is
bounded by four equilateral triangles, has six equal edges with faces
meeting at Ï0° 32', and four trigonal angles. The principal axes join
the middle points of each two opposite edges.—Ex., gray-copper ore,
boracite, and helvine. (2.) The trigonal dodecahedrons (fig. 9) are
bounded by twelve isosceles triangles, and vary in general form from
the tetrahedron to the hexahedron. There are six longer edges
corresponding to those of the inscribed tetrahedron, and twelve shorter
placed three and three over each of its faces ; and four hexagonal and
four trigonal angles.—Ex., gray-copper ore, and bismuth-blende. (3.)
The deltoid-dodecahedrons (fig. 10) are bounded by twelve deltoids, and
vary in general form from the tetrahedron on the one hand, to the
rhombic-dodecahedron on the other. They have twelve longer edges lying
in pairs over the edges of the inscribed tetrahedron ; and twelve
shorter edges, three and three over each of its faces. The angles are
six tetragonal (rhombic), four acute trigonal, and four obtuse
trigonal angles. The principal axes join two and two opposite rhombic
angles.—Ex., gray-copper ore. (4.) The hex-akistetrahedrons (fig. 11)
are bounded by twenty-four
