Abstract
Carbonate mineralization is reasonably well-understood in the Ca–CO2–H2O system but continuously poses difficulties to grasp when Mg is present. One of the outstanding questions is the lack of success in dolomite MgCa(CO3)2 crystallization at atmospheric conditions. The conventional view holds that hydration retards the reactivity of Mg2+ and is supported by solvation shell chemistry. This theory however is at odds with the easy formation of norsethite MgBa(CO3)2, a structural analogue of dolomite, leading to the premise that crystal or molecular structural constrains may also be at play. The present study represents our attempts to evaluate the separate contributions of the two barriers. Crystallization in the Mg–Ba–CO2 system was examined in a non-aqueous environment and in H2O to isolate the effect of hydration by determining the minimal relative abundance of Mg required for norsethite formation. The results, showing an increase from 1:5 to 6:4 in the solution Mg/Ba ratio, represented a ~88% reduction in Mg2+ reactivity, presumably due to the hydration effect. Further analyses in the context of transition state theory indicated that the decreased Mg2+ reactivity in aqueous solutions was equivalent to an approximately 5 kJ/mol energy penalty for the formation of the activated complex. Assuming the inability of dolomite to crystallizes in aqueous solutions originates from the ~40 kJ/mol higher (relative to norsethite) Gibbs energy of formation for the activated complex, a hydration effect was estimated to account for ~12% of the energy barrier. The analyses present here may be simplistic but nevertheless consistent with the available thermodynamic data that show the activated complex of dolomite crystallization reaction is entropically favored in comparison with that of norsethite formation but is significantly less stable due to the weak chemical bonding state.
Highlights
Interests in carbonates trace back to 1870s, with the first recognition that CaCO3 may form different polymorphs [1]
Exclusive formation of norsethite required a strong presence of Mg (Mg/Ba > 7/3); decreasing Mg usually led to co-precipitation of norsethite and witherite first, followed by sole occurrence of witherite (Figure 1)
The Mg–Ba–CO2 system was investigated through crystallization experiments at various conditions in water and a non-aqueous environment to determine the minimal Mg to Ba (Mg/Ba) values at which norsethite can crystallize, and the measured difference was used to estimate the hydration effect on Mg2+ reactivity in the crystallization reactions
Summary
Interests in carbonates trace back to 1870s, with the first recognition that CaCO3 may form different polymorphs [1]. Modern-time motivation to study carbonates lies in the need to understand biomineralization [12] and the unique chemistry of mineral crystallization and dissolution where the thermodynamic equilibria between CO2 (g), HCO3− and CO3= (aq), and alkali earth metals control long-term climate [13,14]. Despite belonging to the same crystal system (trigonal/rhombohedral) as calcite CaCO3 (the most wellstudied member of carbonate minerals) and composing ~50% of the world carbonate formations [16], dolomite MgCa(CO3) has not been shown to crystallize in inorganic systems at ambient conditions. Dolomite (R3) differs from calcite (R3c) only in the absence of the c-glide plane because of the alternation of cation layers along the c-axis. The experimental tests far have shown that this conjecture is nowhere close to reality
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