Crystallization Pressure and Reaction-Induced Cracking in Olivine Carbonation: Insights from Etch Pit Analysis
Authors: Iyare U. C. , Neil C. W. , Boampong L. , Li W. , Meng M. , Frash L. P. , Carey J. W. , Viswanathan H. S. , Detournay E. 
Published: August 2024

ABSTRACT:
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.

1. INTRODUCTION
Carbon mineralization in mafic and ultramafic rocks has emerged as a highly promising and permanent method for CO2 sequestration in geological formations (Johnson et al., 2014; Kelemen et al., 2019; Menefee & Ellis, 2021; Rashid et al., 2022). Mafic, and ultramafic rock formations, mainly composed of olivine, pyroxene, and serpentine, have proven to be more promising in this regard. These rocks are highly reactive in water and have abundant calcium and magnesium which combine with CO2 upon dissolution to form stable carbonate minerals (Menefee & Ellis, 2021). The naturally abundant mineral olivine (Mg2SiO4) has been a focus of carbon mineralization research (Hänchen et al., 2006; Johnson et al., 2014; Kelemen & Matter, 2008). Injecting CO2-rich brine into in olivine dominated rocks rapidly initiate a reaction that results in the dissolution of olivine followed by the precipitation of Magnesite (MgCO3). It is also possible for other precipitation reactions to occur including formation of brucite and serpentine minerals.