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Distribution of cations at two tetrahedral sites in Ca2MgSi2O7-Ca2Fe3+AlSiO7 series synthetic melilite and its relation to incommensurate structure

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Abstract

Synthetic melilites on the join Ca2MgSi2O7 (åkermanite: Ak)-Ca2Fe3+AlSiO7 (ferrialuminium gehlenite: FAGeh) were studied using X-ray powder diffraction and 57Fe Mössbauer spectroscopic methods to determine the distribution of Fe3+ between two different tetrahedral sites (T1 and T2), and the relationship between ionic substitution and incommensurate (IC) structure. Melilites were synthesized from starting materials with compositions of Ak100, Ak80FAGeh20, Ak70FAGeh30 and Ak50FAGeh50 by sintering at 1,170–1,350 °C and 1 atm. The average chemical compositions and end-member components, Ak, FAGeh and Geh (Ca2Al2SiO7), of the synthetic melilites were Ca2.015Mg1.023Si1.981O7 (Ak100), Ca2.017Mg0.788Fe 3+0.187 Al0.221Si1.791O7 (Ak78FAGeh19Geh3), Ca1.995Mg0.695Fe 3+0.258 Al0.318Si1.723O7 (Ak69FAGeh25Geh6) and Ca1.982Mg0.495Fe 3+0.449 Al0.519Si1.535O7 (Ak49FAGeh44Geh7), respectively. Rietveld refinements using X-ray powder diffraction data measured using CuK α -radiation at room temperature converged successfully with goodness-of-fits of 1.15–1.26. The refined Fe occupancies at the T1 and T2 sites and the Mg and Si contents determined by electron microprobe analysis gave the site populations of [0.788Mg + 0.082Fe3+ + 0.130Al]T1[0.104Fe3+ + 0.104Al + 1.792Si]T2 for Ak78FAGeh19Geh3, [0.695Mg + 0.127Fe3+ + 0.178Al]T1[0.132Fe3+ + 0.144Al + 1.724Si]T2 for Ak69FAGeh25Geh6 and [0.495Mg + 0.202Fe3+ + 0.303Al]T1[0.248Fe3+ + 0.216Al + 1.536Si]T2 for Ak49FAGeh44Geh7 (apfu: atoms per formula unit), respectively. The results indicate that Fe3+ is distributed at both the T1 and the T2 sites. The mean T1–O distance decreases with the substitution of Fe3+ + Al3+ for Mg2+ at the T1 site, whereas the mean T2–O distance increases with substitution of Fe3+ + Al3+ for Si4+ at the T2 site, causing decrease in the a dimension and increase in the c dimension. However, in spite of the successful Rietveld refinements for the X-ray powder diffraction data measured using CuK α-radiation at room temperature, each Bragg reflection measured using CuK α1-radiation at room temperature showed weak shoulders, which were not observed in those measured at 200 °C. The Mössbauer spectra of the melilites measured at room temperature consist of two doublets assigned to Fe3+ at the T1 site and two or three doublets to Fe3+ at the T2 site, implying the existence of multiple T1 and T2 sites with different site distortions. These facts can be interpreted in terms of the IC structure in all synthetic melilites at room temperature, respectively. The results of Mössbauer analysis indicate that the IC structure in melilite is caused by not only known multiple T1 site, but also multiple T2 site at room temperature.

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Acknowledgments

We thank Prof. Takuya Ohba of Department of Material Science, Shimane University, and Dr. Taisuke Hayashi of Department of Materials Analysis, CIRS, Shimane University, for their support to use RIGAKU SmartLab X-ray powder diffractometer, Drs. Fujio Izumi of the National Institute for Materials Science and Koichi Momma of the National Museum of Nature and Science for their permission to use RIETAN-FP and VESTA programs, and Dr. Barry P. Roser of Department of Geoscience, Shimane University, for his critical reading of the manuscript. We also thank the cooperation of the Center for Integrated Research in Science for providing the XRD (RIGAKU SmartLab), experimental facility, which was introduced through the Tatara Project supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan. We appreciate Dr. Masanori Matsui, the handling editor, and two anonymous reviewers for their helpful comments and advice. This work was supported by Grant-in-Aid for Scientific Research 22-1791.

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Authors and Affiliations

  1. Department of Materials Creation and Circulation Technology, Interdisciplinary Graduate School of Science and Engineering, Shimane University, Matsue, 690-8504, Japan

    Maki Hamada & Masahide Akasaka

  2. Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan

    Maki Hamada

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  1. Maki Hamada
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  2. Masahide Akasaka
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Correspondence to Maki Hamada.

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Hamada, M., Akasaka, M. Distribution of cations at two tetrahedral sites in Ca2MgSi2O7-Ca2Fe3+AlSiO7 series synthetic melilite and its relation to incommensurate structure. Phys Chem Minerals 40, 259–270 (2013). https://doi.org/10.1007/s00269-013-0566-8

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  • DOI: https://doi.org/10.1007/s00269-013-0566-8

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