P4/n and P4nc long-range ordering in low-temperature vesuvianites

Low-temperature vesuvianites crystallize in the two space groups P4/n or P4 nc due to different arrangements of Ca-dodecahedra and M-2+,M-3+ square pyr amids that form strings along the fourfold axes. Long-range ordered vesuvia nites of acentric P4nc symmetry may have the same diffraction symmetry as c entrosymmetric disordered P4/nnc vesuvianites. In contrast, P4/n long-range ordered vesuvianites exhibit glide plane violating reflections and can eas ily be identified. We report the first successful Xray single-crystal struc ture refinement of a P4nc vesuvianite, a = 15.487(2), c = 11.764(2) Angstro m from the N'chwaning II mine of the Kalahari manganese fields (RSA). This untwinned crystal has Cu2+ and Mn2+, Mn3+ forming the square pyramid and ex hibits an acentric ordering pattern with 85% string A and 15% string B. Thi s is compared to a reinvestigation of the structure of a P4/n vesuvianite f rom Asbestos Quebec (Canada), a = 15.531(2), c = 11.817(2) Angstrom. The cr ystal is composed of a merohedral (110) twin with a close to 1:1 twin ratio and has mainly Fe3+ in square pyramidal coordination. In this centric stru cture, string A is 84% and string B is 16% occupied. Criteria to determine the symmetry of low-temperature vesuvianites, studied by diffraction experi ments, are discussed.