Olivine spinel transformation

Syono, Y., Tokonami, M. Matsui, Y. (1971) Crystal field effect on the olivine-spinel transformation. Phys. Earth Planet. Ineriors, 4,347-52. 

Vaughan, P. J., Green, H. W., and Coe, R. S., 1984. Anisotropic growth in the olivine-spinel transformation of Mg2Ge04 under nonhydrostatic stress. Tectonophysics 108, 299-322. 

In some respects this high-pressure transformation is similar to that of olivine spinel there is a large volume decrease ( 11% cf. 10% for olivine - spinel), but no change in primary coordination number (OZr2Si, ZrOg, Si04). We have already considered the two structure types involved (Sect. 3.4 and Sect. 3.6). 

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... 

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 enthalpy of formation from the oxides of Mg2Sn04 and Co2Sn04 were found by oxide solution calorimetry. Using thermochemical data for the formation of olivine, for olivine - spinel transitions and for the transformation of quartz to stishovite, pressures for the disproportionation of silicate spinels were calculated to be in the range 150 - 200 kbar. 

Figure 9.7 Transition pressures at 1,000 °C for various olivines transforming to the P-phase ( modified spinel or wadsleyite) or Y-phase (spinel or ringwoodite) as a function of the ionic radius ratio divalent cation (Fr+) to Si4 or Ge4 (M4 ) (from Syono et al., 1971). Note that the cations acquiring excess CFSE in spinel over olivine (e.g., Fe2+, Co2+, Ni2+) deviate from a linear trend.

Chapter 2.02 concludes that within broad bounds, an isochemical mantle with composition similar to that inferred from upper mantle rocks is consistent with the seismic properties of the whole mantle. This includes not only wave speeds, but also the appearance of seismic discontinuities at 410 km and 660 km depth that mark the expected phase transformations as upper mantle olivine changes first into spinel and then into perovskite structured high-pressure polymorphs. As discussed in Chapter 2.02, because the phase diagrams for these mineralogical transformations can be determined in the laboratory, the exact depth of the corresponding seismic discontinuities... 

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