Increased Stability of CO2-Host Minerals Under Humid Conditions

Carbon mineralisation provides a safe and effective means of trapping and storing atmospheric CO₂ in highly stable and environmentally benign carbonate minerals, such as magnesite (MgCO₃). However, MgCO₃ formation is kinetically inhibited under Earth’s surface conditions, and while promoting its precipitation to enhance CO₂ trapping efficiency is technologically feasible, it is not currently financially viable owing to low carbon taxes and CO₂ prices.

Hydrated Mg-carbonate minerals, such as nesquehonite (MgCO₃·3H₂O), precipitate readily at the Earth’s surface and are considered promising alternate mineral hosts for CO₂ storage. However, for hydrated Mg-carbonates to be recognised as suitable phases for the capture and storage of anthropogenic CO₂, we need an improved understanding of their stability fields, and subsequent long-term CO₂ security. This is particularly true given the large stockpiles of the minerals that would require long-term storage, or disposal, if the technique were to gain popularity.

Here, we have utilised both in situ X-ray diffraction and thermogravimetric analysis to investigate thermal stability of nesquehonite in humid, relatively low-temperature conditions relevant to the Earth’s surface. These in situ techniques allowed for detailed monitoring of nesquehonite crystallinity and thermal mass loss during short-term decomposition experiments in the presence of water vapour. We found that at both 50°C and 100°C there was a decreased rate of structural decomposition and mass loss when nesquehonite was exposed to humidified gas. We hypothesise that the sorption of water vapour to the mineral surface may impede the outward diffusion of water from the crystal structure, resulting in a higher energy requirement for dehydration to proceed. This has important implications for the disposal and storage of CO₂ in hydrated Mg-carbonate minerals on the millennial timescales required for combating CO₂ pollution. The study is also relevant for tailoring industrial processes to enhance CO₂ storage security via decomposition of nesquehonite to more stable lower hydrates.