In situ X‐ray experiment on the structure of hydrous Mg‐silicate melt under high pressure and high temperature

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.