Garnet solid solutions have been synthesized across the skiagite–Fe-knorringite (Fe32+Fe23+Si3O12–Fe3Cr2Si3O12) binary join. Such compositions reflect a simple Cr-Fe3+ exchange on the octahedral sites, with Fe2+ occupying the adjacent dodecahedral sites. Solid solution is complete across the join and the correct stoichiometry was verified by Mössbauer spectroscopy. A symmetric fit to the molar volume data yields 119.77(2) cm3/mol [unit-cell parameter ao =11.6736(7) Å] for the Fe-knorringite end-member and a small negative excess volume, WV = −0.76(15) cm3/mol. Combining this result with literature data reveals that binary joins involving octahedral site substitutions exhibit significantly different behavior than those where substitution occurs on the dodecahedral sites. In the former case, WV is usually negative, whereas the latter joins have positive deviations from ideal behavior. Therefore, we conclude that the garnet structure responds in a fundamentally different way when accommodating different cations on the dodecahedral or octahedral sites. Mössbauer spectra of skiagite–Fe-knorringite garnets do not exhibit any significant asymmetry in the [8]Fe2+ doublet. Fe3+/ΣFe values determined at room temperature and 80 K confirm the general applicability of the recoil-free fraction correction factors reported by Woodland and Ross (1994) for mixed-valence garnets. Coexisting spinels in some samples are either binary Fe3O4–Fe2SiO4 or Fe3O4–FeCr2O4 solid solutions. Very little mutual solubility is apparent suggesting a significant solvus may exist between the silicate and Cr-bearing spinel series.

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