In situ X-ray diffraction studies of dense hydrous magnesium silicates at mantle conditions

Lizardite was pressurized up to about 28 and 20 GPa and heated at 1310 (±50) and 1100 (±100)°C; antigorite was pressurized at about 25 and 22 GPa and heated at ∼1000°C, with a Nd:YAG laser using the double-side laser heating technique. Samples were then examined in situ by energy dispersive synchrotron X-ray diffraction. Our results show that phase E, superhydrous phase B, and phase D are indeed dense silicate phases that are stable at high P–T conditions and quenchable to ambient conditions. Some unidentified peaks observed in our runs indicate the existence of unknown hydrous phases. The lattice parameter and bulk modulus of phase E are in good agreement with previous measurements. The linear moduli of the three axes of superhydrous phase B are kao=482 (±36) GPa, kbo=544 (±62) GPa, and kco=472 (±66) GPa and the bulk modulus of superhydrous phase B is 156 (±2) GPa. The linear moduli of the three axes of Fe-bearing phase D (phase Dant) are kao=422 (±8) GPa and kco=404 (±4) GPa and the bulk modulus is 134 (±5) GPa, which is ∼19% smaller than the Fe-free phase D reported previously. Our results indicate that phase E, superhydrous phase B, and phase D are indeed thermodynamically stable phases at mantle conditions.