Silicate garnet: A micro to macroscopic (re)view

Silicate garnets, general formula E 3 G 2 Si 3 O 12 , form an important class of rock-forming minerals and, in nature, most are solid solutions. Their crystal-chemical and solid-solution properties are sometimes interpreted in terms of the widely used Pyralspite-Ugrandite classification scheme, and this can lead to erroneous conclusions. In this study, published data are reviewed and analyzed to achieve a synthesis of relevant experimental and computational results and to construct a working “crystal-chemical model” for describing aluminosilicate garnet, E 3 Al 2 Si 3 O 12 , over different length scales. The pyrope-grossular (Py-Gr) solid solution is given special attention, because it has received a great deal of study. It also shows interesting crystal-chemical and thermodynamic mixing behavior. Computational and experimental investigations made on Py-Gr garnets indicate that the shorter Ca/Mg-O2 bond lengths appear to remain roughly constant in length across the binary and can be described as showing “Pauling limit-type” behavior. The longer Ca/Mg-O4 bonds behave differently, because they lengthen with increasing Gr component in the solid solution. Bond behavior in almandine-spessartine (Al-Sp) garnets appears to be partly different, because both Fe/Mn-O2 and Fe/ Mn-O4 bonds show “Pauling limit-type” behavior. E-O bond-length variations are continuous. The bonding type in all aluminosilicate garnet end-members is similar. An analysis shows that various computational simulations on Py-Gr solid solutions are consistent with each other with respect to E-O bond behavior and also with experimental IR, Raman, NMR spectroscopic, and X-ray diffraction results, but not completely with XAS studies made at the Ca edge. Ca/Mg-O4 bond behavior can be used to explain, partly, the nature of various micro/nanoscopic crystal-chemical and strain properties and macroscopic excess thermodynamic mixing behavior of Py-Gr garnets. Micro/nanostrain for the Py-Gr binary is asymmetric in nature, as are the various thermodynamic mixing functions ΔH ex , ΔS ex , and ΔV ex . The widely cited Pyralspite-Ugrandite classification scheme has limited use in terms of explaining many physical and chemical properties of garnet and it should not be used to predict or describe, for example, solid-solution behavior.