Diffusion of Hydroxylated Si Vacancies in Olivine, and Its Relevance to Si Diffusion

Abstract

AbstractThe diffusion coefficient associated with the 3,612 cm−1 infrared absorbance band in H‐bearing olivine, attributed to a defect involving a vacant Si site balanced by four H+ (i.e., ), was determined in H in‐diffusion experiments at 1200°C–1400°C and 1.5–3.0 GPa in piston‐cylinder and multi‐anvil apparatuses. The retrieved values are in good agreement with experimentally determined diffusion coefficients ascribed to the same process from previous experiments, despite those being conducted at atmospheric pressure and representing out‐diffusion rather than in‐diffusion. Overall, the diffusivity of the Si vacancy is several orders of magnitude lower than that of the M‐site vacancy (∼4 orders of magnitude at 1000°C), with a higher activation energy (∼450–500 vs. 200–250 kJ mol−1), when both are charge‐compensated solely by H+. From these data, the diffusion coefficient of Si in H‐bearing olivine can be estimated using the relationship between vacancy concentration, vacancy diffusivity, and diffusivity of an ion moving by a vacancy mechanism. However, these diffusivities are inconsistent with published results for Si diffusivity, with a considerable discrepancy of around two orders of magnitude. This suggests that simple relationships between diffusion coefficients of vacancies and species moving by vacancy diffusion mechanisms may be inappropriate, highlighting again the complexity of H incorporation and diffusion mechanisms in olivine.