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3.3 j Electric-field gradient and mean-squared-displacement tensors in hedenbergite from single crystal Mössbauer measurements (C.A. McCammon and R. Miletich, in collaboration with C. Tennant/Christchurch)

Pyroxene phases constitute a significant portion of the upper mantle, and can provide useful information on oxygen fugacity through determination of relative Fe3+ contents. The Mössbauer milliprobe, developed at Bayerisches Geoinstitut, enables the study of microscopic absorbers with diameters of the order of 100 µm. Previous studies using the technique include measurements on individual grains of polished rock sections and single crystal garnet inclusions in diamonds. In the latter work, component areas of Mössbauer quadrupole doublets were observed to be unequal, which in cubic garnet can normally only arise from anisotropy in the absorber recoilless fractions. This quantity, expressed as the mean-squared-displacement (msd), can be obtained from intenstity measurements from single crystal Mössbauer studies. Simultaneous refinement of angular dependant intensity ratios, related to the electric-field-gradient (efg) tensor, and total intensities, related to the msd tensor, allow unambiguous determination of both tensors. Previously this problem has been regarded as indeterminate in some low symmetry Mössbauer sites. The present work is a first step in addressing many of the above problems.

Single crystals of a natural hedenbergite were cut and polished into thin sections (actual thickness 65 µm, with nominal total iron content 5 mg cm-2) in two different orientations. These were mounted at sequences of 90°C, 75°C and 60°C relative to the -ray direction on a Mössbauer spectrometer, where room temperature spectra were collected over sample areas of 500 µm diameter. Background-corrected dimensionless areas were determined from the spectra, which were then corrected for thickness and and polarisation. The data were refined by a matrix-diagonalisation least squares programme which returns the principal values and directions of the efg and msd tensors in the crystal system.

Both tensors for Fe2+ in the M1 site of hedenbergite are precisely determined. Each has in common the 2-fold axis of the monoclinic unit cell as required by crystal symmetry, but the principal directions in the perpendicular plane are non-coincident. The msd's determined in the Mössbauer experiment exceed those determined from X-ray vibration ellipsoids for Fe2+/M1 by factors of around 3. This indicates that X-ray determined msd's cannot in this instance be used as an approximation in analysing Mössbauer data.

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