Transformation of amorphous precursor to crystalline carbonate: Insights from Mg isotopes in the dolomite-analogue mineral norsethite [BaMg(CO3)2

Abstract

Crystallization from an amorphous precursor is an important pathway of carbonate precipitation in nature. However, the mechanistic details of the transformation from an amorphous phase to a crystalline phase of carbonates remain a topic of intense debate. Two competing mechanisms, including solid-state transition and coupled dissolution-reprecipitation, have been proposed to explain this transformation process. Magnesium is a common element in carbonate crystal lattices and its isotopes may provide unique insights into this problem. In this study, we investigated the transformation of the amorphous carbonate (AC) precursor for norsethite [BaMg(CO3)2], a dolomite analogue mineral, by in situ XRD analysis and isotope exchange experiments using a 25Mg enriched tracer coupled with high precision isotope analyses of δ26Mg and δ25Mg values for aqueous and solid phases. In situ XRD experiments revealed that the AC can transformed to crystalline norsethite at various temperatures (25 °C, 50 °C and 70 °C) and no intermediate mineral formed during the AC transformation process. 25Mg tracers indicated that near-complete Mg isotope exchange occurred in all exchange experiments during AC transformation. More importantly, after the AC transformation, the system showed surprising apparent non-mass dependent fractionation relationship, that the δ25Mg value of solid phase became greater than that of aqueous solution from a lower value, producing positive Δ25Mgsolid-aq fractionation, whereas the Δ26Mgsolid-aq fractionation remained negative. We numerically modeled the behavior of Mg isotopes (in both δ26Mg and δ25Mg) for the experimental system according to the two competing mechanisms of AC transformation. The modeling results suggest that the apparent non-mass dependent isotope behavior can only be explained by the coupled dissolution-reprecipitation process. Therefore, this study does not support the solid-state transition mechanism for AC transformation. Further, this study rigorously proves that norsethite can form by precipitation from aqueous solution without replacement, and implies that Mg2+ in aqueous solutions can be efficiently dehydrated and incorporated into a well ordered dolomite-group mineral (norsethite) under abiotic, low temperature conditions, thus providing new insights for understanding dolomite precipitation in nature.