ABSTRACT

Dense hydrous magnesium silicates (DHMS, synthesized in high-pressure and high temperature experiments) are essential to our understanding of the state of hydration in the Earth’s mantle. There are more than 10 different new dense hydrous magnesium silicates claimed to have been synthesized. A careful analysis and comparison suggests, however, that only the 10-Åphase, phases A,B,C,D and E, and the hydrous β-phase, and the hydrous γ-phase are real. Both phase F and phase G are in reality phase D. Superhydrous phase B may, in reality, be phase C.  The  10-Åphase, phase D, phase E and the hydrous β-phase all form solid solutions having various ranges of the Mg/Si ratio and H₂O content. Among the hydroxyl humite series, only chondrodite, humite and clinohumite have so far been synthesized in high pressure and high temperature experiments. Because the identity and other details of these DHMS’s are poorly understood, the P-T stability fields containing phases B and F and superhydrous phase B determined in most of the experimental studies are likely to be misleading, if not erroneous.