Rusinovite, Ca10(Si2O7)3Cl2, was discovered in an altered carbonate-silicate xenolith enclosed in ignimbrites of the Upper Chegem volcanic caldera. The mineral is named after Vladimir Leonidovich Rusinov (1935–2007), a Russian petrologist and expert in the field of thermodynamics of non-equilibrium mineral systems. A synthetic analogue of rusinovite is also known. The new mineral has an OD structure of which only the average structure could be determined based on strong and sharp reflections recorded by single-crystal X-ray diffraction: space group Cmcm, a = 3.7617(2), b = 16.9385(8), c = 17.3196(9) Å, V = 1103.56(10) Å3, Z = 2. The average structure (R1 = 3.18 %) is characterized by columns of face-sharing disilicate units extending parallel to a. However, in the true structure only each second (Si2O7) unit is occupied. Although rusinovite has a stoichiometry similar to the apatite-group mineral nasonite, Pb6Ca4 (Si2O7)3Cl2, the two structures are considerably different. Rusinovite has following optical properties: α = 1.645(2), β = 1.664(2), γ = 1.675(3); Δ = 0.030, 2Vmeas = −75(10) °; 2Vcalc = −74.6 °; the Mohs hardness is 4–5, the density is 2.91 g/cm3. The mineral forms fibrous crystals often intergrown into spherolites and displays good cleavage parallel to (010). The Raman spectrum of rusinovite strongly resembles that of another skarn calcium-disilicate: rankinite, Ca3Si2O7.