Chapter 6: Optical cells with fused silica windows for the study of geological fluids
Two types of optical cells with fused silica windows are described for study of geological fluids at temperatures (T) up to 6008C and pressures (P) up to 1 kbar. One is the high-pressure optical cell (HPOC), in which fluids of known composition can be loaded directly into the cell and its pressure can be adjusted and measured. The other is the fused silica capillary capsule (FSCC), which contains sample fluid with both ends of the tube flame sealed. Both types of cells can be inserted into a heating-cooling stage (USGS-type, the newly developed stage from INSTEC or that from Linkam) for in situ observations and Raman spectroscopic analyses at various P-T conditions. The HPOC has been applied to measure the solubility and diffusion of methane in water, the solubility of methane hydrate in water, and methane pressures in fluid samples. It is also very useful for providing fluid standards with known composition and pressure for the calibration of Raman spectroscopic systems before quantitative analyses. The FSCC is particularly useful for samples which need to be reacted at elevated P-T conditions for long periods of time (days or weeks). These two types of optical cells with fused silica windows are particularly suitable for the study of organic systems and also for systems containing sulphur.
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