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(also cobalt bloom), an arsenate mineral, with the chemical composition C03[As04]-8H2O. Nickel substitutes iso-morphically for cobalt; admixtures of zinc, magnesium, and iron are also observed. Erythrite crystallizes in the monoclinic system. It is encountered primarily in the form of crusts, concretions, and earthy aggregates. Its color is pinkish red; with increasing nickel content, the color gradually changes to pale pink and gray and then to apple green, which is characteristic of the nickel arsenate annabergite, Ni3[AsO4] · 8H20, which is similar in structure to erythrite but occurs less frequently. Erythrite has a hardness of 1.5–2.5 on Mohs’ scale and a density of 3,100 kg/m3.
Erythrite is the product of the weathering of cobalt and nickelcobalt arsenides and sulfoarsenides. Upon oxidation, it is altered to black-colored hydroxides of trivalent cobalt (heterogenites). Its bright color is an important prospecting guide to the presence of arsenic ores of cobalt and nickel.