Kamenevite, K₂TiSi₃O₉·H₂O, a new mineral with microporous titanosilicate framework from the Khibiny alkaline complex, Kola peninsula, Russia

The new zeolite-like titanosilicate mineral kamenevite K₂TiSi₃O₉·H₂O, isostructural with umbite K₂ZrSi₃O₉·H₂O, is found in two peralkaline pegmatites located at Mt. Suoluaiv (Oleniy Ruchey mine: holotype) and Mt. Rasvumchorr (Rasvumchorr mine) in the Khibiny alkaline complex, Kola peninsula, Russia. It is a hydrothermal mineral associated with lomonosovite, aegirine, villiaumite, pectolite, shafranovskite, ershovite, umbite, potassic feldspar, sodalite, lamprophyllite, etc. Kamenevite occurs as lamellar crystals up to 0.02 × 0.1 × 0.3 mm and their aggregates up to 0.7 mm (Oleniy Ruchey) or grains up to 0.15 mm across (Rasvumchorr). The mineral is transparent, colourless (white in aggregates), with vitreous lustre. It is brittle, the Mohs hardness is ca. 4. Cleavage is (010) good. D_meas = 2.69(2), D_calc = 2.698 g cm⁻³. Kamenevite is optically biaxial (–), α = 1.650(4), β = 1.678(5), γ = 1.685(5), 2V_meas = 60(10)°. The chemical composition of the holotype [wt%, electron microprobe data, H₂O content calculated for 1 (H₂O) pfu] is: Na₂O = 0.48, K₂O = 24.37, CaO = 0.13, Fe₂O₃ = 0.35, SiO₂ = 48.78, TiO₂ = 20.30, ZrO₂ = 0.89, Nb₂O₅ = 0.35, H₂O = 4.85, total = 100.50. The empirical formula calculated on the basis of 10 O atoms per formula unit (apfu) is (K_1.92Na_0.06Ca_0.01)_Σ1.99(Ti_0.94Zr_0.03Fe_0.02Nb_0.01)_Σ1.00S_3.01O₉·H₂O. Kamenevite is orthorhombic, P2₁2₁2₁, a = 9.9166(4), b = 12.9561(5), c = 7.1374(3) Å, V = 917.02(6) Å³ and Z = 4. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 7.92(70)(110), 6.51(47)(020), 5.823(95)(101), 2.988(84)(301, 122), 2.954(100)(041, 320), 2.906(68)(311, 202) and 2.834(69)(141, 212). The crystal structure (single-crystal XRD data, R = 0.0384) is based on a microporous heteropolyhedral framework built by [Si₃O₉]^∞ wollastonite-type chains linked by isolated Ti-centred octahedra. The K⁺ cations and H₂O molecules are located in wide and narrower [001] channels. The mineral is named in honour of the outstanding Russian geologist Evgeniy Arsenievich Kamenev (1934–2017) who made great contribution to the geology of the Khibiny complex.