|
30
|
A PRACTICAL TREATISE ON GEMS.
|
||
|
|
|||
|
the most common variety being
ore. The deltoid-dodecahedron is in like manner the result of the increase of the alternate trigonal groups of faces of the
triakisoctahedron, and its sign is
tetrahedron arises in the development of alternate hexagonal groups of faces in the hexakisoctahedron, and its sign
is
The parallel-faced semitesseral forms are two. (1.) The pentagonal dodecahedrons (fig, 12) are bounded by twelve
|
|||
|
|
|||
|
|
|||
|
|
|||
|
symmetrical
pentagons, and vary in general aspect between the hexahedron and
rhombic-dodecahedron. They have six regular (and in general longer)
edges, lying over the faces of the inscribed hexahedron, and
twenty-four generally shorter (seldom longer) edges, usually lying in
pairs over its edges. The angles are eight of three equal angles, and
twelve of three unequal angles. Each principal axis unites two
opposite regular edges. This form is
derived from the tetrakishexahedron, and its sign is
quently in iron pyrites and cobaltine. (2.) The dyakisdo-
|
|||
|
|
|||