malachite is not as abundant at this locality as it is at the neighbouring Llechweddhelyg Mine, but is still relatively common. At Lletty Evan-Hen malachite typically occurs as discrete spherical sprays (generally <6 mm across) in cavities within quartz veinstone.
  • Lodge Park Copper Trial, Tre’r-ddol, Ceredigion: malachite is common as post-mining coatings of small, pale to mid-green spheroidal aggregates typically with brochantite (Mason & Green, 1996).
  • Machen Quarry, Caerphilly, South Wales: malachite is described as widespread in the upper levels of the quarry, but crystallized specimens are very rare (Plant & Jones, 1995). In common with other South Wales Limestone occurrences malachite is associated with rusty goethite/limonite rich masses derived from the alteration of primary chalcopyrite (T.F. Cotterell, unpublished data).
  • Old Radnor, Presteigne, Powys: Hall (1868) recorded malachite from Old Radnor. Attractive radiating acicular malachite occurs on National Museum of Wales specimen NMW 76.5G.M.2, labelled as from Old Radnor Quarry (Bevins & Sharpe, 1982).
  • Parys Mountain, Anglesey: malachite is recorded by Greenly (1919), but no reference is made by earlier authors (Phillips, 1823; Greg & Lettsom, 1858) suggesting, that malachite was not common, even within the rich gossan in the upper levels of the mine.
  • Penarth and Lavernock, South Wales: malachite occurs fairly commonly as thin pale green coatings on massive gypsum from beds high up in the cliffs between Penarth Head and Lavernock Point. No crystals have been reported.
  • Snowbrook Mine, Llanidloes, Powys: listed by Jones & Moreton (1977). Malachite encrusts weathered dressed chalcopyrite below the main dumps.
  • South Wales Coalfield: Bevins (1994) reported minor amounts of malachite in ironstone nodules from Tirpentwys Colliery, and Llanwynno Tips, at Tylorstown. This is likely to represent superficial alteration of chalcopyrite within the nodules.
  • Wenvoe Quarry, Cardiff, South Wales: malachite occurs in two forms: a specimen in the National Museum of Wales Collection (NMW 87.9G.M.3), collected prior to 1975, consists of a solid mass of goethite (replacing chalcopyrite) encrusted with pale to mid-green slightly botryoidal malachite; more typically, malachite is present as small (<4 mm across) deep green spheroidal aggregates on calcite crystals (NMW 78.33G.M.136).
References:
  • Bevins, R.E., 1994 A Mineralogy of Wales  National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  • Bevins, R.E. & Sharpe, T., 1982 Welsh Minerals.  National Museum of Wales Geological Series No. 4. 28pp.
  • 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.
  • Greenly, E., 1919 The Geology of Anglesey.  Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  • Greg, R.P. & Lettsom, W.G., 1858 Manual of the Mineralogy of Great Britain and Ireland.  John van Voorst, London, 483pp.
  • Jones, A.D., 1987 The Minerals of Llechwedd Helyg.  UK Journal of Mines and Minerals. 3, 25-27.
  • 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.
  • Phillips, W., 1823 An elementary introduction to the knowledge of Mineralogy.  3rd edition. London. 406pp.
  • Plant, S.P. & Jones, I.E., 1995 Minerals of Machen Quarry, Mid Glamorgan, Wales.  Journal of the Russell Society, 6(1), 31-36.
  • 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.