Cocrystallization of isomorphic components in solutions and crystal zoning: An example of the (Ba,Pb)(NO3)2 series

Abstract

The phase equilibrium and growth of mixed (Ba,Pb)(NO 3 ) 2 crystals in aqueous solutions were investigated. The microcrystallization method was adapted to systems with isomorphic components, where crystal composition deviates from the thermodynamic equilibrium even at a low supercooling. The solid phase is characterized by continuous miscibility of the components within the analyzed ranges of temperature and solution composition. The diagram is characterized by substantial nonlinearity of the solubility isotherms and nonuniformity of the solid isocomposite location. Quasiequilibrium paths change their slope depending on solution enrichment in Ba(NO 3 ) 2 and depletion in Pb(NO 3 ) 2 . The area of curvature of quasiequilibrium paths is coordinated with the area of the changing isotherm slope, i.e., the area of "remarkable" points, where the behavior of the system changes drastically. Examples of theoretical zoning of a crystal approximated to a sphere were calculated for paths at a temperature decreasing from 50 to 15 ° C. The content of the Ba component decreases toward the periphery with the consecutive overgrowing of zones. Each zone corresponds to one gram of (Ba,Pb)(NO 3 ) 2 . Crystals grown from different initial solutions consist of different numbers of zones owing to the nonlinearity of solubility isotherms. Specific features of mixed crystal formation should be taken into account in genetic interpretation of natural minerals of mixed composition.

Phase Relations of Mixed Crystals with Solutions: Characterization and Influence on Crystal Formation

“…The use of a nomogram allows determination of the pathways of quasi-equilibrium or equilibrium crystallization for any initial point of any system. Crystals formed in different original solutions have a substantially different character of zoning [5].…”

“…To achieve it, the experimental data were plotted simultaneously on a concentration diagram [2] and a diagram presenting the total salt concentration versus component ratio [19]. Isotherms of solubility were then constructed by means of interpolations made in both diagrams, provided the curves became monotonous after performing interrelated corrections in the manner described in [5]. Fig.…”

“…The mean of the range was accepted as the saturation temperature value. The method was initially developed for compounds of fixed compositions [18] and was adapted [5,16,17] for systems saturated with an isomorphic pair. The adaptation included the following adjustments: (i) It was necessary to minimize the influence of supersaturation upon the crystal composition during all growth stages, otherwise the error of the equilibrium temperature determination can reach 2°C or more.…”

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

“…© Pleiades Publishing, Ltd., 2007 Original Russian Text © A.E. Glikin, 2007, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2007 Crystallochemical investigations of isomorphism have now continued for almost two centuries, starting with the discovery of this phenomenon by E. Mitscherlich in 1819. The constitution and properties can be satisfactorily explained for compounds of variable composition, thanks largely to the profound achievement in the structural analysis of crystals.…”