The crystal structure of Si-deficient, OH-substituted, boron-bearing vesuvianite from the Wiluy River, Sakha-Yakutia, Russia

The crystal structure of a Si-deficient vesuvianite (a 15.678, c 11.828 Aa, space group P4/nnc) from the Wiluy River has been refined to R 0.037. EPMA data show that this vesuvianite has only 16 Si pfu, in contrast to regular vesuvianite which has 18 Si pfu. Site-occupancy refinement showed substantial vacancies at orthosilicate sites. Such vacancies and increased Z(1)-O and Z(2)O bond lengths are consistent with hydrogarnet-type defects, where SiO (sub 4) is replaced by H (sub 4) O (sub 4) tetrahedra. The single crystal investigated shows the highest hydrogarnet-type substitution analysed by structure refinement of vesuvianite. No vacancies were found involving the disilicate group. Along the c axis the increased size of Z(1) tetrahedra is balanced by compression of the adjacent X(3) Ca-bearing dodecahedra. This structural flexibility explains why the c dimension remains largely independent of the hydrogarnet-type substitution. There is no corresponding structural flexibility along the a axis, leading to a systematic increases in a with increasing proportion of vacancies at the Si site.