Magnesite formation during nesquehonite decomposition in the presence and absence of retained self-generated gases and the role of X-ray amorphous materials as essential stores for CO2

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Abstract
Long-term storage of CO2 in nesquehonite (MgCO3·3H2O) relies on its transformation to highly stable magnesite (MgCO3) in a naturally occurring, centuries-long process. Here, we pair in situ X-ray diffraction (XRD) and thermogravimetric analysis to investigate the thermal transformation (30–650 °C, 5 °C/min) of nesquehonite to magnesite, under both open and closed experimental conditions in a supplied atmosphere of CO2 or N2, and the presence or absence of self-generated gases (i.e., CO2, water vapor). We found that following the structural collapse of nesquehonite, magnesite only forms in the presence of gaseous CO2, whether that be externally supplied or self-generated. This is consistent with a dehydration-crystallization mechanism, with increased local accumulation of CO2 (and in a closed system, H2O vapor) shifting thermal events to higher temperatures, allowing for the crystallization of magnesite. Approximately 20 wt% more magnesite formed when nesquehonite was flushed with CO2 gas during heating in an open system, rather than held within a closed, static CO2 atmosphere. We hypothesize that this difference is due to complete dehydration being more difficult to achieve in a closed system, delaying the crystallization of magnesite. Additionally, the distribution of passivating reaction products on unreacted mineral cores may occur in closed systems, where self-generated humidity is retained and the dissolution-precipitation of reaction products may occur at mineral surfaces. We also found that amorphous materials are dominant intermediate stores for CO2, which is significant given they are not typically considered during carbon accounting in natural landscapes or engineered settings. We proposed a novel method to accurately quantify amorphous solids from XRD data during in situ studies where significant gas loss occurs. Our findings further our mechanistic understanding of how magnesite forms from crystalline and amorphous precursors under a range of environmental and industrial conditions, which is key to optimizing stable CO2 storage in Mg-carbonate minerals. In particular, it highlights the importance of considering the role of amorphous phases, atmospheric composition, and self-generated gas retention during magnesite formation.

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