Erythrite

Erythrite, historically nicknamed "cobalt bloom" or "cobalt bluthe" by early miners, is a hydrated cobalt arsenate with the formula Co3(AsO4)2·8H2O. It forms as a secondary mineral in the oxidation zone of cobalt-nickel-arsenic ore bodies, coating and replacing primary arsenides such as skutterudite and safflorite. Its intense crimson, magenta and violet-red tones made it a reliable field indicator that miners used for centuries to trace hidden veins of cobalt ore. Crystallizing in the monoclinic system, Erythrite belongs to the vivianite group and forms a complete solid-solution series with the nickel arsenate annabergite, its apple-green twin; copper-bearing intermediates are now recognized as the distinct species babánekite.

Erythrite most typically occurs as dusty pink encrustations, radiating acicular sprays, reniform crusts and stellate or fibrous aggregates, often associated with cobaltite, cobaltocalcite (sphaerocobaltite), scorodite, adamite and morenosite. Classic historic type material comes from Grube Daniel at Schneeberg in Saxony, Germany, described in 1832, while other well-documented occurrences include Tunaberg in Sweden, Joachimsthal in the Czech Republic, the old cobalt mines of Cornwall (notably Wheal Gorland), Cobalt, Ontario in Canada, the Blackbird mine in Lemhi County, Idaho, and Mount Cobalt in Queensland and the Dome Rock copper mine at Mingary, South Australia. In most of these classic districts, however, Erythrite appears only as dull massive crusts rather than well-formed crystals.

The single locality that transformed Erythrite from a modest crust-forming curiosity into a prized crystal specimen is the Bou Azzer mining district in the Anti-Atlas mountains of Morocco, particularly the workings around Vein 7, where sharp, lustrous, richly colored bladed and prismatic crystals — sometimes reaching several centimeters and often perched on quartz or calcite matrix alongside native silver and other cobalt-nickel minerals — have set the world standard for the species. This Moroccan material, along with rarer Mg-enriched variants documented from the same district, is what most serious collectors now seek when searching for fine Erythrite specimens, valued for a color intensity and crystal habit rarely matched elsewhere, despite the mineral's inherent softness, sectile fracture and unsuitability for jewelry use.