(Sphenoidal).
In this class the only element of symmetry is a single dyad axis,
which is polar in character, being dissimilar at the two ends.
The form {010} perpendicular to the axis of symmetry consists of a
single plane or pedion; the parallel face is dissimilar in character
and belongs to the pedion {01'0}. The pinacoids {100}, {001}, {hol}
and {h'ol} parallel to the axis of symmetry are geometrically the
same in this class as in the holosymmetric class. The remaining forms
consist each of only two planes on the same side of the axial plane
XOZ and equally inclined to the dyad axis (e.g. in fig. 62 the two
planes XYZ and X'YZ'); such a wedge-shaped form is sometimes called a
sphenoid.
[Illustration: FIG. 64.--Enantiomorphous Crystals of Tartaric Acid.]
Fig. 64 shows two crystals of tartaric acid, a a right-handed crystal
of dextro-tartaric acid, and b a left-handed crystal of laevo-tartaric
acid. The two crystals are enantiomorphous, i.e. although they have
the same interfacial angles they are not superposable, one being the
mirror image of the other. Other examples are potassium
dextro-tartrate, cane-sugar, milk-sugar, quercite, lithium sulphate
(Li2SO4.H2O); amongst minerals the only example is the hydrocarbon
fichtelite (C5H8).