Holohedral Class
(Holohedral; Bipyramidal).
Here there are three dissimilar dyad axes of symmetry, each coinciding with a crystallographic axis; perpendicular to them are three dissimilar planes of symmetry; there is also a centre of symmetry. There are seven kinds of simple forms:—
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| Fig. 54. | Fig. 55. |
| Orthorhombic Bipyramids. | |
Bipyramid (figs. 54 and 55). This is the general form and is bounded by eight scalene triangles; the indices are {111}, {211}, {221}, {112}, {321}, {123}, &c., or in general {hkl}. The crystallographic axes join opposite corners of these pyramids and in the fundamental bipyramid {111} the parametral plane has the intercepts a : b : c. This is the only closed form in this class; the others are open forms and can exist only in combination. Sulphur often crystallizes in simple bipyramids.
Prism. This consists of four faces parallel to the vertical axis and intercepting the horizontal axes in the lengths a and b or in any multiples of these; the indices are therefore {110}, {210}, {120} or {hko}.
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| Fig. 56.—Macro-prism and Brachy-pinacoid. | Fig. 57.—Brachy-prism and Macro-pinacoid. |
Macro-prism. This consists of four faces parallel to the macro-axis, and has the indices {101}, {201} ... or {hol}.
Brachy-prism. This consists of four faces parallel to the brachy-axis, and has the indices {011}, {021} ... {okl}. The macro- and brachy-prisms are often called “domes.”
Basal pinacoid, consisting of a pair of parallel faces perpendicular to the vertical axis; the indices are {001}. The macro-pinacoid {100} and the brachy-pinacoid {010} each consist of a pair of parallel faces respectively parallel to the macro- and the brachy-axis.