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The CaGeO3–Ca3Fe2Ge3O12 garnet join: an experimental study

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Abstract

Germanate garnets are often used as isostructural analogues of silicate garnets to provide insight into the crystal chemistry and symmetry of the less accessible natural garnet solid solutions. We synthesised two series of germanate garnets at 3 GPa along the joinVIIICa VI3 (CaGe)IVGe3O12–VIIICa VI3 Fe IV2 Ge3O12 at 900 °C and 1,100 °C. Samples with compositions close to the CaGeO3 end-member consist of tetragonal garnet with a small amount of triclinic CaGe2O5. Samples with nominal compositions between XFe=0.4 and 1.0 consist of a mixture of tetragonal and cubic garnets; whereas, single-phase cubic garnets were obtained for compositions with XFe>1.2 (XFe gives the iron content expressed in atoms per formula unit, and varies between 0 and 2 along the join). Run products which were primarily single-phase garnet were investigated using Mössbauer spectroscopy. Spectra from samples synthesised at 1,100°C consist of one well-resolved doublet that can be assigned to Fe3+ in the octahedral site of the garnet structure. A second doublet, present primarily in samples synthesised at 900°C, can be assigned to Fe2+ at the octahedral sites of the garnet structure. The relative abundance of Fe2+ decreases with increasing iron content. Transmission electron microscopy analyses confirm this tendency and show that the garnets are essentially defect-free. The unit-cell parameters of tetragonal VIIICa VI3 (CaGe)IVGe3O3 garnet decrease with increasing synthesis temperature, and the deviation from cubic symmetry becomes smaller. Cubic garnets show a linear decrease of unit-cell parameter with increasing iron content. The results are discussed in the context of iron incorporation into VIIIMg VI3 (MgSi)IVSi3O3 majorite.

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Acknowledgments

This project benefited through discussions with Friedrich Seifert. Geoffrey Bromiley assisted with the piston-cylinder syntheses, Detlef Krauße with collection of the electron microprobe data, Burkhardt Schmidt with analytical techniques, and Georg Hermannsdorfer with technical aspects of the high-pressure runs. The manuscript was improved through careful reviews by J. Ganguly and an anonymous reviewer. This study was supported by the Sofja Kovalevskaja Award to TBB.

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  1. Gianluca Iezzi

    Present address: Dipartimento Scienze della Terra, Università G. D’Annunzio, 66100, Chieti, Italy

Authors and Affiliations

  1. Bayerisches Geoinstitut, Universität Bayreuth, 95440, Bayreuth, Germany

    Gianluca Iezzi, Tiziana Boffa-Ballaran, Catherine McCammon & Falko Langenhorst

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  1. Gianluca Iezzi
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  2. Tiziana Boffa-Ballaran
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Correspondence to Tiziana Boffa-Ballaran.

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Iezzi, G., Boffa-Ballaran, T., McCammon, C. et al. The CaGeO3–Ca3Fe2Ge3O12 garnet join: an experimental study. Phys Chem Minerals 32, 197–207 (2005). https://doi.org/10.1007/s00269-005-0460-0

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