Transformations isochemical

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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... 

In addition to the phenomena of isochemical crystallization, in the earhest stages of metamorphic transformation in hematite rocks the process of dehydration should occur ... 

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... 

Invariably, however, the hexagonal quartz crystals will transform to the stable tetragonal phase P-spodumene, or as it is sometimes referred to, stuffed keatite. This phase transformation is isochemical with compositions along... 

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