STRUCTURAL STUDIES ON INCOMMENSURATE-NORMAL PHASE TRANSITION IN Co-ÅKERMANITE, Ca₂CoSi₂O₇. K. Hagiya, K. Kusaka, N. Haga, M. Ohmasa and K. Iishi¹⁾. Department of Life Science, Himeji Institute of Technology, Japan; ¹⁾Department of Mineralogical Science and Geology, Yamaguchi University, Japan

Since comparison between the structure of incommensurate phase and that of normal phase in the title compound is expected to be informative to elucidate origin of the modulation, the present studies on change of the structure at elevated temperatures have been carried out. Two-dimensional incommensurate phases of synthetic åkermanite solid solution Ca₂(Mg,Fe)Si₂O₇ were found by Hemingway et al.¹⁾ and Seifert et al.²⁾ independently. They found that åkermanite undergoes a reversible phase transition from the low-temperature incommensurate phase to the high-temperature normal phase. The modulated structures of the Co-analogue (Ca₂CoSi₂O₇) was determined by us³⁾ based on the five-dimensional description. The results revealed that the modulation is caused by the shifts of the constituent atoms.

The specimen of the single crystal was synthesized by one of the authors (K.I.). Temperature dependence of intensities and that of the cell parameters were examined in situ to observe the change of the structure. A fragment of the sample was mounted in a small gas blow heating system installed on an Enraf-Nonius CAD-4 diffractometer. MoK[[alpha]] radiation monochromatized by graphite was used for the measurements. The temperatures, varied from room temperature to 270[[ring]]C with arbitrary steps, were estimated by a thermocouple to regulating the heating system. The intensities were corrected for Lorentz and polarization effects.

The intensity of the satellites steeply weakens to background level at about 220[[ring]]C. In connection with the change, the c-axis contracts at the temperature range between 210[[ring]]C to 230[[ring]]C, while the a-axis expands and the volume of the cell is held almost constant. The structures at the temperature range were analyzed carefully to clarify those phenomena.

1) Hemingway et al. (1986). Can. Mineral. 24, 425-434.

2) Seifert et al. (1987). Phys. Chem. Minerals 14, 26-35.

3) Hagiya et al. (1993). Acta Cryst. B49, 172-179.