Nanoscale Mg-Depleted Layers Slow Carbonation of Forsterite (Mg2SiO4) When Water Is Limited

Abstract

Passivation of silicate surfaces by accumulated reaction products is an obstacle to efficient CO2 mineralization. In this study, we investigate a unique passivation effect during the carbonation of the basalt mineral forsterite (Mg2SiO4) in humid supercritical CO2 (50 °C, 90 bar). Using in situ high-pressure infrared spectroscopy, we demonstrate that dissolution of forsterite into a thin water film slows significantly after reaction for ≈24 h, even under far-from-equilibrium conditions. 29Si magic angle spinning nuclear magnetic resonance spectroscopy detects a highly polymerized amorphous silica at this stage. On the basis of transmission electron microscopy and energy dispersive X-ray spectroscopy, we show that the silica is present as a Mg-depleted layer that is just 2–3 nm thick on the reacted forsterite particles. The decrease in the level of forsterite dissolution in the presence of an extraordinarily thin Mg-depleted layer can be strongly linked to properties of the thin fluid film at the surface, highlighting the importance of water during mineral carbonation. This study furthers our understanding of silicate mineral carbonation under select low-water, humidified fluid conditions relevant to basaltic geologic reservoirs, recovery of critical elements by carbonation of mafic ores, and sequestration of atmospheric CO2 by enhanced rock weathering.