A new hydrated high pressure phase in the MASH system

MgO-Al2O3-SiO2-H2O (MASH) system is well known to a be a simplified model system for the study of water transport and recycling into Earth’s mantle via subduction processes. Therefore any new phase bearing water and arising in P, T conditions compatible with the subducted slab is highly interesting for the comprehension of the complex geological processes determining the dynamical evolution of subducting zones. A new phase with these characteristic have been discovered in experimental runs having clinochlore bulk composition, in the P,T range of 5.2-5.5 GPa and 680-720 °C. It is monoclinic with space group C2/c and unit cell parameters a=9.882(1), b=11.623(1), c=5.081(1), beta=11.1(1)°. This new phase has been identified with conventional electron diffraction. Its structure has been solved using the automated electron diffraction technique [1], a new technique that allows a single crystal electron diffraction experiment on crystals of less than 1um, obtaining quasi kinematical intensities suitable for a conventional direct methods structure solution attempt. To our knowledge this is the first high pressure phase solved in this way. It has chemical composition of Mg2.1Al1.8Si1.1O6(OH)2, and has a pyroxene like structure with 3 cations instead of 2 in the octahedral layer, which force the tetrahedral chains to be more separate. The two anion sites of the extra octahedral cation, that are not shared with tetrahedra are occupied by an hydroxyl. Thanks to the structure solution obtained with TEM investigations it has been possible to synthesise an almost pure sample suitable for powder x-ray diffraction analysis. A Rietveld refinement on the data taken on this sample, and image simulation of high resolution electron microscopy images further confirm this structural model.