The crystal chemistry and Fe^II–site properties of aluminosilicate garnet solid solutions as revealed by Mössbauer spectroscopy and electronic structure calculations

Abstract

 The three binary garnet solid solutions Fe^II ₃Al₂Si₃O₁₂–X^II ₃Al₂Si₃O₁₂ (X^II= Mg^II, Mn^II, Ca^II) have been investigated by ⁵⁷Fe Mössbauer spectroscopy at 298 and 77 K and by electronic structure calculations in the local spin density approximation. The spectra yield isomer shifts and quadrupole splittings that are typical for Fe^II in the dodecahedral X-site of 222 point symmetry and are similar for each of the three binaries recorded. Conversely, electronic structure calculations based on the experimental crystal structure of the different end-member garnets exhibit pronounced variations in some of the electronic properties of Fe^II that are not reflected in the spectroscopic data. These results are interpreted as indicating that the different X–O bonds in garnet solid solutions retain to a large degree the intrinsic lengths that they possess in their respective end members, and that the Fe–O bond does not change greatly as a function of composition. This is evidence for the state of alternating bonds and not for the virtual crystal approximation in describing the X–O bond types or lengths in aluminosilicate garnet solid solutions. The observed degree and behavior of the Fe^II doublet asymmetry in the Mössbauer spectra for the three solid solution series do not indicate major variations in the anisotropic recoil-free fraction of Fe^II. Variations in doublet asymmetry are more likely a result of complex next-nearest X-site neighbor interactions and/or some degree of short-range cation ordering, though doublets representing different local X-site cation configurations cannot be resolved or fitted to the experimental spectra.