Crystallization of solid solutions from aqueous solutions in a porous medium: zoning in (Ba, Sr)SO₄

Abstract

Barite-celestite solid solutions have been synthesized under controlled conditions by the counter-diffusion of Ba2+, Sr2+and SO42–ions through a porous transport medium (silica gel), to investigate the factors which control compositionalzoning. The equilibrium compositions of solid solution and aqueous solutionhave been determined from the relative solubilities of barite and celestite, predicting that virtually pure barite should precipitate from Sr-rich solutions. However, nucleation and growth in a porous medium, where mass transport is by diffusion, takes place at very high supersaturations. The threshold supersaturation for nucleation is inversely related to the solubility and is therefore much higher for barite than celestite. The balance between the thermodynamics and kinetics of nucleation and growth at high supersaturations leads to zoning patterns the reverse of those predicted at equilibrium. At high supersaturations the zoning is periodic and sector-controlled with many of the general features observed in natural minerals. Oscillatory zoning with compositional gaps can take place without the need to invoke miscibility gaps or periodic variations in externally controlled intensive parameters.

Molecular-scale surface processes during the growth of calcite in the presence of manganese

“…For the Mn-Ca-COrH 2 0 SS-AS system, the function f3(x) that expresses the supersaturation of aqueous solutions with respect to solid solutions has the following form (e.g., Prieto et al, 1993;Putnis et al, 1995):…”

Nucleation of solid solutions crystallizing from aqueous solutions

“…Only relatively recently have there been significant advances in understanding the thermodynamics of such solid-solution -aqueous-solution (SS -AS) systems (Lippmann 1980(Lippmann , 1982Glynn & Reardon 1990). The way in which the supersaturation of a given aqueous solution should be defined, relative to all possible solid-solution compositions, is still contro versial (Prieto et al 1993;Pina et al 2000;Astilleros et al 2002). For the thermo dynamic representation of SS -AS systems, the mixing properties of the solid solution as well as the aqueous solution must be known.…”

A combined in situ AFM and SEM study of the interaction between celestite (0 0 1) surfaces and carbonate-bearing aqueous solutions

“…In the case of the celestite-strontianite system, we can also conclude that determining overall reaction rates for processes which in volve simultaneous dissolution and crystallization reac tions must be done taking into account both the spatial and temporal relationships between these reactions on a molecular scale. 6. Concluding remarks 1.…”

“…6 3 [5]) determines that the geochemical conditions that would allow to precipitate intermediate solid solutions are very restrictive. Thus, the chemical bimodality of this solid solution can be considered a consequence of the important differences in crystallization kinetics resulting from the high difference in solubility between the two end-members [6,7], It has also been pointed out that crystals of celestite barite solid solution could act as precursors for the forma tion of other phases relatively common in nature, mainly carbonates corresponding to the Ba"Srl_xC03 solid solu tion [8]. The fact that intermediate compositions of…”