WADSLEYITE II: A NEW HIGH PRESSURE HYDROUS PHASE IN THE PERIDOTITE-H2O SYSTEM

Abstract

A new hydrous silicate phase with formula Mg1.71Fe0.177Al0.01Si0.965H0.332O4 has been synthesized at 1400°C and 17.5 GPa from a hydrous iron-rich mantle peridotite composition. The space group is Imma and unit cell parametersa = 5.6884(4) b = 28.9238(15) and c = 8.2382(6) #9, 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 3.513 g/cm3. 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 a 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.