THE CHEMISTRY OF VESUVIANITE

Abstract

The crystal chemistry and chemical variations in 76 vesuvianite samples from 54 different localities have been examined using electron-microprobe analyses i supple­ mented by analysis of selected samples for Fe.+ IFe 3 +, Li, Be and H20. Results of analyses were reduced to formula units on the basis of 50 cations excluding B. A general formula for boron-free vesuvianite can be written as XI9Y13ZIS068WIO, where X are cations occupying [8]-coordinate sites, Yare cations occupying [6]- and [5]-coordinate sites, Z are cations occupying [4]-coor­ dinate sites, and Ware monovalent and divalent anions. For 388 boron-free analyses, the mean Si content is 18.0(2) a.p.f.u. (atoms per formula unit), with skew and kurtosis of -0.03 and 0.32, respectively, indicating that only Si occurs at the Z-group sites. The mean (Ca + Na + Ln + Pb + Bi + Th) content is 19.0(3) a.p.f.u., with skew and kurtosis of -0.38 and 0.22, respectively. This indicates that only these cations occupy the X-group sites. The mean (AI + Mg + Fe + Ti + Cr + Mn + Cu + Zn) content is 13.0(2) a.p.f.u., with skew and kurtosis of 0.64 and 0.87, respectively; this indicates that only these cations occupy the Y-group sites. In addition, the individual means for each group of sites support a 50-cation scheme of renormalization. This procedure is recommended for boron-free vesuvianite. The principal cationic substitutions in boron-free vesuvianite occur at the Y-group sites, and involve homovalent )e.g., Mg .. Fe2+ , AI .. Fe3+) and heterovalent (e.g., AI .. Ti, Mg