Meeting room "Glova", 4 floor, Integrated Research Building
Abstract
Microscopic structure of molten silicate at high pressure gives important insights into the fundamental understanding of the physicochemical properties of magma in the Earth's interior. In particular, in-situ observations of liquid are necessary for the realistic response of the structure to pressure because the information through the glass is always under-estimated due to the kinetic effect and the reversible property of the structure during decompression. A local structural change in AlO4 tetrahedra is thought to happen at much lower pressures than in the SiO4 tetrahedra, which influent to various properties of aluminous silicate melt. In addition, it is widely known that water changes the structure and properties of silicate melt dramatically. Here, we investigate the difference in local structure between hydrous and anhydrous albite (NaAlSi3O8) melt with high-pressure x-ray diffraction.
A sample of anhydrous albite was prepared with a mixture of NaCO3, Al2O3 and SiO2. The mixture was decarbonized at 1000°C for 9 hours, then melted at 1200°C, and finally quenched at room temperature to make a glass. Hydrous albite was made from sodium silicate glass (Na2Si6O13) plus Al(OH)3 gibbsite, which has 9.3-wt% water content. High-pressure x-ray diffraction experiments have been conducted at NE5C beamline in PF-AR, Tsukuba, Japan. A 6-6 type compression with a DIA-type press, MAX80, was used for the high-pressure and temperature experiments. Diffractometry of the molten sample for the structural analysis was performed with polychromatic x-rays with the energy-dispersive method. Diffraction patterns of the sample were collected from 2θ= 3° to 30° to cover a wider range of Q (Q= 4πEsinθ/12.398 [A-1], where E and θ are the energy of the x-rays and the Bragg angle, respectively).
The pressure dependence of the peak-shift of the first peak in S(Q), which represents the intermediate range ordering of aluminous silicate network, for the hydrous melt is much smaller than for the anhydrous melt although the position of the peak is located on the higher-Q side. This structural change in intermediate range ordering can be closely related with the previous results from the viscometry on the hydrous and anhydrous albite melts (e.g., Suzuki et al., 2002; Kato et al., 2004). The shoulder peak on the right side of the Si-O peak in G(r) appears from 4 GPa in the hydrous melt, which becomes intense with pressure. This shoulder peak can be assigned to a highly coordinated Al-O atomic pair. This striking peak, which reflects the appearance of AlOn polyhedra, was not seen in the anhydrous melt up to 6.2 GPa. Further discussion will be presented in the presentation.
For inquiry：Taku Tsuchiya TEL:（089）927-8198
E-mail takut@sci.ehime-u.ac.jp
