Local structure of ferric iron-bearing garnets deduced by IR-spectroscopy

Powder IR absorption spectroscopy has been used to characterise cation substitutions in three garnet solid solutions: grossular–andradite, skiagite–andradite and skiagite–almandine. The wavenumber shift of the highest energy mode associated with tetrahedral vibrations depends on the type of cation occupying the adjacent sites in the structure. The wavenumber shifts exhibit positive deviations from linearity that correlate closely with the variations of the Si–O bond distances for all three garnet solid solutions. The autocorrelation function has been used to determine an effective line width (Δcorr) of the absorption bands over a given spectral region. Non-linear behaviour of Δcorr was found for all three solid solutions. An empirical calibration between Δcorr excess and calorimetric enthalpy of mixing data gives an estimate for the symmetric Margules parameters WspecH of the three solid solutions. Comparison with the systematics of aluminosilicate garnets in terms of WspecH vs. ΔV2, where ΔV represents the difference in molar volume between the end members in a binary system, reveals that such a relationship is not generally applicable to garnet solid solutions with an octahedral cation other than Al.