MODELS OF GROWTH ZONING IN CRYSTALS OF SOLID SOLUTIONS. P.Azimov (Inst. of Precambrian Geology and Geochronology, Russia), A.Shtukenberg (St.Petersburg State University, Russia).

We have suggested expression for the supersaturation of solution containing several isomorphically cocrystallizing salts. Based on it the mathematical models were constructed for formation of growth zoning in mixed crystals. In the models considered we assume kinetic regime of crystallization; influence of mass transfer is not taken into account. The growing crystal is considered to be a three-dimensional polyhedron.

Three models relate the arising zoning to partition of components between solution and crystal in the closed system.

Model of isothermic desupersaturation. Initial supersaturation decreases and asymptotically vanishes. This process produces monotonic zoning. Concentration of the more soluble salts increases, while that of the less soluble one drops from the core to rim of the crystal.

Model of isothermic solvent evaporation. Under the constant rate eva-poration the supersaturation goes through two maxima and decreases with a limiting value above the equilibrium one. Salt molalities in the solution change nonmonotonically as well, thus producing complex zoning profiles.

Model of isohydric cooling of solution in closed system. The crystal growth goes simultaneously with the cooling. The time dependence of supersaturation for the exponentially slowing cooling has one maximum and asymptotically vanishes. Zoning is produced here by the partition between crystal and solution with changing distribution coefficient which temperature dependence strongly affects the zoning profiles.

Model of cooling of solution in open system. One more model considers crystallization in system having constant composition of the solution. Zoning is generated by variation of the distribution coefficient with temperature. Composition differences between core and rim of the crystal are small, but their trend may be opposite to the zoning trend of partition.