Garnets phase compositions

Table 5.22 Bulk modulus and thermal expansion for various compositional terms of the garnet phase. The experimental range for bulk moduli is relative to a common value of K = 4. The calculated values are from Ottonello et al. (1996). References for thermal expansion are as follows (1) Ottonello et al. (1996) (2) Skinner (1956) (3) Suzuki and Anderson (1983) (4) Isaak et al. (1992) (5) Armbruster and Geiger (1993). ...

Figure 4 Plot of garnet composition versus radius for garnet growth between 500 °C and 577 °C at 6 kbar. Assemblage and chemical system are the same as in Figure 3. Dotted lines show trace of equilibrium compositions, which result from phase equilibrium effects. Solid lines show fractional crystallization model. Similarity of curves for all components (not just Mn) indicates that phase equilibria strongly control garnet chemical compositional changes, but significant differences between curves, especially for Mn, indicate that fractional crystallization is also important (source Spear, 1993).

For auother possible reactiou mechauism for the formatiou of mixed oxides, binary alkoxide (glycoxide) may be formed prior to the formation of the mixed oxide. In fact, various binary glycoxides have been prepared and their crystal structures have been elucidated. Formation of BaTiOj by the glycothermal reaction may be explained by this mechanism, because Ba-Ti binary alkoxide is well established. However, formation of garnet phases (Sections III.C.l through III.C.3) cannot be explained by this mechanism because addition of seed crystals, in some cases, gives a product with a chemical composition completely different from that of the product obtained without the addition of the seed crystals. 

The mixing properties of garnet components and many other rock-forming solid solutions is an on-going problem, and we can leave resolution of the problems of grossular-pyrope to the experts. The point made here is simply that determinations of excess properties can be made by calorimetry, measuring phase compositions, and X-ray work, and that these can be combined into values of Wc and hence of activity coefficients. None of this is easy to do. 

Table 5.19 summarizes the main occurrences of the various compositional terms. Because garnets are solid mixtures, the listed components merely indicate the main compositional terms of the phase. 

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