Anorthosite dissolution as a function of pH at 60 and 120 °C: Implications for subsurface carbon mineralization

Carbon mineralization in reactive rocks is a promising approach for mitigating carbon dioxide (CO₂) emissions by converting CO₂ into stable carbonate minerals. Anorthosites, abundant igneous rocks composed primarily of calcium-rich plagioclase, hold significant potential for CO₂ capture and storage due to their rapid dissolution rates in acidic environments thereby promoting the formation of stable carbonate minerals. In this study, the dissolution behavior of anorthosites collected from Yanbu, Saudi Arabia, was evaluated at field-relevant conditions. Element release rates were experimentally measured in mixed-flow reactors in aqueous fluids at pH ranging from 2 to 12 and temperatures of 60 and 120 °C. The results show that silicon release rates are consistent with those reported for intermediate plagioclase in the literature. A pronounced preferential initial calcium release was observed at all investigated conditions. Mass balance calculations suggest this preferential release is driven by calcium ion exchange with sodium ions and/or ammonium ions at the plagioclase surface. The preferential release of calcium continued throughout all experiments performed at pH greater than 3, where some experiments lasted up to 550 hours. The preferential release of calcium, combined with the observed rapid precipitation of aluminum-oxyhydroxide phases at near to neutral conditions, facilitates the formation of calcium carbonate minerals. Given the global abundance of anorthosites, these findings underscore their potential as host rocks for subsurface mineral carbon disposal, providing a robust and scalable solution for long-term CO₂ capture and storage.