In situ redox control and Raman spectroscopic characterisation of solutions below 300 °C

Abstract

Redox reactions often occur and significantly affect many geological processes. To simulate redox reactions in low temperature (T < 400 °C) hydrothermal experiments, fused silica was used as a hydrogen membrane to impose an externally fixed H2 pressure (PH2) on a fused silica capillary capsule (FSCC; 150 μm inner diameter, 375 μmouter diameter and ∼6mmlong) to define the redox state of the sample in the FSCC. At 300 °C, it required less than 7 hours to reach osmotic equilibrium. In this study, a constant PH2 was imposed on an FSCC, which originally contained a 0.5 m (mole/kg H2O) SnCl4 0.5 m HCl aqueous solution, at 300 °C and vapour saturation pressure. In situ Raman spectra of the sample solution collected at 300 °C show that the reduction rate of SnIV to SnII species increased substantially with an increase of 1.1 bar of PH2.We characterised precipitation and dissolution of cassiterite under various P-T-pH-PH2 conditions and greatly increased our capabilities for performing rigorous hydrothermal experiments at temperatures below 400 °C, in which redox control is difficult to ensure without in situ approaches.