Magnesium silicate dissolution investigated by 29Si MAS, 1H–29Si CPMAS, 25Mg QCPMG, and 1H–25Mg CP QCPMG NMR

Abstract

Olivine—(Mg,Fe)₂SiO₄—has been the subject of frequent investigation in the earth sciences because of its simple structure and rapid dissolution kinetics. Several studies have observed a preferential release of the divalent cation with respect to silicon during weathering under acidic conditions, which has been correlated to the formation of a silicon-rich leached layer. While leached layer formation has been inferred through the changing solution chemistry, a thorough spectroscopic investigation of olivine reacted under acidic conditions has not been conducted. The pure magnesium end member of the olivine series (forsterite—Mg₂SiO₄) was chosen for detailed investigations in this study because paramagnetic iron hinders NMR investigations by providing an extra mode of relaxation for neighboring nuclei, causing lineshapes to become significantly broadened and unobservable in the NMR spectrum. For reacting forsterite, spectroscopic interrogations using nuclear magnetic resonance (NMR) can elucidate the changing magnesium coordination and bonding environment. In this study, we combine analysis of the changing solution chemistry with advanced NMR techniques (²⁹Si MAS, ¹H–²⁹Si CP MAS, ²⁵Mg QCPMG, and ¹H–²⁵Mg CP QCPMG NMR) to probe leached layer formation and secondary phase precipitation during the dissolution of forsterite at 150 °C.