Effect of water on the structure of rhyolite glasses - X-ray diffraction and Raman spectroscopy studies

Abstract

Wide-angle X-ray diffraction and Raman spectroscopy investigations of the structure of several water-containing silicate glasses with rhyolite compositions quenched from 2 kbar pressure and 1000°C temperature have been carried out. It is shown that the main difference between the diffraction patterns is a displacement of the first sharp diffraction peak (FSDP) to higher Q values ( Q = 4 π sin(θ)/λ) with increasing water content. At the same time, a comparison of the reduced radial distribution functions shows that the first and the second nearest neighbour distances do not change position with composition. The electron density correlation functions clearly indicate a decrease of the correlation length and an increase of the local packing of the structural units with increasing the total water content. The Raman spectroscopy results indicate that the glasses have an almost completely polymerized three-dimensional network. This fact is confirmed by calculation of the number of non-bridging oxygens per tetrahedron (NBO/ T ) from the bulk chemical composition. It is suggested that the structure of water-bearing rhyolite glasses is best described as consisting of locally densified, fully polymerized anhydrous microdomains separated by water-rich disordered regions. This structural model is consistent with previous Raman spectroscopy studies and molecular dynamics simulations of permanently densified amorphous SiO 2 .