Crystal structure of a new spinelloid with the wadsleyite structure in the system Fe 2 SiO 4 -Fe 3 O 4 and implications for the Earth's mantle

Abstract

A new spinelloid polytype with a composition Fe2.45Si0.55O4 has been synthesized at 1100 degrees C and 5.6 GPa that is isostructural with wadsleyite [beta-(Mg,Fe)(2)SiO4]. The refined parameters (space group Imma) are: a = 5.8559(2) Angstrom, b = 11.8936(4) Angstrom, c = 8.3684(2) Angstrom, V = 582.84(2) Angstrom(3). Tetrahedrally coordinated Fe3+ and Si are completely disordered and the substitution of Fe3+ for nearly one-half of the Si results in a significant expansion of the tetrahedra. This is the first direct evidence that significant amounts of Fe3+ can be incorporated into the wadsleyite-type structure. Because the beta form of Fe2SiO4 is unstable, the implication is that Fe3+, by the 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 influence the exact position of the "410 km" discontinuity. The apparent compatibility of Fe3+ in the wadsleyite structure, suggests that available Fe3+ will be readily incorporated in the modally dominant phase in the upper parts of the transition zone, thereby leading to a low f(O2) in this region of the mantle.