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}.
[Illustration: FIG. 56.--Macro-prism and Brachy-pinacoid.]
[Illustration: 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.
Figs. 56-58 show combinations of these six open forms, and fig. 59 a
combination of the macro-pinacoid (a), brachy-pinacoid (b), a prism
(m), a macro-prism (d), a brachy-prism (k), and a bipyramid (u).
[Illustration: FIG. 58.--Prism and Basal Pinacoid.]
[Illustration: FIG. 59.--Crystal of Hypersthene.