New Al-Bearing Hydrous Mg-Silicate in the Deep Upper Mantle

Subducting slabs are supplying water into the deep mantle, so the stabilities of hydrous minerals under high pressure and high temperature are important issue to clarify the water transportation mechanism into deeper part of the Earth's mantle. Here we reported a new Al-bearing hydrous Mg-silicate, which was synthesized at 10 GPa and 1000ºC in the system MgO-Al2O3-SiO2-H2O (phase A (Mg7Si2O8(OH)6) + Al2O3 + H2O system). The composition of this new phase was determined to be Mg11Al2Si4O16(OH)12, and it contains about 12 wt% water. Powder X-ray diffraction and electron diffraction showed that this new phase has a hexagonal structure, with a = 5.1972(2) Å, c = 22.991(4) Å, and V = 537.8(2) Å3, and the possible space group is P‾6c2, P63cm, or P63mcm. The calculated density is 2.761 g/cm3, which was determined by assuming that the formula unit per cell (Z) is 1. This crystal structure is quite unique among mantle minerals in having an extraordinarily long c axis. We confirmed the stability P-T region of this new phase, and found that its stability region is very similar to that of phase A which is the high pressure hydrous phase in serpentine (Mg6Si4O10(OH)8) composition. We further confirmed that this new phase was stable in the chlorite (Mg5Al2Si3O10(OH)8) composition at 10 GPa and 1000ºC. The present results indicate that this new phase will be stable in Al-rich subducting slab instead of phase A and transport water into the deep upper mantle or even into the upper part of the transition zone.