The relationship between the supersaturation at the point of crystallization and the rate at which supersaturation increases has been studied from nucleation experiments on barite BaSO₄, strontianite SrCO₃, witherite BaCO₃ and gypsum CaSO₄.2H₂O. The crystallization experiments have been carried out by the counter-diifusion of cations and anions through a column of porous silica gel transport medium. Nucleation is suppressed in a finely-porous medium resulting in very high values of supersaturation before crystallization from the solution begins. This threshold supersaturation for nucleation depends on the solubility of the salt, the porosity of the medium and the supersaturation rate. Nucleation inhibitors were used to extend the range of supersaturation attainable. In all cases the experimental data fits the general expression: rate of change of supersaturation ∝ (threshold supersaturation)^m. These results are compared to previous work from the field of chemical engineering on the relationship between supersaturation, volume and cooling rate in aqueous salt solutions. These experiments have important implications to supersaturation in natural fluids and subsequent crystallization in relation to geological problems including crystallization in low temperature sedimentary environments and fluid-rock ratios in hydrothermal mineral deposits.

Barite-celestite solid solutions have been synthesized under controlled conditions by the counter-diffusion of Ba₂₊, Sr₂₊and SO₄^2–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.