Compound Crystals. Several molecular crystals contain more
than one component. Substances crystallizing with solvent (mostly water)
molecules have already been mentioned. There are several other examples,
such as crystals containinga-andb-lactose in the stoichiometrya 5 b 3. If the
composition of the crystals can vary continuously, be it in all proportions or
within a limited range, one speaks ofsolid solutions. These can occur if the
molecules involved are very similar and their packing in the crystal is not
very tight. The prime example is, again, acylglycerols.
FIGURE15.5 Variation in crystal morphology for identical unit cells. (a) Crystals
in the rhombohedral system that only have faces present in the unit cell: (001), (010),
and (100). (b) Cubic system; the leftmost picture is a cube, the rightmost one a
regular octahedron; in between are intermediate shapes. (c) Examples of the various
shapes that ana-lactose monohydrate crystal (monoclinic) can assume in practice. In
(a) and (b) the shapes are shown in perspective. In (c) we have projections (all in the
same direction with respect to the axes of the unit cell).