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The mineral wadsleyite, (Mg,Fe)SiO₄, is the dominant phase in the transition zone of the Earth between 410 and ~520 km depth. The frequent but incorrect use of the name "beta-spinel" for this phase derives from the close relationship between its structure and that of spinel. Both are comprised of spinel-like sheets of structure containing SiO₄ tetrahedra and (Fe,Mg)O₆ octahedra. One way of stacking these layers generates the structure of spinel. Another regular stacking sequence generates the structure of wadsleyite. It has long been known that in other chemical systems additional regular stacking sequences of these same spinel-like layers give rise to further stable structures. Collectively these related structures are termed "spinelloids".

These other spinelloid structures do not occur in the (Mg,Fe)SiO₄ system. But some years ago workers at the Bayerisches Geoinstitut synthesised the spinelloid-V structure in the system magnetite-fayalite (Fe₃O₄-Fe₂SiO₄) at high pressures and temperatures in the multi-anvil press. A.B. Woodland has now determined the complete phase equilibria in this system over a wide range of temperatures and pressures. At pressures from 3 to 6 GPa and temperatures of 900-1200°C he has shown that at intermediate compositions stability fields for the spinelloid structures II, III, and V exist. The crystal structures of phases II and III in this system have been determined for the first time by single-crystal X-ray diffraction.

Both spinelloids contain Fe (of mixed oxidation state) in the octahedral sites of their structures, and Fe³⁺ and Si⁴⁺ in the tetrahedral sites. There is only one symmetrically distinct tetrahedral site in the spinelloid-III structure, joined in pairs to form T₂O₇ groups. There is no evidence from the X-ray diffraction data of any significant ordering of Fe³⁺ and Si⁴⁺ on the tetrahedral sites. In the spinelloid-II structure there are two distinct tetrahedral sites, T1 and T2, arranged to form T₃O₁₀ groups. Structure refinement to the X-ray data shows that the central T2 site is smaller and enriched in Si (~72% Si + 28% Fe) compared to the outer larger T1 sites(~29%Si + 71% Fe).

Wadsleyite itself also has the spinelloid-III structure. Our discovery of the same structure type in the magnetite-fayalite system immediately suggests the possibility of at least partial solid solution between them, based upon the coupled substitution of 2Fe³⁺ for Fe²⁺+Si⁴⁺ in wadsleyite. Because the spinelloid-III structure in the Fe₃O₄-Fe₂SiO₄ system is stable at much lower pressures than wadsleyite this substitution could act to stabilise wadsleyite to lower pressures. Thus the 410 km discontinuity within the Earth that is caused by the transformation of olivine to wadsleyite could be shifted to shallower depths merely through a change in redox state. In addition, the large modal abundance of wadsleyite in the upper portions of the transition zone implies that the available Fe³⁺ will be distributed throughout a relatively large volume, rather than being concentrated in modally minor phases, as is the case in the upper mantle. This would lead to a significant lowering of the fO₂ in the transition zone, thereby increasing the feasibility of "redox melting" as a viable process for melt generation in the deeper portions of the upper mantle.