Wadsleyite II: A new high pressure hydrous phase in the peridotite-H2O system

A new hydrous silicate phase with formula Mg1.71Fe0.177Al0.01Si0.965H0.332O4 has been synthesized at 1400°C and 17.5GPa from a hydrous iron-rich mantle peridotite composition. The space group is Imma and unit cell parameters a = 5.6884(4); b = 28.9238(15); and c = 8.2382(6)Å, so that a and c are approximately those of wadsleyite, whereas b is 2.5 times that of wadsleyite. The calculated density of this phase is 3.513g/cm. In addition to five Mg/Fe octahedra and two silicate tetrahedra, the structure contains one partially occupied Mg/Fe octahedron and one partially occupied silicate tetrahedron. These two partially occupied sites may be the result of decompression of a single Mg/Si octahedron. The synthesis of this phase which has not been previously noted or described indicates that the crystal chemistry of H incorporation into high pressure spinelloid structures is more complex than previously thought. Whereas the transition zone of the Earth’s mantle may serve as very large reservoir for hydrogen, further work will be required to outline the relative stabilities and physical properties of the various hydrous silicates that may exist there.