Emerging investigator series: kinetics of diopside reactivity for carbon mineralization in mafic-ultramafic rocks

The ongoing use of fossil fuels to supply modern energy demands has necessitated research on combating carbon dioxide (CO₂) emissions and climate change. Carbon storage via mineral trapping in basalt and related rocks is a promising strategy. However, mineralization rates depend on the variable minerology that makes up these rock formations. Diopside (CaMgSi₂O₆) is a common pyroxene mineral in ultramafic and mafic rocks including basalt, but relatively little work has been done to understand its carbon mineralization kinetics using hydrated supercritical CO₂, which induces the formation of reactive nanoscale interfacial water films. In situ XRD experiments at 50-110 °C and 90 bar indicate that diopside transforms into a myriad of Mg/Ca carbonates, including huntite [Mg₃Ca(CO₃)₄] and very high magnesium calcite (VHMC, i.e., protodolomite). Through ex situ characterization, we were able to constrain reaction pathways for the dissolution-precipitation transformation process including metastable intermediate precipitates. Experiments performed at variable temperatures enabled Avrami-derived rate constants and an apparent activation energy of 97 ± 16 kJ mol⁻¹, implying the dissolution of diopside is the rate-limiting step. Density functional theory (DFT) calculations, used to gain molecular insight into the surface stability of the diopside during dissolution, suggest that exposed calcium cations are susceptible to dissolution when put in contact with water given their coordination environment. The collective results point to the high CO₂ mineralization potential of diopside in basalts, which could help guide parameterization of reactive transport models needed to design and permit commercial-scale subsurface carbon storage operations.