posnjakite

Posnjakite is stoichiometrically identical with langite and both minerals are monoclinic (Back 2022).

Paar (1973) described langite from Tsumeb and compared the occurrence with a langite-posnjakite paragenesis at Brixlegg in Austria. It is interesting that he makes no allusion to posnjakite at Tsumeb.

The first published report of posnjakite at Tsumeb is by Bartelke (1976) who made clear that XRD was used to distinguish the mineral from langite. He recorded that "Posnjakite forms small, light blue, flat prismatic crystals whose surfaces are poorly developed." He also noted the likelihood that the two minerals – posnjakite and langite – might be found together at Tsumeb but that, at that time, XRD evidence for this association was "… still pending".

Pinch and Wilson (1976) described posnjakite as "… deep blue-green tabular crystals mixed with green acicular brochantite, and associated with malachite, devilline and serpierite as well. It has also been observed on tennantite pseudomorphs after azurite and on cerussite."

Keller (1977a) proposed a paragenetic sequence of (mainly) sulphate minerals which he believed formed under acidic conditions. His sequence "II/3" is as follows:

II/3: primary sulphides >> (brochantite + posnjakite + malachite) >> linarite >> serpierite

Keller (1984) later added that langite is an occasional member of this paragenesis which is typically encountered on the surface of tennantite pseudomorphs after enargite.

Gebhard (1999) reiterated the close similarity of posnjakite and langite and noted that both minerals occur together on tennantite pseudomorphs after enargite.

anglesite; arsentsumebite; brochantite; cerussite; chalcocite; connellite; devilline; enargite; langite; linarite; malachite; mimetite; olivenite; serpierite; tennantite-(Cu) (?); tennantite-(Zn)