Crystal Growth
CRYSTAL GROWTH. The growth of a crystal starts with a nucleus and proceeds by the deposition of layers of the same substance on its surface. This contrasts with the growth of living cells which takes place internally. In a growing crystal the rate of deposition on all faces is usually constant, so that its actual shape does not change during growth. In some cases, however, certain faces grow at a greater rate than others, and the crystal continually changes its shape during growth. If during crystallisation the material deposited on the crystal faces varies in colour or opacity, the completed crystal will have a zoned structure, by means of which its growth pattern can now be studied (Plate 74). There is usually a very gradual change in the composition, concentration, pressure and temperature of the solution in which crystals are growing, and a change in colour or opacity within a crystal is thus usually best seen in those which have taken a long time to form. In this way certain long prismatic minerals are formed, whose ends show a variation in colour or opacity. Such long, pencil-like crystals are commonly found in druses. They are often opaque at their base, becoming gradually clearer upwards until at their apex they are completely transparent (Plates 2, 6). If the supply of the mother liquor is cut off completely, crystal growth will cease. In the flat, tabular aggregates shown in PL. 49, for instance, the growth of all the numerous needle-like crystals came to an end at the same time.