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)degrees C; antigorite was pressurized at about 25 and 22 GPa and heated at similar to 1000 degrees 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 k(ao) = 482 (+/- 36) GPa, k(bo) = 544 (+/- 62) GPa, and k(co) = 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 D-ant) are k(ao) = 422 (+/- 8) GPa and k(co) = 404 (+/-4) GPa and the bulk modulus is 134 (+/- 5) cpa, which is similar to 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. (C) 2000 Elsevier Science B.V. All rights reserved.