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3.3 f. Evidence from Mössbauer spectroscopy for the incommensurate-normal phase transition in åkermanite (C.A. McCammon, J.D.C. McConnell, R.J. Angel and F. Seifert)

Åkermanite, Ca2(Fe,Mg)Si2O7, crystallises in the melilite structure. At high temperature it exhibits a normal (N) structure with space group P-421m, which transforms reversibly to an incommensurate (IC) structure with decreasing temperature. Theoretical analysis combined with results from X-ray diffraction has indicated that there are two component structures within the modulation with symmetries P-4 and P21212 (Annual Report 1997). Mössbauer spectroscopy is able to resolve the two components as separate subspectra; hence we have used it to study the IC phase and the IC-N transition.

The Mössbauer spectrum of the N phase consists of a single quadrupole doublet, which changes to two quadrupole doublets below the IC-N transition. To observe the transition as a function of temperature and pressure, we recorded Mössbauer spectra of a synthetic sample of Ca2Mg0.9757Fe0.03Si2O7 at (1) temperatures from 4.2 to 425 K at atmospheric pressure; and (2) pressures from 0 to 3.4 GPa at room temperature. The transition from the IC to the N phase occurs at 366 ± 3 K at 0 GPa, and at 1.8 ± 0.2 GPa at 293 K. Mössbauer spectroscopy at 4.2 K confirms that only two components of the IC structure modulation exist as predicted by theory. The variation of centre shift with temperature was fit to a Debye model, and combined with observations of the variation of quadrupole splitting with temperature, indicates that the effect of temperature on the distortion of the two sites is quite different. In contrast, pressure appears to have a more uniform effect. These results, coupled with information on site occupancies, can be correlated with structural details and theory to provide a comprehensive description of the IC phase and the IC-N transition in Fe-åkermanite.

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