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 m agnesium-rich hedenbergite, approximate composition CaFe0.54Mg0.46 (SiO3)(2 ), in which each of the electric-field gradient and mean-squared displaceme nt 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 ax is of the monoclinic unit cell, but the principal directions of the two ten sors in the perpendicular plane art: non-coincident. The mean-squared displ acements determined in the Mossbauer experiment exceed those determined fro m the X-ray vibration ellipsoids for Fe2+/M1 by a factor of 1.6; the anisot ropy in the mean-squared displacement tensor from the Mossbauer measurement s exceeds that from X-ray by a factor of around 5. The ramifications of the se differences are discussed.