In general the growth rate of a surface will be controlled by a combination of structurally related factors, such as intermolecular bonds and dislocations, and external factors such as supersaturation, temperature, solvent and impurity concentration. The individual crystal faces each have their own growth-rate dependence on temperature and supersaturation. Therefore, altering the temperature and supersaturation history can affect crystal habit. The extent of changes that can be induced in this way can be investigated by changing experimental conditions. As a general rule, crystals become more extreme in habit as supersaturation levels increase.
Impurities operate by binding to growth sites, and thereby reducing the crystal growth rate.
Since different crystal faces can have different characteristics due to the orientational order imposed by the crystal lattice, specific impurities can bind effectively to some faces, but not others. These face-specific interactions result in modification of the crystal habit, and some impurities can have significant effects even at trace levels. Comparison of crystallization from pure and impure solutions may indicate whether or not significant habit modification is caused by the impurities in the test solutions. A granular or prismatic crystal habit is usually desired, but there are also specific occasions when other morphologies, such as plates or needles, may be wanted. The trace presence of foreign cations can exert an influence on the crystal habit of inorganic salts.