Structure of a hydrous silicate melt with a forsterite-enstatite-H2O composition was investigated by in-situ X-ray diffraction with synchrotron radiation. The high temperature hydrous melt was successfully enclosed in a single-crystal diamond capsule under high pressure, up to 6.5 GPa. Results showed that the first sharp diffraction peak (FSDP), which reflects the intermediate-range structure of the complex melt, shifted drastically toward higher Q (scattering vector [Å-1]) at pressure below 2.2 GPa. This observation indicates that the silicate network in the melt was depolymerized by the influence of water molecules and partly replaced by Si-OH units under those high-pressure conditions. In contrast, between 3-5 GPa, such a peak shift was not clearly detected, suggesting that the lengthening of the intermediate-range ordering of ...-Si-O-Si-... linkage (i.e., polymerization of silicate network) occurred.
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