Diffuse reflections and the symmetry of vesuvianite

Vesuvianite is a common rock-forming silicate mineral with the general chem ical formula X(19)Y(13)Z(18)T(0-5)O(68)W(10), 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 physi cal properties that indicate deviations from this symmetry. Many vesuvianit e samples are optically biaxial; a sector-zoned crystal from Quebec shows { 100}, {010} and {001} sectors with low (0 degrees to 5 degrees), intermedia te (approximately 36 degrees) and high (approximately 60 degrees) 2V values , respectively. X-ray precession photographs of fragments from each of the sectors show that the number and intensity of glide-violating reflections i ncrease in the sequence {101} --> {001} --> {100}; in addition, deviations from 4/mmm Laue symmetry were also apparent. Diffuse streaking is associate d with the violating reflections. A combination of the optical and X-ray ev idence indicates that vesuvianite is monoclinic. However, attempts to refin e the crystal structure in monoclinic space groups have been inconclusive.