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)(NO3)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(NO3)2 and depletion in Pb(NO3)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)(NO3)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.