Arseniate of Copper

This beautiful variety is described by Count Bournon in the Philosophical Transactions for 1801, and we cannot do better than profit by his description. “It is in very thin hexaëdral laminæ, the six sides alternating in an inclined position, with the broad hexaëdral planes on either side at an angle of about 135°, and the third at 115°, on the opposite side.” See fig. 1. The crystals are more or less piled on each other, and are often to be divided, or split parallel to their surfaces, in the same manner as Mica. They are very brittle, mostly of an emerald green, and as transparent as the best glass, their lustre resembling the thin glass called frosting; or, as the Count expresses it, the lustre of those coloured metal plates known by the name of foil, and are most splended when the light falls on the broad planes. The edges are more opaque, partly from the contrary direction of the crystal, and party from the striæ in the direction of the laminæ. Fig 2. is a general group of crystals. Fig. 3. shows a variety in my possession of yellower tint^*.

The lower geometrical figures show, according to Count Bournon’s measurement, that if the inclined sides were to be increased by a regular set of decreasing plates placed upon the surface till they formed an equilateral triangle, they would become oblique octaëdrons, (see right hand figure;) and if they further continued on these planes till they were lost, they would produce a rhomboidal prism, which, as it seems to agree with the fragments, may be the primitive form. I should have observed that it not only splits into laminæ on the broad planes, but that it also readily does so with the side facets. Its fracture is sometimes irregularly conchoidal and glassy. Spec. grav. 2,548. Mr. Chenevix found it to contain oxide of copper 58, arsenic acid 21, water 21.