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.

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