Si-DEFICIENT, OH-SUBSTITUTED, BORON-BEARING VESUVIANITE FROM THE WILUY RIVER, YAKUTIA, RUSSIA

A low-temperature, Si-deficient variety of vesuvianite occurs in porous tetrahedral “achtarandite” pseudomorphs consisting of hibschite, along the banks of the Wiluy River, Yakutia, Russia, the type locality of grossular and wiluite. The (H4O4)-for(SiO4) hydrogarnet-type substitution is evident in the vesuvianite, a substitution that allows it to be considered an analogue of hibschite. This variety of vesuvianite belongs to a new series in the vesuvianite group, as expressed by the formula X19Y13T0–5(Si2O7)4(SiO4)10–x(OH)4xW10. The filling of the X, Y, and T positions in this Si-deficient vesuvianite, where x varies from 0.67 to 2.89, is analogous to that in vesuvianite and wiluite. The Si-deficient vesuvianite is characterized by increased unitcell parameters, a 15.688(3), c 11.860(3) Å and by lower indices of refraction, 1.691(1), 1.668(1). In the OH-region, the FTIR and Raman spectra differ sharply from those of low-temperature vesuvianite from rodingites, but are similar to the spectra of hibschite. A line near 3620 cm–1 indicates that the substitution occurs only in the isolated tetrahedra. More than 25% of these can be substituted by (H4O4).