Single-crystal IR spectroscopy of pyrope-almandine garnets with minor amounts of Mn and Ca

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 = (Mg_x-Fe_1-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.