Electric-field gradient and mean-squared-displacement tensors in hedenbergite from single-crystal Mossbauer milliprobe measurements

We report Mossbauer milliprobe measurements on small single-crystals of a magnesium-rich hedenbergite, approximate composition CaFe0.54Mg0.46 (SiO3)(2), in which each of the electric-field gradient and mean-squared displacement tensors for Fe2+ in the M1 site of the crystal are precisely determined. Each tensor has in common, as required of crystal symmetry, the twofold axis of the monoclinic unit cell, but the principal directions of the two tensors in the perpendicular plane art: non-coincident. The mean-squared displacements determined in the Mossbauer experiment exceed those determined from the X-ray vibration ellipsoids for Fe2+/M1 by a factor of 1.6; the anisotropy in the mean-squared displacement tensor from the Mossbauer measurements exceeds that from X-ray by a factor of around 5. The ramifications of these differences are discussed.