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.

The nucleation and growth of CaCO 3 phases from aqueous solutions with SO 4 2À :CO 3 2À ratios from 0 to 1.62 and a pH of $10.9 were studied experimentally in batch reactors at 25°C. The mineralogy, morphology and composition of the precipitates were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and microanalyses. The solids recovered after short reaction times (5 min to 1 h) consisted of a mixture of calcite and vaterite, with a S content that linearly correlates with the SO 4 2À :CO 3 2À ratio in the aqueous solution. The solvent-mediated transformation of vaterite to calcite subsequently occurred. After 24 h of equilibration, calcite was the only phase present in the precipitate for aqueous solutions with SO 4 2À:CO 3 2À 6 1. For SO 4 2À :CO 3 2À > 1, vaterite persisted as a major phase for a longer time (>250 h for SO 4 2À :CO 3 2À = 1.62). To study the role of sulfate in stabilizing vaterite, we performed a molecular simulation of the substitution of sulfate for carbonate groups into the crystal structure of vaterite, aragonite and calcite. The results obtained show that the incorporation of small amounts (<3 mole%) of sulfate is energetically favorable in the vaterite structure, unfavorable in calcite and very unfavorable in aragonite. The computer modeling provided thermodynamic information, which, combined with kinetic arguments, allowed us to put forward a plausible explanation for the observed crystallization behavior.

We investigated the influence of the porosity of the growth medium on the crystallization of calcium carbonate in hydrogels with different gelatin solid contents (2.5, 5, and 10 wt %). In all experiments, the precipitate consisted of calcite with occasional occurrences of some vaterite and aragonite. The calcite grew as compact radial intergrowths of crystals that show rhombohedral external faces. The crystal surfaces consist of identical 1−10 μm sized rhombohedral sub-blocks. Electron backscatter diffraction (EBSD) uncovered the radial intergrowth structure of the aggregates. EBSD also documented the internal microscale mosaicity and mesocrystal-like constitution of the gel-grown calcite. Raman spectroscopy and thermogravimetric analysis confirmed the presence of gelatin within the crystals. It reached up to ∼2 mass % in the calcite-gelatin composites that formed in hydrogels with 10 wt % gelatin content. Calcite morphology and mosaicity varied with the gelatin content of the hydrogel, such that an increase in gelatin content initiated the growth of smaller crystal aggregates having progressively rougher surfaces, increasing amounts of incorporated gel, and increasing degrees of misorientation in the internal mosaic structure. Apart from biospecific morphology, the gel growth experiment successfully mimics many characteristics of calcite biomineralization such as formation of a hierarchical hybrid composite, crystal mosaicity, and mesocrystal-like constitution.

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We investigate the effect of gelatin, agarose, and silica hydrogel with and without magnesium in the growth medium on calcite single crystal growth and aggregate formation. We characterize the hydrogel and the mineral by cryo-scanning electron microscopy (SEM), high-resolution SEM, and electron backscatter diffraction (EBSD). We image the pristine hydrogel fabric and the fabric of hydrogel incorporated into the mineral. We visualize the hydrogel–mineral interface and investigate the effect of the hydrogels on calcite micro- and mesostructure in the gel/calcite composits. We compare hydrogel fabrics in biomimetic hybrid composites with biopolymer matrices and networks in biological carbonate tissues of bivalves, gastropods, brachiopods, and corraline red algae. In Mg-free environments, silica gel has very little effect on crystal morphology and arrangement; the gel/calcite composite that forms is a single gradient mesocrystal. Agarose and gelatin hydrogels influence mineral organization in gel/calcite aggregates, and these consist of very few subunits separated by hydrogel membranes. With Mg added to the growth medium, large and small angle boundaries highly increase in number: silica gel/calcite aggregates consist of partial spherulites with mesocrystalline subentities; agarose, gelatin gel/calcite aggregates are regular spherulites, and their subentities are single crystals. Thus, calcite crystal organization is influenced by accumulative split growth provoked by incorporation of magnesium.

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.

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