Carbonate mineral paragenesis and reaction kinetics in the system MgO–CaO–CO2–H2O in presence of chloride or nitrate ions at near surface ambient temperatures

Abstract

The reaction kinetics and solid phase products following the dissolution of Mg(OH)2 by CO2 sparging in the presence of calcium salts at 35°C, over a thirty day period have been studied. Experiments [A] and [B] were conducted with CaCl2 salts with [Mg2+(aq):Ca2+(aq)] molar ratios 5:1 and 10:1 respectively. Experiment [S] employed [Mg2+(aq):Ca2+(aq)=5:1] ratio but was seeded with hydromagnesite. Experiment [N] employed calcium nitrate [Mg2+(aq):Ca2+(aq)=5:1]. Results from all experiments show that magnesian calcite is the initial anhydrous carbonate to form, but with time this reacts and is replaced by aragonite formation. Towards the end of experiments formation of calcite/magnesian calcite is mildly increasing at the expense of aragonite. Aragonite production is coeval with the generation and progressive decomposition of nesquehonite [Mg(HCO3,OH)·2H2O] forming Mg5(CO3)4(OH)2·xH2O mineral phases (where x=8 and 5H2O) in conjunction with subordinate barringtonite [MgCO3·2H2O]. The latter mineral is interpreted as an indicator of incongruent dissolution of nesquehonite. Experiments [A] and [B] document a short lived episode of chlorartinite [Mg2(CO3)Cl(OH)·3H2O] production, interpreted as an unstable intermediate between Mg(OH)2 and Mg(HCO3,OH)·2H2O. Chlorartinite is not detected in experiment [S] indicating that either accelerated reaction rates in the seeded environment make the phase extremely short lived, or the direct path from [Mg(OH)2] to nesquehonite is kinetically favoured. Seeding also stimulated hydromagnesite growth. However it was insufficient to adequately ease supersaturation resulting in coeval nesquehonite formation and transformation. Aragonite formation in experiment [N] was delayed relative to the other experiments. This time delay suggests that until nitrate depletion was achieved through nitro-magnesium carbonate [Mg(NO3)2·6H2O] formation, precipitation of aragonite is suppressed. Based on all the experimental data, it is suggested that carbonate mineral paragenesis is driven by geochemical feedback between a range of calcium and magnesium carbonate dissolution–precipitation events and is a sensitive function of the experimental conditions.