Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Geiger, C. A. Search for Related Content GeoRef GeoRef Citation

Silicate garnet: A micro to macroscopic (re)view

Institut für Geowissenschaften, Abteilung Mineralogie, Universität Kiel, Olshausenstr. 40, D-24098 Kiel, Germany

Correspondence: ^* E-mail: chg{at}min.uni-kiel.de

Silicate garnets, general formula E₃G₂Si₃O₁₂, 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, E₃Al₂Si₃O₁₂, 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 H^ex, S^ex, and V^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.

Key Words: Garnet • bonding • crystal chemistry • thermodynamics • spectroscopy • X-ray diffraction • computations • strain and solid solutions