Olivine cell volume

Fig. 10. Unit cell volume vs. rjj for M2SiS4 and M2GeS4 olivine compounds...

Table 5.6 Structure of olivine compounds (from Smyth and Bish, 1988). Molar volnme is expressed in cm /mole molar weight in g/mole density in g/cm cell edges and cell volume are in A and A, respectively.

High time-resolution is afforded with energy dispersive diffraction (see above). While systematic errors are problematic, reliable structural refinements are possible for a limited class of experiment. The software and method to enable structure refinement using the Rietveld method and ED powder diffraction data are now well-established (Chen and Weidner 1997, Larson and von Dreele 1986). In a recent study of the partitioning of iron between the olivine and ringwoodite polymorphs of (Mg,Fe)2Si04, energy dispersive data were sufficiently accurate to allow derivation of unit cell volumes... 

Equations 5.15 and 5.16 give only approximate values for the volumes of olivine compounds. More accurate study of binary mixtures outlines important deviations from ideal behavior, which result in slight curves in cell parameter vs. composition plots, as shown in figure 5.7. The greatest deviations regard cell edges and are particularly evident for the (Mg,Ni)2Si04 mixture. 

Figure 5.7 Molar volumes and cell edges in binary olivine mixtures (from Akimoto et al., 1976 modified). Note conspicuous deviations from linearity in cell edges of (Mg,Ni)2Si04 mixture, satisfactorily reproduced by site-occupancy calculations based on parameters in table 5.7.

In olivine, 24 independent interatomic distances must be evaluated in order to define the structure. For all 24 distances (Z)y) plus cell edges and volume, regressions of the following type may be proposed ... 

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