Sulphuret of Copper

We continue this modification with a very curious variety of the same substance, probably less rich in copper, although differing very little in the lustre or the colour of the fresh fracture.

The outward respect resembles the grey or vitreous copper ores, being of a dull grey colour, and very roughly formed, as it were of certain nuclei, which viewed in the direction of the solid points, or angles, give a peculiar bright shining glare. Its faces are more determined and flatter than in the last: the three trapezoidal ones of the same are not finished (see the right-hand middle figure), but leave a triangular face, transverse to the original face of the tetraëdron, forming one equilateral and three isosceles triangular faces on each side, which makes altogether a new sixteen-sided figure. This is a modification very different from any before mentioned. Haüy only finds the modification from the bevelling of the edges of the tetraëdron, as in his grey and copper pyrites, tab 70 and 71, fig. 78 to 89. However, the faces on o on figs. 81, 85, 86, 87, 88, 89 approach it; but the angle of the incidence is that of the rhomboidal dodecaëdron, or 120°; whereas ours is that of the dodecaëdron with isosceles triangular faces. Most of the crystals in this group are truncated at the edges, like fig. 27 of Romé de l’Isle (see our geometrical figure at the bottom). These sloping truncations add 12 narrow pentagonal faces; and thus we have a new figure with 28 faces.

Copper pyrites may be known from iron pyrites by its brassy colour, smooth fracture, and not striking fire with steel.