A possible new Al-bearing hydrous Mg-silicate (23 Aa phase) in the deep upper mantle

A new Al-bearing hydrous Mg-silicate that we named as 23 Aa phase was synthesized at 10 GPa and 1000 degrees C, while also coexisting with diaspore and pyrope in the following system: phase A [Mg (sub 7) Si (sub 2) O (sub 8) (OH) (sub 6) ]+Al (sub 2) O (sub 3) +H (sub 2) O. The chemical composition of this new 23 Aa phase is Mg (sub 11) Al (sub 2) Si (sub 4) O (sub 16) (OH) (sub 12) , and it contains about 12.1 wt% water. Powder X-ray diffraction and electron diffraction patterns show that this new 23 Aa phase has a hexagonal structure, with a=5.1972(2), c=22.991(4) Aa, and V=537.8(2) Aa (super 3) , and the possible space group is P6c2, P6 (sub 3) cm, or P6 (sub 3) /mcm. The calculated density is 2.761 g/cm (super 3) accordingly, 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. Several experiments revealed that its stability region is very similar to that of phase A. We further confirmed that this new 23 Aa phase was stable in the chlorite composition at 10 GPa and 1000 degrees C. The present results indicate that this new 23 Aa hydrous phase will form in an Al-bearing subducting slab, and transport water together with Al into the deep upper mantle or even into the upper part of the transition zone.