adamite

azurite; conichalcite (?); malachite

A block of mineralised feldspathic sandstone is covered on one face with a circa 10 mm thick layer of tightly intergrown adamite crystals. The adamite crystals have a sub-vitreous lustre with reasonable translucency and an unusual brownish-green colour. Broken surfaces, however, reveal a plum-coloured blush (Exhibit 2).

This variety of adamite is unusual at Tsumeb and has been colloquially described as "cobaltian adamite". The colour is indeed due to the presence of cobalt, but only as a trace element, with less than 0.05 wt% Co known to be sufficient to cause this pink colouration (Southwood et al. 2020).

The feldspathic sandstone matrix hosts minor amounts of azurite, botryoidal aggregates of conichalcite (?), and sparse malachite. A little adamite is also intergrown with the adamite (Exhibit 2).

This specimen was number 4318 in the collection of Austrian industrialist Hans von Karabacek, part of who’s collection was purchased for Harvard University in 1935.