Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 2nd UK recording
Chemical Composition: Basic hydrated copper sulphate
Chemical Formula: Cu12(SO4)2(OH)20·2H2O
Method(s) of Verification: XRD.
- Supergene: post-mining oxidation & weathering deposits
Introduction: redgillite has only recently been described as a mineral (Pluth et al., 2004), although to collectors of microminerals the distinctive, pale-green lathlike crystals have become a familiar site from a number of localities since its first discovery in the 1980s. It is one of a number of rare supergene sulphates that apparently exist only within the post-mining environment - where the chemistry is dictated by rapid superficial chemical weathering of broken mixed sulphide intergrowths. Associated minerals typically include covellite and sulphate minerals such as langite, wroewolfeite, brochantite and substances which, despite having a discrete chemical composition, are as yet uncharacterised as minerals.
Occurrence in Wales: first noted by S.A. Rust on specimens collected in the late 1980's at Esgairhir Mine, a number of other sites have since been discovered in Central Wales: indeed, the majority of UK sites for redgillite are within this area. Some fine, if small, specimens have been collected from a number of sites, although, given that the mineral has only just (2004) been accepted as a new species (with the type locality being Silver Gill in the Caldbeck Fells, Cumbria), there is currently little published information available.
- Bwlchrhennaid Mine, Goginan, Ceredigion: lathlike crystals to 0.3 mm (S.A. Rust Collection).
- Darren Mine, Pen-bont Rhydybeddau, Ceredigion: lathlike green crystals (S.A. Rust Collection).
- Dylife Mine, Penegoes, Powys: green laths associated with serpierite and wroewolfeite (S.A. Rust Collection).
- Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: green laths (S.A. Rust Collection); additionally as scattered acicular to lathlike crystals and radial sprays to 1 mm, on corroded primary sulphide matrices (and often separated from the primary sulphide by a thin film of covellite). Associated minerals include brochantite and an uncharacterised mid-blue finely botryoidal phase (specimens in the National Museum of Wales Collection).
- Esgairhir Mine, Tal-y-bont, Ceredigion: noted by Cooper & Stanley (1990), redgillite was found here in the late 1980's by S.A. Rust and subsequently by other micromineral specialists. Associated with langite and covelline on corroded chalcopyrite, scattered redgillite laths can cover areas to several square centimetres. Sprays of crystals to 1 mm have also been noted.
- Frongoch Mine, Devil's Bridge, Ceredigion: redgillite occurs rarely as small groups of divergent green laths in association with langite, schulenbergite, caledonite, hemimorphite and hydrocerussite (listed as Unknown F2 by Green et al., 1996).
- Hendrefelin Mine, Ysbyty Ystwyth, Ceredigion: as tufts of acicular green crystals to 0.5 mm (S.A. Rust Collection) associated with an uncharacterised blue substance.
- Llechweddhelyg Mine, Penrhyncoch, Ceredigion: green laths (S.A. Rust Collection).
- Ponterwyd Mine, Llywernog, Ceredigion: green laths (S.A. Rust Collection).
- Cooper, M.P. & Stanley, C.J., 1990. Minerals of the English Lake District - Caldbeck Fells. Natural History Museum Publications, 160p.
- 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.
- Pluth, J.J., Steele, I.M. & Kampf, A.R., 2005. Redgillite, a new mineral from Caldbeck Fells, Cumbria, England. European Journal of Mineralogy (submitted MS).