In situ X-ray di¡raction studies of dense hydrous magnesium silicates at mantle conditions

Lizardite was pressurized up to about 28 and 20 GPa and heated at 1310 ( þ 50) and 1100 ( þ 100)3C; antigorite was pressurized at about 25 and 22 GPa and heated at V10003C, 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 ( þ 36) GPa, kbo = 544 ( þ 62) GPa, and kco = 472 ( þ 66) GPa and the bulk modulus of superhydrous phase B is 156 ( þ 2) GPa. The linear moduli of the three axes of Fe-bearing phase D (phase Dant) are kao = 422 ( þ 8) GPa and kco = 404 ( þ 4) GPa and the bulk modulus is 134 ( þ 5) GPa, which is V19% 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. ß 2000 Elsevier Science B.V. All rights reserved.