Introduction: although not a common mineral globally, ullmannite is important locally in areas where base-metal (ie. Cu-Pb-Zn) sulphide veins carry enhanced nickel levels. Depending on the levels of Ni, Co, Sb, As and S present, a wide range of Co-Ni sulpharsenides/sulphantimonides and sulphides may accompany ullmannite together with common base-metal sulphides such as galena, chalcopyrite and sphalerite. Ullmannite belongs to the cobaltite group of minerals: with increasing cobalt content it passes into the species willyamite and with increasing arsenic content it passes into gersdorffite. Ullmannite closely resembles other cobaltite-group minerals and where present as microscopic inclusions requires chemical analysis to fully confirm its presence. Larger euhedral crystals bear a superficial resemblance to galena: however the superior hardness is diagnostic, as is the brighter lustre and the imperfect cubic cleavage.

Occurrence in Wales: ullmannite was first discovered in Wales by Ashton (1982) who described it in material collected from Goginan Mine during his PhD research. Subsequently a wider-ranging study of the ore mineralisation of Central Wales revealed ullmannite to be widespread in two distinct generations (Mason, 1994; 1997; 1998; Bevins & Mason, 1997). The mineral has yet to be confirmed from other areas of Wales, although some polished sections of galena from the Llangynog orefield reveal microscopic inclusions resembling ullmannite. The first generation of ullmannite in Central Wales is restricted in its occurrence to the main (A1-c) polymetallic mineral assemblage (Mason, 1994, 1997), which is dominated by galena. Ullmannite occurs, along with a considerable number of other minerals, as a microscopic (tens of microns in size) inclusion-forming phase in the galena, where it may be abundant. The distribution of ullmannite therefore follows the distribution of this galena generation, which principally occurs within the northern and western parts of the orefield. It is worth noting that the records of arsenopyrite in Central Wales galenas by Raybould (1974) were all references to this generation of ullmannite, as evidenced by a re-examination of his sections. The second ullmannite generation is present as much coarser crystals and some relatively rich specimens of ullmannite have been collected from occurrences of this distinctive assemblage, the A2-b, in the classification of Mason (1994, 1997). Ullmannite forms euhedral cubic crystals, generally 1-2 mm but up to 10 mm in size, sometimes nucleated upon chalcopyrite and often overgrown epitaxially by later galena. This sulphide assemblage occurs in a vuggy matrix of white to clear, often rather friable, crystalline quartz. Occurrences of this assemblage are scattered across the western parts of the orefield.

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