Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Chemical Composition: Nickel antimony sulphide
Chemical Formula: Ni9Sb2S8
Method(s) of Verification: all localities cited - EMPA (British Geological Survey, Nottingham, D.J. Bland) & XRD (P.H.A. Nancarrow)
- Hydrothermal: mesothermal polymetallic veins
Tucekite in polished section from Henfwlch Mine in Central Wales. The euhedral crystal, 0.5 mm long, is embedded in galena (grey) and chalcopyrite (yellow). The pale buff colour of the tucekite is characteristic. Image: J.S. Mason.
Tucekite crystal (0.2 mm long) in polished section,viewed under crossed polars. The three images show the diagnostic polarisation colours. Sample fron Nantycagl Mine, Central Wales. Image: J.S. Mason.
Introduction: in its few reported worldwide occurrences tucekite appears to be capable of occurring in a range of settings, illustrated by the fact that the mineral was first described from two geologically diverse localities, namely the auriferous conglomerates of the Witwatersrand and a mineralized Archaean schist in Queensland, Australia. More recently a few occurrences have been noted from the polymetallic ore deposits of Siegerland in Germany, while the Welsh occurrences are from polymetallic hydrothermal veins. Associated minerals include galena, chalcopyrite, gold and various Co-Ni sulphides, sulpharsenides and sulphantimonides. Tucekite is the antimony analogue of the more widespread nickel bismuth sulphide, hauchecornite.
Occurrence in Wales: Tucekite was originally misidentified as gudmundite (another nickel antimony sulphide) by Mason (in Rust & Mason, 1988) in a sample from Esgairhir mine in Central Wales. Subsequently, electron microprobe analyses of an optically similar mineral occurring at the nearby Nantycagl mine revealed the mineral to be a nickel antimony sulphide of unusual composition and X-ray diffraction analysis of the same mineral grain finally determined it to be tucekite (Mason, 1998). At the time this was only the third occurrence worldwide of the mineral. Subsequently, examples were found in polished sections of ores from several nearby mines: the localities form a tight cluster outside of which it has not been observed. In its known Welsh occurrences, it is only visible in polished section, where its distinctive display of colours under crossed polars is a diagnostic aid to identification.
- Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: tucekite is not uncommon at this site, where it forms euhedral overgrowths on chalcopyrite and is in turn overgrown by galena. It occasionally contains inclusions of electrum. Crystals reach 0.5 mm in length but are difficult to detect except in polished sections.
- Esgair Fraith Mine, Tal-y-bont, Ceredigion: as microscopic overgrowths on siegenite and millerite, associated with chalcopyrite; also as inclusions in galena.
- Esgairhir Mine, Tal-y-bont, Ceredigion: rare, as microscopic overgrowths on siegenite and millerite, associated with chalcopyrite.
- Henfwlch Mine, Ceulanymaesmawr, Ceredigion: rarely observed in polished sections as euhedral inclusions, up to 0.5 mm, in galena, associated with chalcopyrite.
- Hyddgen Mine, Uwchygarreg, Powys: very rare as microscopic inclusions in galena, associated with pyrite and chalcopyrite.
- Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
- Mason, J.S., 1998. Tucekite, a mineral new to Britain, and other rare ore minerals from the Central Wales Orefield. UK Journal of Mines and Minerals, 19, 30-36.
- Mason, J.S., 1997. Regional polyphase and polymetallic vein mineralisation in the Caledonides of the Central Wales Orefield. Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), 106, B135-B144.
- Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.