Arseniate of Iron

This was mostly confounded with arseniate of copper until the celebrated Chenevix, by analysis, ascertained it to be arseniate of iron: see Phil. Trans. for 1801. Count Bournon observes that it crystallizes in cubes rarely a little flattened: i may add rarely lengthened. I, however, have it a little so, perhaps the fourth of its diameter; and his figure conveys that idea, although I suppose unintentionally. The sides, he observes, are smooth and brilliant. I am happy to add another character, that they are diagonally striated in alternate order on each face; this is readily seen in most of my specimens^*. They are often a little concave in the centre, and rising to the edges in the longitudinal direction of the straiæ, and also show signs of being formed on cubical nuclei. I have them from a light yellowish green to a bight perfect green, apparently neither inclining to yellow or blue, passing on to deepish blue green, and thence to an olive colour, being heightened, as it were, with red; then, the yellow and red prevailing, they are of a brownish resin colour: some are very pellucid and transparent, and all so in some degree. The upper figure shows them of their common natural size in a gangue of quartz mixed with oxides of copper and iron, &c. The middle figure is magnified to show their construction more readily; and the right hand geometrical figure shows the striæ. In the left hand bottom figure, the only modification known of this substance, according to count Bournon (to use his own words), “Four of the eight solid angles of the cube are replaced by an equal number of equilateral triangular planes, situated in such a manner that every one of the sides of the cube becomes an elongated hexagon, having two angles of 90° each, and four of 135°. Crystals modified in this way are very scarce. I have never seen but one specimen, which is in the collection of sir John St. Aubin. Its crystals are pretty large and well defined.” I therefore consider as a great rarity a specimen in my museum, which exposes two crystals thus truncated. It is easily scratched with a pin, but it scratches uncommon calcareous spar. By Chenevix’s analysis it was found to contain