J.S. Mason - Central Wales Orefield: Siegenite and cobalt
||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.
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.
CENTRAL WALES MENU