(Hemimorphic).
Here there is only one dyad axis in which two planes of symmetry
intersect. The crystals are usually so placed that the dyad axis
coincides with the vertical crystallographic axis, and the planes of
symmetry are also vertical.
The pyramid {hkl} has only four faces at one end or other of the
crystal. The macro-prism and the brachy-prism of the last class are
here represented by the macro-dome and brachy-dome respectively, so
called because of the resemblance of the pair of equally sloped faces
to the roof of a house. The form {001} is a single plane at the top of
the crystal, and is called a "pedion"; the parallel pedion {001'}, if
present at the lower end of the crystal, constitutes a different form.
The prisms {hko} and the macro- and brachy-pinacoids are geometrically
the same in this class as in the last. Crystals of this class are
therefore differently developed at the two ends and are said to be
"hemimorphic."
[Illustration: FIG. 60.--Crystal of Hemimorphite.]
[Illustration: FIG. 61.--Orthorhombic Bisphenoid.]
Fig. 60 shows a crystal of the mineral hemimorphite (H2Zn2SiO5) which
is a combination of the brachy-pinacoid {010} and a prism, with the
pedion (001), two brachy-domes and two macro-domes at the upper end,
and a pyramid at the lower end. Examples of other substances belonging
to this class are struvite (NH4MgPO4.6H2O), bertrandite (H2Be4Si2O9),
resorcin, and picric acid.