CRYSTAL-STRUCTURE OF A NEW SPINELLOID WITH THE WADSLEYITE STRUCTURE IN THE SYSTEM FE2SIO4-FE3O4 AND IMPLICATIONS FOR THE EARTHS MANTLE

A new spinelloid polytype with a composition Fe2.45Si0.55O4 has been s ynthesized at 1100 degrees C and 5.6 GPa that is isostructural with wa dsleyite [beta-(Mg,Fe)(2)SiO4]. The refined parameters (space group Im ma) are: a = 5.8559(2) Angstrom, b = 11.8936(4) Angstrom, c = 8.3684(2 ) Angstrom, V = 582.84(2) Angstrom(3). Tetrahedrally coordinated Fe3and Si are completely disordered and the substitution of Fe3+ for near ly one-half of the Si results in a significant expansion of the tetrah edra. This is the first direct evidence that significant amounts of Fe 3+ can be incorporated into the wadsleyite-type structure. Because the beta form of Fe2SiO4 is unstable, the implication is that Fe3+, by th e substitution mechanism: 2Fe(3+) = Si4+ + Fe2+, acts to stabilize the wadsleyite structure. It is possible that the addition of Fe3+ could stabilize (Mg,Fe)(2)SiO4 wadsleyites to lower pressures, which would i nfluence the exact position of the ''410 km'' discontinuity. The appar ent compatibility of Fe3+ in the wadsleyite structure, suggests that a vailable Fe3+ will be readily incorporated in the modally dominant pha se in the upper parts of the transition zone, thereby leading to a low f(O2) in this region of the mantle.