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In laboratory experiments, the precipitation of dolomite at ambient temperature is virtually impossible due to strong solvation shells of magnesium ions in aqueous media and probably also due to the existence of a more intrinsic crystallization barrier that prevents the formation of long-range ordered crystallographic structures at ambient surface conditions. Conversely, dolomite can easily form at high temperature (>100 °C), but its precipitation and growth requires several days or weeks depending on experimental conditions. In the present study, experiments were performed to assess how a single heat-ageing step promotes the formation of dolomite under high-carbonate alkaline conditions via dissolution-precipitation reactions. This reaction pathway is relevant for the so-called hydrothermal dolomite frequently observed in carbonate platforms, but still ill-defined and understood. Our precipitation route is summarized by two main sequential reactions: (1) precipitation of Mg-calcite at low temperature (∼20 °C) by aqueous carbonation of synthetic portlandite (Ca(OH) 2 ) in a highly alkaline medium (1 M of NaOH and 1 M of MgCl 2 ), leading to precipitation of oriented nanoparticles of low- and high-Mg calcite (∼79 wt%) coexisting with aragonite (∼18 wt%) and brucite (∼3 wt%) after 24 h; (2) fast dolomitization process starting from 1 h of reaction by a single heat-ageing step from ∼20 to 200, 250 and 300 °C. Here, the Mg-calcite acts as a precursor that lowers the overall kinetics barrier for dolomite formation. Moreover, it is an important component in some bio-minerals (e.g. corals and seashells). Quantitative Rietveld refinements of XRD patterns, FESEM observations and FTIR measurements on the sequentially collected samples suggest fast dolomite precipitation coupled with dissolution of transient mineral phases such as low-Mg calcite (Mg < 4 mol%), high-Mg calcite (Mg > 4 mol%), proto-dolomite (or disordered dolomite; Mg > 40 mol%) and Ca-magnesite. In this case, the dolomite formation rate and the time-dependent mineral composition strongly depend on reaction temperature. For example, high-purity dolomitic material (87 wt% of dolomite mixed with 13 wt% of magnesite) was obtained at 300 °C after 48 h of reaction. Conversely, a lower proportion of dolomite (37 wt%), mixed with proto-dolomite (43 wt%), Ca-magnesite (16 wt%) and high-Mg calcite (4 wt%), was obtained at 200 °C after 72 h. The present experiments provide an additional mechanism for the massive dolomite formation in sedimentary environments (ex. deep sea organic-rich carbonate-sediments) if such sediments are subjected to significant temperature variations, for example by hot fluid circulations related to volcanic activity. In such systems, organic degradation increases the carbonate alkalinity (HCO 3 − ) necessary to induce the dolomitization process at low and high temperature. [ABSTRACT FROM AUTHOR]

The microstructural evolution of CaSiO3 wollastonite subjected to carbonation reactions at T = 90 °C and pCO2 = 25 MPa was studied at three different starting conditions: (1) pure water; (2) aqueous alkaline solution (0.44 M NaOH); and (3) supercritical CO2. Scanning and transmission electron microscopy on reacted grains prepared in cross-section always revealed unaltered wollastonite cores surrounded by micrometer-thick pseudomorphic silica rims that were amorphous, highly porous, and fractured. The fractures were occasionally filled with nanometer-sized crystals of calcite and Ca-phyllosilicates. Nanoscale chemical profiles measured across the wollastonite-silica interfacial region always revealed sharp, step-like decreases in Ca concentration. Comparison of the Ca profiles with diffusion modeling suggests that the silica rims were not formed by preferential cation leaching (leached layer), but rather by interfacial dissolution-precipitation. Extents of carbonation as a function of time were determined by quantitative Rietveld refinement of X-ray diffractograms performed on the reacted powders. Comparing the measured extents of carbonation in water (condition 1) with kinetic modeling suggests that carbonation was rate-controlled by chemical reactions at the wollastonite interface, and not by transport limitations within the silica layers. However, at conditions 2 and 3, calcite crystals occurred as a uniform surface coating covering the silica layers, and also within pores and cracks, thereby blocking the connectivity of the originally open nanoscale porosity. These crystals ultimately may have been responsible for controlling transport of solutes through the silica layers. Therefore, this study suggests that pure silica layers were intrinsically non-passivating, whereas silica layers became partially passivating due to the presence of calcite crystallites. [ABSTRACT FROM AUTHOR]

Abstract: Carbonation reactions of wollastonite have been investigated in experiments conducted at conditions relevant to geologic CO2 sequestration in subsurface environments (T =90 °C and pCO2 =25 MPa). Experiments were performed in batch reactors on single-mineral powders of selected grain size, either in supercritical CO2 or in aqueous solutions with different alkalinities. The main reaction products were calcite and amorphous silica. Calcite occurred either as a compact, continuous coating of small crystals in the case of supercritical CO2 and in circum-neutral pH aqueous solutions, or as isolated larger crystals in acidic aqueous solutions. Measured extents of carbonation were faster in aqueous solutions than in supercritical CO2. Extents of carbonation in aqueous acidic solutions were modelled using a geochemical code incorporating kinetic parameters for wollastonite dissolution and calcite precipitation taken from the literature. Comparison with experimental results shows that, in aqueous solutions, wollastonite dissolution is the rate-limiting step of the carbonation reaction and that silica layers have only a minor passivation effect on the overall reaction rate. In acidic solutions, secondary calcite coating has only a minor effect on carbonation extent, whereas the experimental data in circum-neutral pH aqueous solutions are consistent with dissolution rates inhibited by the observed dense calcite coating. A geochemical code incorporating a modified reactive surface model successfully accounted for this passivation effect. The more efficient carbonation in acidic than in circum-neutral pH solutions is thus ascribed to differences in microstructures of the secondary calcite layers due to differences in supersaturation states of calcite when its precipitation begins. Inhibitory effects of dense calcite coating are also invoked for limiting carbonation extents of wollastonite in supercritical CO2. Secondary calcite precipitations are thus shown to play a major role both as coatings of reactive surfaces and by maintaining large undersaturation with respect to wollastonite dissolution. [Copyright &y& Elsevier]