Abstract
Powder X-ray diffraction (XRD) patterns of phase B (Mg12Si4O21H2), anhydrous B (Mg14Si5O24) and superhydrous B (Mg10Si3O18H4) were calculated using recently published structural data to facilitate phase identification by powder XRD technique. Comparison of these patterns with those of the previously reported hydrous phases revealed that superhydrous B and phase C are actually an identical phase. The XRD patterns were then used to identify the phases which appeared in the previous high-pressure experiments by the present author. A newly derived dehydration reaction of phase A (Mg7Si2O14H6) is as follows: 3 phase A = 2superhydrous B+Mg(OH)2+4H2O. The reaction was observed at 14 GPa and 1100°C. Superhydrous B coexisting with silica-rich phase was also detected in the 2Mg(OH)2+SiO2 system at ≥17 GPa. Therefore, superhydrous B might be stabilized in subducting slab at those depths.
