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A commensurate-incommensurate phase transition in iron-bearing åkermanites

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Abstract

Synthetic melilites on the join Ca2MgSi2O7 (åkermanite) — Ca2FeSi2O7 (iron åkermanite) with Fe/(Fe+Mg) from 0.0 to 0.7 exhibit, at room temperature, an incommensurate phase with a rectangular modulation of a wavelength of about 19 Å in the [110] direction. Upon increase of temperature, they transform to a commensurate melilite structure at about 80° C for Fe/(Fe+Mg)=0.0 and about 250° C for Fe/(Fe+Mg)=0.6. In addition to the T(2) positions of the melilite structure filled by Si, the incommensurate phase exhibits two distinguishable T(1) sites containing the Mg and Fe2+. These two sites merge into one site during the phase transition from the incommensurate to the commensurate phase. A structural model for the incommensurate phase is based on the misfit between the tetrahedral (Mg, Fe2+)Si2O 4−7 sheets and the Ca2+ ions.

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Authors and Affiliations

  1. Bayerisches Geoinstitut, Universität Bayreuth, Postfach 101251, 8580, Bayreuth, West Germany

    F. Seifert

  2. Mineralogisches Institut, Universität Kiel, Olshausenstr. 40, 2300, Kiel, West Germany

    M. Czank, B. Simons & W. Schmahl

Authors
  1. F. Seifert
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  2. M. Czank
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  3. B. Simons
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  4. W. Schmahl
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Seifert, F., Czank, M., Simons, B. et al. A commensurate-incommensurate phase transition in iron-bearing åkermanites. Phys Chem Minerals 14, 26–35 (1987). https://doi.org/10.1007/BF00311145

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  • DOI: https://doi.org/10.1007/BF00311145

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