In situ X-ray diffraction study of chrysotile at high P–T conditions: transformation to the 3.65 Å phase

The behavior of chrysotile Mg 3 (Si 2 O 5 )(OH) 4 in water medium at simultaneously high pressure and high temperature was studied by in situ synchrotron X-ray diffraction using a diamond anvil cell. In contrast to previous ‘dry’ experiments, chrysotile in water-saturated conditions undergoes two-phase transitions and exhibits higher thermal stability. At 260 °C / 3.7 GPa the initial chrysotile (phase I) transforms to the ‘chrysotile-like’ phase II, followed by the appearance of the ‘chrysotile-like’ phase III at 405 °C / 5.25 GPa. Phase III is characterized by enlarged interlayer distances, presumably resulting from the H 2 O intercalation into the interlayer space. During further compression, the ‘chrysotile-like’ phase III is decomposed to the 10 Å phase Mg 3 (Si 4 O 10 )(OH) 2 · x H 2 O, the 3.65 Å phase MgSi(OH) 6 , phase D, forsterite, enstatite and coesite or stishovite. The 3.65 Å phase appears at 8.8 GPa / 500 °C. The series of transformations leads to a water deficiency in the system, restricting the complete transformation from the 10 Å phase to the 3.65 Å phase. These data emphasize the crucial role of excess water in the stabilization of the high-pressure hydrous phases. The present study is the first in situ observation of sequential transformations of hydrous phases: serpentine → 10 Å phase → 3.65 Å phase, important as a potential water transport mechanism to the deep mantle.