%0 Journal Article
%A Fernández González, Ángeles
%A Prieto Rubio, Manuel
%A Putnis, Andrew
%A López De Andrés, María Sol
%T Concentric zoning patterns in crystallizing(Cd,Ca)CO3 solid solutions from aqueous solutions
%D 1999
%@ 0026-461X
%U https://hdl.handle.net/20.500.14352/57179
%X Otavite-calcite solid solutions have been synthesized to investigate the factors which control thepatterns of compositional zoning. The equilibrium partitioning of otavite between the aqueous and thesolid phase can be described by means of a function XCdCO3(XCd,aq)eq, which relates the solid molefraction and the aqueous activity fraction of Cd. According to this function, there is a strongpreferential partitioning of Cd towards the solid. A set of experiments starting with parent solutionswith different Cd2+/Ca2+ ratios was carried out. In these experiments nucleation takes place at highsupersaturations and the Cd2+ and Ca2+ ions tend to be laid down in a ratio which deviates from theequilibrium distribution. Experimental nucleation data can be fitted to a curve XCdCO3(XCd,aq)ef of thesame functionality as that for equilibrium.After nucleation, supersaturation decreases as the crystals grow. During this process, substituting ionsare not incorporated into the solid in the same stoichiometric proportion as in the aqueous solution.Therefore, crystal and fluid compositions tend to vary as growth proceeds and this evolution isregistered as a compositional zoning. The curves XCdCO3(XCd,aq)eq and XCdCO3(XCd,aq)ef confine thereaction path corresponding to the growth process. Here, these curves are used to account for theobserved zoning patterns.
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