Sulphuret of Copper

This copper ore is not uncommon; but the form of the crystallization in this specimen is either rare, or has been hitherto overlooked by most mineralogical writers. Tetraëdrons have been spoken of, but not with lenticular convex faces, which seems a character of this ore when crystallized in tetraëdrons; at least most British specimens have some inclination to convexity. These appear to be always inclined to tarnish, very often assuming a coat, either of the colour of blued steel, or blueish black; and it often has the green patina, or oxide of copper, on the surface, which count Bournon speaks of in his description of yellow copper; Phil. Trans. for 1801. When fresh broken it is of a bright greenish yellow colour with a metallic lustre, and the flaws tarnish to the various colours of what is commonly called Peacock Copper Ore. The fracture is smoothish, having more or less of a fine-grained surface, sometimes like the finest sand, as count Bournon has observed. The crystals are brittle, and too tender to strike fire with steel.

The left hand sides of the two figures show the inclination to form three trapezoidal fraces on the triangular ones; and the figure between two columns of quartz shows them more plainly, as it does also the signs of the triangular laminæ of superposition. This is taken from another Cornish specimen. The geometrical figure shows the somewhat obtuse tetraëdron, each face of which is replaced by three trapezoidal ones, making a dodecaëdron. The nearest modification to this kind is in Romé de l’Isle, tab. 1. fig. 28. but this has twelve additional isosceles triangular faces. Haüy has a crystal something like this in sulphuret of zinc, which he derives from the rhomboidal dodecaëdron. See his fig 197. The rounded tetraëdral crystals are therefore passing to the dodecaëdron, in an almost imperceptible manner, as the three figures on the second line show. This specimen has some more perfectly marked, and some truncated like the two left-hand figures.