J.S. Mason - Central Wales Orefield: Siegenite and cobalt pentlandite

Siegenite ((Ni,Co)3S4) was discovered in Central Wales back in the mid 1980s. It is widespread, and again is restricted to early, complex polymetallic assemblages. Locally, it is common (with samples of selected veinstone assaying up to 3% Co+Ni); given that it was regarded as worthless in the 19th century - there is some evidence that is was regarded mistakenly as pyrite or arsenopyrite - it would be interesting to see how much remains underground at one locality!

In rich samples crystals to 2mm or so are observed. In polished section it has the distinctive pinkish tinge seen here, and the beautiful intergrowths of cobalt pentlandite (yellow lamellae) may be seen, especially if you have oil immersion lenses.

Cobalt pentlandite ((Ni,Co)9S8) is a rare mineral in global terms and Central Wales hosts the only British occurences known so far. It either occurs as these distintive trellislike lamellar intergrowths with siegenite, or more rarely as flamelike bodies in the darker yellow chalcopyrite, particularly evident above the top siegenite grain in the image L. The trellis lamellae are in the order of 10-40 microns wide usually. Identical textures are reported from Cobalt, Ontario (reading a paper on the mineralisation there was how I worked out what the mineral was, prior to probing it). Not the easiest mineral to photograph!

The samples are from Erglodd (above) and Loveden mines, both near Talybont, in an area where siegenite and associates are particularly noticeable.