In this work, we aimed to experimentally study the nucleation and growth of CaCO₃ phases precipitated from supersaturated aqueous solutions in the presence of varying concentrations of sulphate oxyanion. The experiments were conducted under pH conditions close to neutral (7.6) and by considering a wide range of initial (SO₄²⁻)/(CO₃²⁻) ratios (0 to approx. 68) in the aqueous solution. We paid special attention to the evolution of the precipitates during ageing within a time framework of 14 days. The mineralogy, morphology, and composition of the precipitates were studied by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and EDX microanalysis. The concentration of sulphate ions in the reacted aqueous solution was studied by ICPs. The experimental results showed that the mineral composition of the precipitate recovered in each run varied with the (SO₄²⁻)/(CO₃²⁻) ratio in the parental solution, which influenced the mineral evolution of the precipitates during ageing. We observed that high concentrations of sulphate in the aqueous solution stabilized the vaterite precipitates and inhibited calcite formation. Furthermore, aragonite never precipitated directly from the solution, and it was only formed via a dissolution-precipitation process in solutions with a high (SO₄²⁻)/(CO₃²⁻) ratio after long reaction times. Finally, gypsum only precipitated after long ageing in those aqueous solutions with the highest concentration of sulphate. The reaction pathways during ageing, the morphology of the calcite crystals, and the composition of vaterite and calcite were discussed considering both kinetic and thermodynamic factors. These results showed a considerably more complex behavior of the system than that observed in experiments conducted under higher pHs and supersaturation levels and lower (SO₄²⁻)/(CO₃²⁻) ratios in the aqueous phase.

Barite (BaSO₄) and celestite (SrSO₄) are the end-members of a nearly ideal solid solution. Most of the exploitable deposits of celestite occur associated with evaporitic sediments which consist of gypsum (CaSO₄·2H₂O) or anhydrite (CaSO₄). Barite, despite having a broader geological distribution is rarely present in these deposits. In this work, we present an experimental study of the interaction between gypsum crystals and aqueous solutions that bear Sr or Ba. This interaction leads to the development of dissolution-crystallization reactions that result in the pseudomorphic replacement of the gypsum crystals by aggregates of celestite or barite, respectively. The monitoring of both replacement reactions shows that they take place at very different rates. Millimeter-sized gypsum crystals in contact with a 0.5 M SrCl₂ solution are completely replaced by celestite aggregates in less than 1 day. In contrast, only a thin barite rim replaces gypsum after seven days of interaction of the latter with a 0.5 M BaCl₂ solution. We interpret that this marked difference in the kinetics of the two replacement reactions relates the different orientational relationship that exists between the crystals of the two replacing phases and the gypsum substrate. This influence is further modulated by the specific crystal habit of each secondary phase. Thus, the formation of a thin oriented layer of platy barite crystals effectively armors the gypsum surface and prevents its interaction with the Ba-bearing solution, thereby strongly hindering the progress of the replacement reaction. In contrast, the random orientation of celestite crystals with respect to gypsum guarantees that a significant volume of porosity contained in the celestite layer is interconnected, facilitating the continuous communication between the gypsum surface and the fluid phase and guaranteeing the progress of the gypsum-by-celestite replacement.

The sorption of dissolved inorganic pollutants into the structure of minerals is an important process that controls the mobility and fate of these pollutants in the Earth’s crust. It also modifies the surface structure and composition of the host mineral, affecting its crystallization kinetics. Here, we investigate the effect of hexavalent chromium, Cr(VI), on the nucleation and growth of gypsum by conducting two types of experiments: (i) in situ atomic force microscopy (AFM) observations of the growth of gypsum {010} surfaces in the presence of Cr(VI) and (ii) gypsum precipitation experiments by mixing aqueous solutions containing variable amounts of Cr(VI). Gypsum precipitation is progressively delayed when occurring from solutions bearing increasing Cr(VI) concentrations. Chemical analyses of gypsum precipitates show that gypsum incorporates small Cr(VI) amounts that correlate with the content of this ion in the aqueous solution. Gypsum cell parameters variation reflects this incorporation. At the molecular scale, Cr(VI) induces a slowdown of step advance rates on gypsum {010} surfaces accompanied by the roughening of nanostep edges and the so-called “template effect”. This effect involves the reproduction of the original nanotopography after the completion of individual advancing monolayers and appears as a general nanoscale phenomenon occurring during growth of solid solutions from aqueous solutions even in the case of compositionally-restricted solid solutions.

Epitactic crystal growth plays a main role in the development of mineral processes and in the synthesis of advanced materials. Celestite (SrSO₄) forms epitactic overgrowths on anhydrite (CaSO₄) (100), (010) and (001) surfaces upon interacting with Sr-bearing aqueous solutions. Two populations of differently oriented celestite crystals related by symmetry operators of substrate are identified on (001)_Anh and (100)_Anh anhydrite substrates by SEM observations and synchrotron X-ray diffraction analysis. Substrate-induced twins arise after the coalescence of individuals belonging to these populations. Progressing growth results in a marked morphological evolution of epitactic celestite, whose crystals undergo sustained branching and loss of co-orientation that result in the formation of sheaf-like aggregates, on (100)_Anh, and swan-like aggregates, on (001)_Anh. We relate this evolution to celestite growth in a Ca-rich environment due to continued anhydrite dissolution and incorporation of small amounts of Ca into celestite structure. This incorporation would induce lattice strain which would be released through the formation of dislocations. The regular arrangement of these dislocations in small-angle boundaries would result in progressive splitting, driving the evolution from celestite single crystals to aggregates. Sharp compositional gradients in the boundary layer could explain the anisotropic development that leads to the formation of the swan-like celestites.