Crystal Growth of Osmium(IV) Dioxide in Chlorine-Bearing Hydrothermal Fluids

Abstract

A mineral’s morphology is usually related to its growth process and environment. This study reported crystal growth of OsO2 through hydrolysis experiments of K2OsCl6 at 150–550 °C and 100 MPa to investigate the growth mechanism of OsO2 and the transport and enrichment of Os in chlorine-bearing hydrothermal fluids. Time-series experimental results showed that the OsO2 crystals grow from 40–150 nm irregular nanoparticles to 150–450 nm nanospheres with time. As the temperature and initial solution concentrations increase, OsO2 can form more uniform and larger OsO2 nanosphere crystals, suggesting a positive effect of temperature and initial solution concentration on the crystal growth of OsO2. The results indicate that the nucleation and aggregate growth driven by the hydrolysis of Os–chloride complex controls the early growth of OsO2 crystals for a short duration; however, after the hydrolysis reaches equilibrium, the growth process of OsO2 nanosphere crystals is dominated mostly by the Ostwald ripening, where the diffusion of Os ions along the fluid–nanocrystal boundary facilitates the coarsening. Given that the transport and cycle of Os from the lithosphere to the hydrosphere is controlled mainly by the stability of the Os–chloride complex, OsO2 nanosphere crystals could occur in seafloor hydrothermal vent systems.