Abstract
Computer simulations of the Mg2+ dehydration mechanism show that solution additives can stabilise undercoordinated Mg2+ hydration configurations, opening up coordination sites on the central Mg2+ ion, promoting Mg-carbonates nucleation and growth.
Highlights
Simulations of hydrated Mg2+, in the absence and presence of several solution additive anions, show that in pure liquid water Mg(H2O)62+ is the only stable coordination state; yet anions may stabilise undercoordinated five-hydration configurations
Due to the highly hydrated character of Mg2+ in aqueous solutions (ΔhydG° = −455 kcal mol−1),[8] the difficulty in precipitating magnesite has long been ascribed to the slow kinetics of Mg2+ dehydration.[9]
Geological records show that magnesite, MgCO3, and dolomite, CaMg(CO3)[2], form during weathering of ultramafic rocks (Mg-rich and low silica) and sedimentary processes taking place at low-T,10 which are different from the conditions necessary to stimulate the synthesis of the anhydrous forms of Mg-carbonates
Summary
New insights into the role of solution additive anions in Mg2+ dehydration: implications for. Due to the highly hydrated character of Mg2+ in aqueous solutions (ΔhydG° = −455 kcal mol−1),[8] the difficulty in precipitating magnesite has long been ascribed to the slow kinetics of Mg2+ dehydration.[9] geological records show that magnesite, MgCO3, and dolomite, CaMg(CO3)[2], form during weathering of ultramafic rocks (Mg-rich and low silica) and sedimentary processes (precipitation and/or replacement) taking place at low-T,10 which are different from the conditions necessary to stimulate the synthesis of the anhydrous forms of Mg-carbonates This conundrum, known as the “dolomite problem”, represents one of the most long-standing questions in low-temperature geochemistry.[11]. By increasing the temperature from 300 K to 700 K, a gradual stabilisation of Mg(H2O)52+ occurs and the activation barrier between six- and five-coordinated Mg2+ decreases (Fig. 1B, in orange), which is consistent with the experimental evidence that higher temperatures promote the direct precipitation of magnesite.[6,7]
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