О ХОДЕ СОКРИСТАЛЛИЗАЦИИ ИЗОМОРФНЫХ ВЕЩЕСТВ В РАСТВОРАХ И ЗОНАЛЬНОСТИ КРИСТАЛЛОВ НА ПРИМЕРЕ РЯДА (BA,PB)(NO3)2

Abstract

For mixed crystals (Ba,Pb)(N03)2 were investigated phase equilibrium and processes of their growth in aqueous solutions. Micro-crystallization method was adapted for systems with isomorphic components, where a crystal composition deviates from the thermodynamic equilibrium even at a low supercooling. The solid phase is characterized by a continuous miscibility of the components within the analyzed ranges of temperature and solution composition. The diagram is characterized by a substantial nonlinearity of the solubility isotherms and a non-uniformity of the solid isocomposite location. Quasi-equilibrium trajectories change their slope versus Ba(N03)2 decrease and РЬ(ЫОз)2 increase in solution. The area of a curvature of the quasi-equilibrium trajectories is coordinated with the area of isotherm steepness change, i. e. the area of «remarkable» points where system features change sharply. The examples of theoretical zonality of a crystal approximated to a sphere were calculated for the trajectories at the temperature decrease from 50 to 15 °C. The content of the Ba-componcnt decreases toward the periphery with the sequence overgrowing of zones; each zone corresponds to 1 gram of the substance (Ba,Pb)(N03)2. Substantially, crystals grown in different initial solutions consist of different numbers of the zones. A non-linearity of the solubility isotherms causes these variations. A specificity of mixed crystal formation causes the discussed phenomena that are urgent to be taken into account at genetic interpretations of natural minerals characterized by a mixed composition.