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Ch. 1: Form of Minerals

Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals Text size:minus plus Restore normal size   Mail page  Print this page
42
A PRACTICAL TREATISE ON GEMS.
second prism o=P2, producing an equiangular twelve-sided prism, which always represents the combination,
and cannot occur as a simple form. An example of a more complicated combination is seen in fig. 39, of a crystal of apatite, whose sign with the corresponding letters is
Hexagonal minerals more frequently crystallize in those series of hemihedral forms that are named rhombohedral, from the prevalence in them of rhombohedrons. These are bounded by six rhombs (tig. 40), whose lateral edges do
not lie in one plane, but vise and fall in a zig-zag manner. The principal axis unites the two trigonal angles, formed by three equal plane angles, and in the most common variety the secondary axes join the middle points of two opposite edges. When the polar edges form an angle of more than 90°, the rhombohedrons are named obtuse; when of less, acute. Hexagonal scalenohedrons (fig. 41) are bounded by twelve scalene triangles, whose lateral edges do not lie in one plane. The principal axis joins the two hexagonal angles, and the secondary axes the middle points of two opposite lateral edges.
The rhombohedron is derived from the first kind of hexagonal pyramid by the hemihedric development of its
alternate faces. Its general sign should therefore be
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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