Sulphuret of Zinc, Blend

Blende (commonly called Black Jack by the miners) is often found crystallized, but generally in a very confused manner, and most frequently of a deep jet-black. The tetraëdral variety, here figured, has something of a less deep black lustre, and approaches to lead-like appearance. Except one or two solitary crystals, they are generally in plated clusters or groups; the plates, for the most part diminishing from the edges to the centre of the triangular faces, forming three faces, as on the dark side of the right hand figure. Occasionally, each face of the tetraëdron will differ; and one may either be seen plain, as at the base of the lowermost geometrical figure, or have the above-mentioned three faces terminating in a point like the dotted faces on the distant side, or with another triangular face like the right or left hand sides of the same figure. The truncation of the four solid angles, in the left hand and the geometrical figure, are parallel to the octaëdron. These modifications are all evident in the present specimen, which is a Cornish one. They rest on a greenish chlorite, on a light sort of schist or slaty rock, commonly called killas by the Cornish miners. They are often accompanied with rock crystals and copper pyrites. The nucleus is a rhomboidal dodecaëdron, and the integrant molecule is a tetraëdron with isosceles triangular faces, according to Haüy. The modification called encadré by that author, among his sulphurets of copper, resembles this very much; but he does not seem to have known such in sulphuret of zinc. Its specific gravity is 4.1665 according to Brisson. It may be scratched with a knife, and it will scratch sulphate of barytes, but not fluor. Refraction simple, Haüy.