Measurement of the Near-Infrared Spectra of H 2 O-SiOH-Bearing Siliceous Materials at High Pressures

Abstract

Techniques for measuring OH-combination bands of water and other OH-group bearing solid or liquid materials in the near-infrared (6000 to 4000 cm⁻¹) at high pressures by means of a modified diamond anvil cell, adapted to a single beam micro-spectrometer, have been developed and applied to opals, i.e., minerals in the system SiO₂-H₂O containing relatively high amounts of water and SiOH. Liquid embedding, using the gasket technique and poly-fluoro-carbon oils as embedding liquids, which provide a hydrostatic pressure environment at least up to 50 kbar, is applied for solid materials. The high pressure near-infrared spectra of gel-like, spherulitic opal-AG show a strong relative decrease of the high energy components of the complex (v₁ + v₂)H₂O-band system at around 5200 cm⁻¹ in favor of the low energy components of this band system. This is tentatively interpreted as due to an increase of fully hydrogen bonded water at the expense of water in which only one OH-group is H-bonded. The corresponding effect in opal-AN with a glass-like continuous SiO_4/2-tetrahedral network is much smaller.