Vesuvianite from the Somma-Vesuvius Complex: New Data and Revised Formula

Abstract

At present, the vesuvianite group of minerals consists of eight members, six of which are distinguished by the dominant cation in the Y1(A,B) five-coordinated site. We investigated two vesuvianite samples from the type locality by electron microprobe analysis, Mössbauer and infrared spectroscopy, TGA/DSC, MAS NMR, single-crystal and powder X-ray diffraction. The crystal structures of these samples (# 27844 and 51062 from the Vesuvius collection, Fersman Mineralogical Museum, Moscow) have been refined to R1 = 0.027 and R1 = 0.035, respectively. Both samples have the space group P4/nnc; a = 15.5720(3) and 15.5459(3), c = 11.8158(5) and 11.7988(4), respectively. In both samples low-occupied T1 and T2 sites are populated by minor B and Al, which agrees with their high-temperature origin. According to our experimental results, the general revised crystal-chemical formula of vesuvianite can be written as VII−IXX19VY1VIY12(Z2O7)4(ZO4)10(W)10, where X are seven- to nine-coordinated sites of Ca with minor Na, K, Fe2+ and REE impurities; VY has a square pyramidal coordination and is occupied predominantly by Fe3+ with subordinate Mg, Al, Fe2+ and Cu2+; VIY has octahedral coordination and is predominantly occupied by Al with subordinate Mg, Fe2+, Fe3+, Mn2+, Mn3+, Ti, Cr and Zn; ZO4 = SiO4, sometimes with subordinate AlO4 and/or (OH)4, and W = OH, F, with minor O and Cl. The idealized charge-balanced formula of the vesuvianite end-member without subordinate cations is Ca19Fe3+(Al10Me2+2)(Si2O7)4(SiO4)10O(OH)9, where Ме = Fe2+, Mg2+, Mn2+.