The complex mechanism of Ti4+ incorporation into litidionite from the Somma-Vesuvius volcano, Italy

For this study, the rare Cu-bearing silicate fumarolic assemblages from the Somma-Vesuvius volcano, Italy, characterised by the rare mineral litidionite, CuKNaSi4O10, were investigated. We report new data about Cu- and Ti-bearing phases found in these mineralisations, in which Ti-bearing litidionite occurs together with kamenevite, perovskite and rutile. Ti-bearing litidionite appears on the latest stages of partial crystallisation of Ti-bearing silica glass. Incorporation of Ti4+ into the litidionite crystal structure was investigated in detail. The Raman spectra of Ti-bearing litidionite contains an intense band at 597 cm(-1) related to anti-symmetric bending vibrations of Si-O bonds or overlapping stretching vibrations of Ti-O bonds. The bands in the range 350-500 cm(-1 )correspond to symmetric bending vibrations of O-Si-O bonds and overlapping stretching vibrations of Ti-O bonds. The crystal structure of Ti-litidionite has been refined in the P (1) over bar space group, a = 6.9699(7), b = 7.9953(10), c = 9.8227(10) angstrom, alpha = 105.186(9), beta = 99.458(8) and gamma = 114.489(10) to R-1 = 0.064 for 1726 unique observed reflections. The refinement of the site-occupation factors confirmed the presence of Ti at a five-coordinated M site. The mean bond distance of 2.125 angstrom for the M site agrees with its refined occupancy (Ti0.32Cu0.30Ca0.29Fe0.09)(1.00). The incorporation of Ti into the litidionite structure is accompanied by the complex heteropolyhedral substitution according to the scheme Ti-V(4+) + (VII-VIII)square + Al-IV(3+) <-> Cu-V(2+) + (VII-VIII)(Na,K)(+) + Si-IV(4+). Two possible configurations for the phase with maximal TiO2 content (12.06 wt.% or 0.56 Ti apfu) CuTiK square Na2Si7AlO20 (Z = 1) or CuTiK2Na square Si7AlO20 (Z = 1) have been proposed.