HR: 1340h
AN: MR13C-1412    [Abstracts]
TI: Physicochemical states of water at high temperatures in the microcrystalline silica (chalcedony) by in-situ infrared spectroscopy
AU: * Fukuda, J
EM: jfukuda@ess.sci.osaka-u.ac.jp
AF: Department of Earth and Space Science, Graduate School of Science, Osaka University, 1- 1 Machikaneyama-cho, Toyonaka-shi, 560-0043, Japan
AU: Nakashima, S
EM: satoru@ess.sci.osaka-u.ac.jp
AF: Department of Earth and Space Science, Graduate School of Science, Osaka University, 1- 1 Machikaneyama-cho, Toyonaka-shi, 560-0043, Japan
AB: Water in rocks is present mainly as H₂O in grain boundaries and pores, and as hydroxyl species in crystal structures. At higher temperatures, physicochemical states of water in rocks would be changed and dehydration is also expected. One of the representative materials in the crust including both H₂O and hydroxyl is the microcrystalline silica, chalcedony. Water in chalcedony from Oshamanbe, Hokkaido, Japan was investigated at high temperatures by in-situ infrared (IR) spectroscopy. First, distribution of water in the sample was investigated by polarized optical microscope (POM) observation and IR measurement. Under the POM, the sample exhibits typical aspects of chalcedony, showing length-fast and rhythmic extinctions. The sizes of optical domains caused by the preferred orientation of crystallites are ranging from one micron (fine) to several microns (coarse) and heterogeneous in millimeter to centimeter scales. IR spectra of these domains show a broad band from 3800 to 2500cm^-1 due to O-H stretching of H₂O, with a shoulder at 3585cm^-1 due to Si-OH species. Less water was contained by IR measurement in coarser grain domains. Second, the sample was heated from room temperature to 400° C at 50° C intervals under the IR microscope. Every 50° C, the peak position of H₂O (3430cm^-1 at 25° C) shifted to higher frequency by about 20cm^-1 and that of Si-OH (3585cm^-1 at 25° C) shifted to higher frequency by about 2.5cm^-1. The peak shift of H₂O can be explained by the decrease of average coordination numbers of H₂O. The peak shift of Si-OH can be due to the increase of hydrogen bond distance of Si-OH--- O. Third, the sample was heated isothermally from 400° C to 700° C under the IR microscope. By analyzing the OH band area, the dehydration kinetics of chalcedony was quantitatively investigated.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3630 Experimental mineralogy and petrology
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting