Evidence from ²⁹Si Solid-State Nuclear Magnetic Resonance of Dissolution Reactions of Forsterite

We have used solid-state nuclear magnetic resonance (NMR) of ²⁹Si to monitor the reaction of forsterite (Mg₂SiO₄) with isotopically enriched ¹³CO₂. Fate of silicon that comes from dissolution of forsterite in the presence of water or NaCl brine was monitored as a function of depth in a packed bed reactor, which used elevated-temperature and -pressure conditions to mimic underground geological sequestration conditions. Silicon provides an important window into the reaction since the precipitation of amorphous silica can affect forsterite dissolution, and solid-state magic-angle spinning NMR can readily distinguish between ²⁹Si in forsterite and amorphous silica. Furthermore, differences between Q³ and Q⁴ species of amorphous silica were observed as a function of depth. Results differed between water- and brine-based reactions, with enhanced dissolution of forsterite observed in the brine.