MEMOIRS, &c. 
I. — On the Crystallisations of Copper-Pyrites. 
By W. Haidingee, Esq. of Freyberg, Member of the 
Wernerian Society, &c. 
TThE regular forms of Pyramidal Copper-Pyeites^ 
a mineral so well known as an ore of copper, and so gene- 
rally spread over the earth, have, till of late, been described 
very imperfectly by mineralogical authors. The crystals of 
this mineral were taken for octahedrons and tetrahedrons, 
modified by the cube, the rhomboidal dodecahedron, &c. ; 
and the cleavage exhibited in the massive varieties was sup- 
posed to take place in the direction of the faces of the octa- 
hedron. 
A group of very distinct crystals, which' I had the op- 
portunity of examining in the year 1817, shewed, however, 
forms entirely incompatible with that of the regular octa.. 
hedron, and with such as can be derived from it. These 
crystals had almost the shape of tetrahedrons, with trun- 
cated angles, as in fig. 1. (PL I.) The solid angle H was re- 
placed by a face of cleavage, the intersections EF and FG 
of which, with the adjacent faces, were parallel to the lines 
AC and BD in these faces, the said lines being perpendi- 
cular to the edges IH and KL. If by planes parallel to 
VOL. IV. A 
