A new hydrous Al-bearing pyroxene as a water carrier in subduction zones

A new Hydrous Al-bearing PYroxene (HAPY) phase has been synthesized at 5.4 GPa, 720 degrees C in the MgO-Al(2)O(3)-SiO(2)-H(2)O model system. It has the composition Mg(2.1)Al(0.9)(OH)(2)Al(0.9)Si(1.1)O(6), a C-centered monoclinic cell with a = 9.8827(2), b = 11.6254(2) c = 5.0828(1) angstrom and beta = 111.07(1)degrees. The calculated density is 3.175 g/cm(3) and the water content is 6.9% H(2)O by weight. Its structure has been solved in space group C2/c by the recently developed automated electron diffraction tomography method and refined by synchrotron X-ray powder diffraction. HAPY is a single chain inosilicate very similar to pyroxenes but with three instead of two cations in the octahedral layer, bonded to four oxygens and two hydroxyl groups. The Si tetrahedra are half occupied by Al and cation ordering appears in the octahedral layer with two sites occupied by Mg and one by Al. The stability of such previously unknown hydrous silicate beyond the chlorite pressure breakdown may significantly promote the H(2)O transport in the subduction channel to depths exceeding 150 km.