On the genetic nature of isomorphism: Mechanism of component selection during crystal growth

Abstract

The concept of mixed-crystal formation, which has developed since the early 1980s, suggests the leading role of the metasomatic exchange between the crystal and solution. Compositions of solid and liquid phases shift to meet one another up to achieving thermodynamic equilibrium. This mechanism provides the regulation of the crystal composition during the process of isomorphism. The principles of the proposed assembly mechanism of selecting isomorphic components in the proportions dictated by a given combination of conditions (composition of the solution, supercooling or superheating, and temperature) are set forth as follows. The particle assemblages characterize a certain statistical distribution by the composition precipitate on the surface. Only those assemblages, which satisfy the aforementioned conditions, survive, whereas the other assemblages are dissolved, initiating the salting-out of the former assemblages. The validity of the proposed model is corroborated by the development of the assembly structure of autoepitaxial (K,Rb)HC8H4O4 nuclei on a KHC8H4O4 crystal observed with a Park CP atomic force microscope. The nuclei are widely scattered in composition, degree of faceting, and size, demonstrating their different stability and growth rate. Another example is the distribution of spontaneously precipitated mixed K(Br,Cl) crystals by isomorphic composition and size. The small and large fractions are enriched in KBr in comparison with medium-size fractions, while the volumetric proportions of the fractions reveal a bimodality with the complete absence of fractions 30–50 μm. The traditionally discussed statistical selection of particular ions, which is probably second in importance; however, it may become crucial when the amount of one of the isomorphic components is rather small. While oversaturation is fairly high, crystallization without selection may be expected.