Phase transitions olivine -> spinel

These two structures have been described above (Sect. 2.6.3 and Sects. 2.8.1, 3.2) and are depicted in Figs. 16 and 29. We have already suggested (Sect. 4) that in forsterite (a-Mg2Si04, olivine type) the repulsive forces between O... O (d 2.6 A) are less than those between Mg... Mg (d = 3.0 A) which, in turn, are less than those between Mg... Si (d 2.7 A). One would therefore expect that in the phase transition a-Mg2Si04 (olivine) — y-Mg2Si04 (spinel) the principal result would be an increase in the last of these distances, in order to relieve the largest non-bonded repulsions . And this is indeed the case the mean non-bonded (next-nearest neighbour) distances are as shown... 

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... 

Nanoparticles may also be important within planetary interiors. For example, phase transitions within the deep Earth may generate materials that are composites of nanoparticles (e.g., within the spinel phase at the olivine-spinel transition at the 400-km discontinuity). These grain sizes may affect both kinetics and rheology (e.g., of ice in planetary interiors Stern et al. 1997). Chemical reactions in the deep Earth, perhaps between metal and silicate near the core-mantle boundary, may be impacted by nanocrystals. 

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.

At about 500 km depth, within the mantle transition zone, olivine undergoes a further phase change from /3-Mg2Si04 to spinel structured Mg2Si04 - y-Mg2Si04 (ringwoodite - Fig. 3.1). 

These include mostly transition metal oxides and sulfides that can be classified in several ways, e.g., according to their red-ox potentials 2, 3, 4, 5 V vs. Li/Li ", or according to their structure, e.g., layered, spinel, inverse spinel, olivine. Typical 2-3 V materials are LixMoeOg chevrel phase (0 < x < 4), TiS2, M0S2. Typical 3-4V materials are Li Mn02 (0 < x < 1), Li VO , LixCo02,... 

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