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.

Osmium Transport and Enrichment From the Lithosphere to the Hydrosphere: New Perspectives From Hydrothermal Experiments and Geochemical Modeling

“…The precipitates from the sample containers were in situ analyzed using a WITec alpha 300R Confocal Laser Raman Spectrometer (Germany) that was equipped with three lasers (488, 532, and 633 nm), three gratings (300, 600, and 1,800 grooves/mm), and four Zeiss objectives (5×, 20×, 50×, and 100×) with high spectral resolution (≤0.8 cm −1 ) at the hydrothermal laboratory of the Guangzhou Institute of Geochemistry. Raman spectra of the precipitates were acquired using the 532 nm laser, the grating with a density of 300 grooves/cm, a Zeiss objective with a magnification factor of 50×, a laser powder of 10 mW, an acquisition time of 6 s, and cumulative frequencies set to be recorded for ten times in this study (Yan et al., 2022).…”

“…Theoretically, the Os(IV) cation is expected to undergo completer hydrolysis in hydrothermal solutions due to its high polarization force ( z 2 / r > 20) (Turner et al., 1981), which thus causes the hydrolysis of Os(IV)‐Cl complex (Yan, Sun, et al, 2021; Yan et al., 2022). Considering that K + has negligible effects on the hydrolysis of Os‐Cl complexes, the stepwise hydrolysis reaction of OsCl 6 2− in this study can be expressed as follows (Xiong & Wood, 2000): $$${{\text{OsCl}}_{6}}^{2-}+{\mathrm{H}}_{2}\mathrm{O}\leftrightarrow \text{Os}(\text{OH}){{\text{Cl}}_{5}}^{2-}+\text{HCl}.$$$ $$$\text{Os}(\text{OH}){{\text{Cl}}_{5}}^{2-}+{\mathrm{H}}_{2}\mathrm{O}\leftrightarrow \text{Os}{(\text{OH})}_{2}{{\text{Cl}}_{4}}^{2-}+\text{HCl}.$$$ $$$\text{Os}{(\text{OH})}_{2}{{\text{Cl}}_{4}}^{2-}+{\mathrm{H}}_{2}\mathrm{O}\leftrightarrow \text{Os}{(\text{OH})}_{3}{{\text{Cl}}_{3}}^{2-}+\text{HCl}.$$$ $$$\bullet \bullet \bullet \phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\text{Os}{{(\text{OH})}_{6}}^{2-}\leftrightarrow \text{Os}{(\text{OH})}_{4}+2{\text{OH}}^{-}.$$$ …”

“…Theoretically, the Os(IV) cation is expected to undergo completer hydrolysis in hydrothermal solutions due to its high polarization force (z 2 /r > 20) (Turner et al, 1981), which thus causes the hydrolysis of Os(IV)-Cl complex (Yan, Sun, et al, 2021;Yan et al, 2022). Considering that K + has negligible effects on the hydrolysis of Os-Cl complexes, the stepwise hydrolysis reaction of OsCl 6 2 in this study can be expressed as follows (Xiong & Wood, 2000): The hydrolysis reaction (Equation 1) can be simplified and rewritten as OsCl 6 2 (aq) + 2H 2 O (aq) ↔ OsO 2 (s) + 4HCl (aq) + 2Cl (aq).…”