Diffuse reflections and the symmetry of vesuvianite

Vesuvianite is a common rock-forming silicate mineral with the general chemical formula X₁₉Y₁₃Z₁₈T_0.5O₆₈W₁₀ , where X = Ca, Na, Ln, Pb, Sb; Y= Al, Mg, Fe, Ti, Mn, Cu, Zn; Z = Si; T= B; W= (O, OH, F). Vesuvianite is ideally tetragonal with space group P4/nnc, but many samples show physical properties that indicate deviations from this symmetry. Many vesuvianite samples are optically biaxial; a sector-zoned crystal from Quebec shows {100}, {010} and {001} sectors with low (0° to 5°), intermediate (approximately 36°) and high (approximately 60°) 2V values, respectively. X-ray precession photographs of fragments from each of the sectors show that the number and intensity of glide-violating reflections increase in the sequence {101} →{001} →{100}; in addition, deviations from 4/mmm Laue symmetry were also apparent. Diffuse streaking is associated with the violating reflections. A combination of the optical and X-ray evidence indicates that vesuvianite is monoclinic. However, attempts to refine the crystal structure in monoclinic space groups have been inconclusive.