THE SYMMETRY OF VESUVIANITE

The vesuvianite structure has ideal symmetry P4/nnc, but many vesuvian ite samples show physical properties that indicate deviations from thi s symmetry. Here we examine 76 samples of vesuvianite from approximate ly 50 different localities, and focus on their physical and chemical p roperties that are affected by symmetry. Three groups are recognized, on the basis of optical properties: (i) normal crystals of vesuvianite show uniform extinction and small (0-5 degrees) 2V; (ii) blocky cryst als of vesuvianite show irregularly shaped areas of variable birefring ence in a (001) section, with 2V values in the range 5-35 degrees; (ii i) sector-zoned crystals of vesuvianite show {001}, {101} and {100} se ctors with low (-5 degrees), intermediate (20-35 degrees) and high (40 -60 degrees) values of 2V, respectively; some crystals may be more com plex, with {110} zones. X-ray precession photographs of fragments from each of the sectors show the number and intensity of the ''glide-viol ating'' reflections to increase in the sequence {101} --> {001} --> {1 00}; in addition, deviations from 4/mmm Laue symmetry also were appare nt. Diffuse streaking is associated with the ''violating'' reflections . Possible derivative space-groups can be derived from the P4/nnc pare nt group using group-reduction techniques. This shows that several spa ce groups previously suggested are not possible. In addition, several of the derivative space-groups can be eliminated, as the observed opti c orientations are not compatible with these symmetries. A combination of optical and X-ray-diffraction evidence indicates that the symmetry of vesuvianite is P2/n (or Pn). We suggest that there is a continuous (or near-continuous) ferroelastic phase transition between a high-tem perature P4/nnc structure and a low-temperature P2/n or Pn structure. Differential order of cations over pairs of sites in the channels of t he structure cannot drive the transition to P2/n symmetry, as the asso ciated order-parameter does not transform as the active irreducible re presentation of this transition. The variety of optical types of vesuv ianite are a result of different relationships between the temperature interval of crystallization and the temperature of the phase transiti on.