AbstractFor 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$\bar{1}$ space group, a = 6.9699(7), b = 7.9953(10), c = 9.8227(10) Å, α = 105.186(9), β = 99.458(8) and γ = 114.489(10) to R₁ = 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 Å for the M site agrees with its refined occupancy (Ti_0.32Cu_0.30Ca_0.29Fe_0.09)_1.00. The incorporation of Ti into the litidionite structure is accompanied by the complex heteropolyhedral substitution according to the scheme ^VTi⁴⁺ + ^VII–VIII□ + ^IVAl³⁺ ↔ ^VCu²⁺ + ^VII-VIII(Na,K)⁺ + ^IVSi⁴⁺. Two possible configurations for the phase with maximal TiO₂ content (12.06 wt.% or 0.56 Ti apfu) CuTiK□Na₂Si₇AlO₂₀ (Z = 1) or CuTiK₂Na□Si₇AlO₂₀ (Z = 1) have been proposed.