The crystal structure of a vesuvianite withP4/n symmetry

The crystal structure of green vesuvianite from Val d'Ala (Piemonte, Italy): Ca19Al4 (Al,Mg,Fe3+)8(Fe3+,Fe2+)(SiO4)10(Si2O7)4(OH,O)10,Z=2, ā=15.543(1),c=11.818(1) A, has been refined in the space groupP4/n. The finalR was 0.034 for the 2858 observed reflections, and 0.059 for all 4197 unique reflections. The lowering of the symmetry from that usual in vesuvianites is due to the ordering of two atoms (Ca and Fe), which in the space groupP4/nnc are alternately and statistically located in two sites with multiplicities double the number of such atoms in the unit cell. The possible occurrence of vesuvianites withP4nc symmetry is also discussed, in reference to an attempt to refine the crystal structure of the yellow vesuvianite from Canzocoli (Predazzo, Italy).