CHARACTERIZATION AND STABILITY OF PHASE 'A', A HIGH PRESSURE PHASE IN THE SYSTEM MAGNESIUM OXIDE - SILICON DIOXIDE - WATER (MANTLE)

The existence of "Phase A" was first reported by Ringwood and Major (1967) and Sclar, et al. (1967) who concluded that the Mg/Si ratio of Phase A was 1.5 to 2 and 2.0, respectively. Subsequently, Yamamoto and Akimoto (1974) proposed that Phase A has a Mg/Si atomic ratio of 3.5. Mixtures of brucite and dehydrated silicic acid with Mg/Si ratios ranging from 1-4 were reacted at pressures between 6 and 11.5 GPa and temperatures between 500 and 1000(DEGREES)C. The experimental results are in accord with a 2:1 Mg/Si atomic ratio for Phase A. Seventeen X-ray diffraction maxima assigned to Phase A were indexed on a hexagonal cell with a = 7.867 (+OR-) 0.002 (ANGSTROM) and c = 9.589 (+OR-) 0.008 (ANGSTROM) which is in accord with the cell edges reported by Yamamoto and Akimoto (1974). Thermogravimetric analysis shows that Phase A dehydroxylates above 600(DEGREES)C at 0.1 mm of Hg and that the total weight loss at 900(DEGREES)C was 10.8 percent. Scanning electron microscopy reveals the presence of hexagonal prisms. EDS spectra taken with a STEM indicates that the relative peak heights of Mg to Si in Phase A and forsterite are similar. Infrared absorption spectroscopy suggests that the silicon and magnesium in Phase A are tetrahedrally and octahedrally coordinated respectively, and that Phase A contains structurally bound (OH) groups. The stoichiometric formula for Phase A suggested by the experimental data is (2MgO:SiO(,2):H(,2)O) which can be represented by {MgSiO(,3)(OH)(,2)} or {MgSiO(,3)(.)Mg(OH)(,2)}. The stability field of Phase A appears to be in excess of 6.0 GPa, and Phase A appears to persist as high as 11.5 GPa. For a bulk chemistry of 2:1 Mg/Si, below 500(DEGREES)C, the assemblage 10 (ANGSTROM) phase + brucite appears to be stable, and above 800(DEGREES)C forsterite + water appear to be stable. Phase A could be a very important phase with respect to the earth's mantle.