Cobalt Arsenates

The ortho-arsenate, Co₃(AsO₄)₂.8H₂O, occurs in nature as erythrite or cobalt bloom in the form of monoclinic crystals isomorphous with vivianite, Fe₃(PO₄)₂.8H₂O, and perhaps with nickel ochre, Ni₃(AsO₄)₂.8H₂O. Its crystallographic elements are:

a:b:с = 0.7937:1:0.7356. В = 74° 51'

The pyro-arsenate Co₂As₂O₇, is known, as also various ammonium arsenates.

An impure basic arsenate is known under the name of zaffre, and is used in painting porcelain. It may be obtained by roasting iron-free cobaltite, or by addition of sodium carbonate to a solution of cobaltite in nitric acid, when the zaffre is obtained as a precipitate.