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A new spinelloid polytype with a composition Fe 2.45 Si 0.55 O 4  has been synthesized at 1100 °C and 5.6 GPa that is isostructural with wadsleyite [β-(Mg,Fe) 2 SiO 4 ]. The refined parameters (space group Imma) are: a = 5.8559(2) Å , b = 11.8936(4) Å , c = 8.3684(2) Å , V = 582.84(2) Å 3 . Tetrahedrally coordinated Fe 3+  and Si are completely disordered and the substitution of Fe 3+  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 Fe 3+  can be incorporated into the wadsleyite-type structure. Because the b form of Fe 2 SiO 4  is unstable, the implication is that Fe 3+ , by the substitution mechanism: 2Fe 3+  = Si 4+  + Fe 2+ , acts to stabilize the wadsleyite structure. It is possible that the addition of Fe 3+  could stabilize (Mg,Fe) 2 SiO 4  wadsleyites to lower pressures, which would influence the exact position of the ‘‘410 km’’ discontinuity. The apparent compatibility of Fe 3+  in the wadsleyite structure, suggests that available Fe 3+  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.

american mineralogist

LETTER. 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