The olivine — spinel phase transition

1 The olivine spinel phase transition Experimental phase equilibrium studies have confirmed deductions from seismic velocity data that below 400 km, olivine and pyroxene, the major constituents of Upper Mantle rocks, are transformed to denser polymorphs with the garnet, y-phase (spinel) and P-phase (wadsleyite) structures (fig. 9.2). In transformations involving olivine to the P- or y-phases, transition pressures 

2 Partitioning of iron in post-spinel phases in the Lower Mantle 

The transformation of olivine to spinel which delineates the onset of the Transition Zone in the Mantle is isochemical. However, in the Lower Mantle the (Mg,Fe)2Si04 stoichiometry appears to be unstable with respect to denser oxide structure-types. One post-spinel transformation that has been the subject of numerous investigations is the spinel to periclase plus perovskite transition 

The strong partitioning of iron into magnesiowiistites is the result of higher CFSE of Fe2+ ions in the periclase structure. 

Various estimates have been made of the CFSE of Fe2+ in dense oxide structures modelled as potential Mantle mineral phases (Gaffney, 1972 Bums, 1976a). All estimates indicate that octahedrally coordinated Fe2+ (in periclase, for example) has a considerably higher CFSE than Fe2+ ions in the eight- to twelve-coordination sites in the perovskite structure. Thus, the CFSE of Fe2+ in 

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