Dolomite and Magnesite Mineralization: the roles of Mg-hydration

H. Henry Teng (Department Chemistry, George Washington University)

Magnesium is the second most widely occurring metal component in carbonates next only to calcium. Mg-bearing carbonate minerals play critical roles in the health of Earth system as they constitute a significant fraction of lithosphere carbon reservoir and build skeletal structures for the majority of marine invertebrate organisms. Despite the wide occurrence, high-Mg and sole- Mg phases such as dolomite ([Ca,Mg]CO₃) and magnesite (MgCO₃) prove to be virtually impossible to crystallized under ambient conditions. For long it is believed that Mg hydration is the culprit for this geological mystery because of the smaller size and, consequently, higher charge density of Mg²⁺ relative to Ca²⁺. The seemingly clear understanding in the role of water leads to an interesting question: will magnesite precipitate if the formation of hydration shell is breached or prevented? Meanwhile, a plethora of literature data documented the occurrence of dolomite in the presence of sulfur reducing bacteria SRB, leaving no lucid explanation for the kinetic effect of water. The ultimate goal of our research is to address these issues through experimentally testing the follwoing hypotheses: (1) weakend solvation shell around Mg²⁺ will lead to MgCO₃ crystallization if dehydration is the kinetic hindrance and (2) sulfide is the ultimate mediating agent for SRB facilitated dolomite formation. Crystallization experiments were carried out in dry organic and organo-aqueous binary solvents and in the presence of sulfide ions. Experimental results indicate (1) Anhydrous MgCO₃ precipitates readily under dry conditions but appears only in amophous form. (2) Crysalline anhydrate only forms when Mg/Ca in the organic solvent goes below 1. At Mg:Ca = 1:1, proto-dolomite with ~37% MgCO₃ was detected. (3) Tri-hydrate MgCO₃ (nesqhehonite) is the dominant phase in binary solvent, similar to the result in pure aqueous environments. However, the presence of organic solvent greatly accelerate the crystallization kinteics. (5) Input of solutions with Mg/Ca = 5 instantly halts the growth on seeded calcite crystals in saturated solutions. However, in situ AFM observations on the (104) faces reveals the inhibation tackles primarily nulceation but not step flow despite the change of hillock morphology. (6) The retardation effect of Mg is greatly alleviated, if not eliminated at all, when the input solutions contains 0.5 mM sulfide. Initial effect of Mg changes the hillock from the rhombic to a tear-drop mophology, but continued growth leads to step bunching in the < 441 > directions and ultimately a return to the cleavage form. The observations indiate that there may exsit a previous unrecognized and more intrinsic mineralization barrier for the formation of dolomite and magnesite at ambient conditions, and suggest that the conventional view in the inhibitory effect of water and Mg-hydration on the crystallization of Mg-containing carbonates may need to to be re-evaluated.

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