Precipitation of ordered dolomite via simultaneous dissolution of calcite and magnesite: New experimental insights into an old precipitation enigma
Résumé
In the present study, we demonstrate that ordered dolomite can be precipitated via simultaneous dissolution of calcite and magnesite under hydrothermal conditions (from 100 to 200°C). The temperature and high-carbonate alkalinity have significantly co-promoted the dolomite formation. For example, when high-purity water was initially used as interacting fluid, only a small proportion of disordered dolomite was identified at 200°C from XRD patterns and FESEM observations. Conversely, higher proportion of ordered dolomite, i.e. clear identification of superstructure ordering reflections in XRD patterns, was determined when high-carbonate alkalinity solution was initially used in our system at the same durations of reaction. For this latter case, the dolomite formation is favorable therefrom 100°C and two kinetic steps were identified (1) proto-dolomite formation after about five days of reaction, characterized by rounded sub-micrometric particles from FESEM observations and by the absence of superstructure ordering reflections at 22.02 (101), 35.32 (015), 43.80 (021), etc. 2thetha on XRD patterns; (2) proto-dolomite to dolomite transformation, probably produced by a coupled dissolution-recrystallization process. Herein, the activation energy was estimated to 29 kJ/mol by using conventional Arrhenius linear-equation. This study provides new experimental conditions to which dolomite could be formed in hydrothermal systems. Temperature and carbonate alkalinity are particularly key physicochemical parameters to promote dolomite precipitation in abiotic systems.
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