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

Abstract

For this study, the rare Cu-bearing silicate fumarolic assemblages from the Somma-Vesuvius volcano (Italy), characterized by the rare mineral litidionite, CuKNaSi 4 O 10 , were investigated.Recently other Cu-and Ti-bearing phases were found in these associations and here we report new data about these mineralizations, in which Ti-bearing litidionite occurs together with kamenevite, perovskite and rutile. Ti-bearing litidionite appears on the latest stages of partial crystallization of Ti-bearing silica glass. Incorporation of Ti 4+ into litidionite crystal structure was investigated in detail. The Raman spectra of Ti-bearing litidionite contain intense band at 597 cm −1 related to antisymmetric 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 ̅ space group, a = 6.9699(7), b = 7.9953(10), c = 9.8227(10) Å, α = 105.186(9), β = 99.458(8), γ = 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 five-coordinated M-site.The mean bond distance of 2.125 Å of M-site agrees with its refined occupancy (Ti 0.32 Cu 0.30 Ca 0.29 Fe 0.09 ) 1.00 . The incorporation of Ti into litidionite structure is accompanied by the complex heteropolyhedral substitution according to the scheme V Ti 4+ + VII-VIII □ + IV Al 3+ ↔ V Cu 2+ + VII-VIII (Na,K) + + IV Si 4+ . Two possible configurations for the phase with maximal TiO 2 content (12.06 wt% or 0.56 Ti apfu) CuTiK□Na 2 Si 7 AlO 20 (Z = 1) or CuTiK 2 Na□Si 7 AlO 20 (Z = 1) have been proposed.

A new mineral cuprodobrovolskyite Na4Cu(SO4)3 from the Tolbachik volcano (Kamchatka, Russia) and relationships in the family of natural anhydrous Na–Cu sulfates

“…It is noteworthy, that the recently IMA-approved mineral enricofrancoite (IMA No. 2023-002) with the ideal formula KNaCaSi 4 O 10 from the Somma-Vesuvius volcanic complex (Naples, Italy) is an analogue of litidionite KNaCuSi 4 O 10 with Ca instead of Cu (Balassone et al , 2023). This find is more indirect evidence that post-volcanic processes can contribute to Ca–Cu 2+ isomorphic substitution.…”