Publication Date:
2015-10-02
Description:
A new Al-bearing hydrous Mg-silicate that we named as 23 Å phase was synthesized at 10 GPa and 1000 °C, while also coexisting with diaspore and pyrope in the following system: phase A [Mg 7 Si 2 O 8 (OH) 6 ] + Al 2 O 3 + H 2 O. The chemical composition of this new 23 Å phase is Mg 11 Al 2 Si 4 O 16 (OH) 12 , and it contains about 12.1 wt% water. Powder X-ray diffraction and electron diffraction patterns show that this new 23 Å phase has a hexagonal structure, with a = 5.1972(2), c = 22.991(4) Å, and V = 537.8(2) Å 3 , and the possible space group is P c 2, P 6 3 cm , or P 6 3 / mcm . The calculated density is 2.761 g/cm 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 Å phase was stable in the chlorite composition at 10 GPa and 1000 °C. The present results indicate that this new 23 Å 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.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
