Crystallization Pressure and Reaction-Induced Cracking in Olivine Carbonation: Insights from Etch Pit Analysis

Carbon mineralization in mafic and ultramafic rocks has emerged as a highly promising method for permanent CO₂ sequestration. It is hypothesized that stress concentration, caused by the volume expansion and mineral transformation during carbonation, can cause damage and create new fractures. To evaluate the stress generated during this process, we conducted one-week carbonation experiments on millimeter-sized natural olivine grains in a high-pressure batch reactor (139 atm) at 185°C, using aqueous CO₂ and a solution of NaHCO₃ and NaCl. Our experimental results indicate distinct spatial separation between dissolution and precipitation, with most of the precipitation occurring at locations separate from dissolution sites. Post-experiment analysis using scanning electron microscopy (SEM) revealed microstructural changes, including the appearance of etch pits, dissolution channels, and chemical alterations due to magnesite precipitation. Magnesite was found to precipitate within the etch pits, surfaces, and dissolution channels. To gain further insights, we propose a linear elastic fracture mechanics (LEFM) model to estimate the critical crystal growth pressure required to promote mode I (tensile) crack propagation at the tips of the etch pits. The LEFM model suggests that tensile cracks can propagate when etch pits are loaded by a crystal growth pressure on the order of 0.1 GPa. It is worth noting that this pressure range falls within the theoretically expected values for crystallization pressure during olivine carbonation. However, in our experimental observations, we did not observe any fracturing of the olivine grains. Perhaps due to the relatively short duration of our experiments. Given more time, it is plausible that fractures can eventually propagate since the crystallization pressure exceeds the minimal critical pressure for cracking. The findings in this study suggest key corrective experimental designs for further investigation of reaction-induced cracking during carbon mineralization.