Minerals in low-water environments are covered in Ångstrom-to-nanometer thin H2O films with unique properties that can lead to mineral transformation pathways that are very different from those in bulk water. For example, H2O films have extraordinarily high mineral-surface-to-water-volume ratios; thus, relatively minor amounts of mineral dissolution can rapidly yield high degrees of supersaturation with respect to secondary phases. Despite numerous examples of H2O-limited domains in nature, we currently lack a fundamental understanding of coupled mineral dissolution, nucleation, and growth reactions in confined adsorbed H2O films. Here, we present results of an investigation of the low temperature and low H2O activity carbonation pathway of forsterite (Mg2SiO4) to magnesite (MgCO3) in thin H2O films in humidified high-pressure CO2. Experiments were conducted on nanometer-sized forsterite at 25, 40, and 50 °C in 90 bar CO2 at a relative humidity 62%, which corresponds to a H2O film thickness of 1.78 monolayers. In situ infrared (IR) spectroscopy was used to monitor forsterite carbonation as a function of time, and post reacted samples were characterized ex situ by scanning electron microscopy (SEM), high-energy X-ray diffraction (heXRD), and pair distribution function (PDF) analyses. These combined results evidenced for the first time a carbonation pathway of forsterite to magnesite through an amorphous magnesium carbonate (AMC) metastable phase. Chemometric modeling of the IR spectroscopic data quantified the dissolution of forsterite and the formation of both AMC and magnesite as a function of time. Fitting of these data with a kinetic geochemical model in PHREEQC showed that magnesite grew in thin H2O films following a pseudo zero-order rate law, and magnesite remained supersaturated by two to three orders of magnitude. This study furthers an understanding of divalent silicate carbonation under low-H2O CO2-dominated fluids as part of a comprehensive effort to predict the fate of CO2 stored in geologic reservoirs.

Low temperature magnesite growth during forsterite carbonation in thin H2O films

ACS Spring Meeting 2021

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