Mg2+-Fe2+ substitution in the dodecahedral interstice of eight natural single crystals and two synthetic polycrystalline garnets was investigated by measurement of mid- and far-infrared (IR) reflectance spectra across the pyrope-almandine [Py-Al = (Mgx-Fe1-x)3Al2Si3O 12] binary. The effect of minor Mn and Ca substitution was investigated by comparison with spectra of four additional natural garnets, Py8Al82Sp10 and Py10Al76Sp14, where Sp denotes spessartine (Mn3Al2Si3O12), and Py72Al18Gr10 and Py54Al36Gr10, where Gr denotes grossular (Ca3Al2Si3O12). Data obtained from the two synthetic polycrystals are in excellent agreement with data from the single crystals. Frequencies of all 17 IR-active fundamental modes were observed in all specimens except pyrope. Of the 17 modes, 16 show linear correlations between frequency and composition along the pyrope-almandine join. The remaining mode, assigned to translations of the divalent cations against the O framework, exhibits two-mode behavior. The trends show that two modes in pyrope are accidentally degenerate. Frequencies of garnets with up to 10 mol% spessartine and grossular contents lie on the trends defined by the pyrope-almandine samples with negligible impurities. However, one to three additional modes are seen for some intermediate compositions, apparently resulting from either resonances among combination modes or two-mode behavior due to high Ca contents. The nearly linear behavior of the modes indicates that mixing should be close to ideal for the pyrope-almandine series because the lattice contribution dominates heat capacity, entropy, and compressibility.
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