Admixtures on Crystal Shape

Under conditions of extremely slow growth, the shape of a crystal Is determined by thermodynamics the crystal tends to grow to a shape of a polyhedron having minimwn siaface energy [75) for faces holds 

Such equilibrium shape can be affected by admixtures In the case that they change the specific surface energy of individual faces In a different way. This condition is. however, met only exceptionally, as crystals usually grow under highly non-equlllbrlum conditions. 

Crystal shape Is usually determined by the growth rates of individual crystal faces. According to the prtnc jAe of ooerlappingfaces 169 the faces that grow more slowly remain In the final crystal shape whereas the 

As the Interatomic spaclngs and electric fields are different for different crystal surfaces [82], selective adsorption of individual admixtures occurs on the most favoured ones [11,31,35,163,195]. It Is therefore often 

Example How would the addition of Al3+ affect the habit of ammonium sulphate crystals The lattice dimensions of ammonium sulphate are a - 0.595 nm. b -1.056 nm. c - 0.773 nm. and that of aluminium ammonium sulphate is a = 1.22 nm. Each basal surface of ammonium sulphate cells contains two molecules. Their mean distances are as follows  

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The effectiveness of an admixture Is closely bound to the given system and cannot be simply generalized. For the activity of additives on the crystal shape, Buckley (32] defined the measure of the effectiveness of the additive as the number of weight units of the crystallizing substance per one unit of the additive that causes a certain shape modification. Another way of measurement and evaluation of the effectiveness of admixtures has also been described in the literature (219. The influence of admixtures drops with Increasing temperature and growth rate (32). 

The behaviour of crystalline solids is not always defined explicitly, because their properties depend on structure, size and shape of crystals and on the admixture of contaminating components. The following account is aimed mainly at the characteristic properties of pure substances (minerals) with reference to the effect of impurities, wherever the respective relationships are of more general validity. 

Crystallization Is one of the oldest separation operations In chemical industries. It serves not only to separate and purify substances, but also to produce crystals with a required shape. Both of these aspects are closely connected with the presence of admixtures In the solution. Among the many factors affecting the process of crystallization 1172.226], (e.g.. temperature, supersaturation, agitation), admixtures often exhibit the most pronounced effect. Even traces of admixtures can Influence the nucleatlon, crystal growth, shape and size of product crystals, and also other properties (caking, hygroscoplclty, etc.). On the other hand, they may be entrained into crystals and lower their purity. 

Another theory 1201) supposes that admixtures can affect the shape of crystals only when they are incorporated into the crystalline body. An admixture particle adhering to a crystal face acts on the deposition of the macrocomponent independently of its position If it Is Incorporated into the lattice. It can affect Just particles lying very close to It. As the admixtures are adsorbed on different faces in a different amount, they affect the growth of these faces in a different degree. 

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