Eliseevite, Na1.5Li[Ti2Si4O12.5(OH)1.5]·2H2O, is a new microporous titanosilicate of the lintisite-kukisvumite family [monoclinic, C2/c, a = 27.48(1), b = 8.669(4), c = 5.246(2) Å, β = 90.782(8)°, V = 1249.7(9) Å3, Z = 4]. The mineral is found in two different peralkaline veins in an ijolite–foyaite–malignite differentiated complex of the Lovozero alkaline massif, Kola Peninsula, Russia. At Mt. Alluaiv, eliseevite occurs in an aegirine-eudialyte-sodalite-microcline vein as long-prismatic to fibrous crystals (up to 2 mm long) growing in voids of natrolitized sodalite in close association with albite, analcime, catapleiite, chabazite-Ca, gmelinite-K, manganoneptunite, microcline, murmanite, and an ussingite. At Mt. Punkaruaiv, it is found within a ussingite-aegirine-microcline vein as long-prismatic crystals (up to 0.8 mm long) in close association with chabazite-Ca, chkalovite, eudialyte, manganoneptunite, punkaruaivite, rhabdophane-(Ce), sodalite, sphalerite, and steenstrupine-(Ce). It is a late-stage, hydrothermal mineral formed as a result of alteration of murmanite. The mineral is transparent, pale creamy to colorless, with a vitreous luster and a white streak. Cleavage is perfect along {100}, fracture is splintery. Mohs hardness is about 5. In transmitted light, the mineral is colorless, biaxial (−): α = 1.665(2), β = 1.712(2), γ = 1.762(5) (for λ = 589 nm); Y = b, Z^a = 8–12°. Dispersion is medium, r < v. Dcalc = 2.706 g/cm3, Dmeas = 2.68(4) g/cm3. The mean chemical composition (n = 7) determined by the Penfield method (water), ICP-MS (Li), and electron microprobe (other elements) is (wt%): H2O 10.50, Li2O 2.85, Na2O 9.15, K2O 0.08, CaO 0.05, Fe2O3 0.21, Al2O3 0.08, SiO2 46.87, TiO2 29.40, Nb2O5 0.72, total 99.91. The empirical formula calculated on the basis of Si = 4 apfu is: (Na1.51K0.01Ca0.01)∑1.53Li0.98[(Ti1.89Nb0.03Fe3+0.01Al0.01)∑1.94Si4O12.26(OH)1.74]·2.12H2O. The simplified formula taking into account the results of a single-crystal study is Na1.5Li{Ti2O2[Si4O10.5(OH)1.5]}·2H2O. The six strongest reflections in the X-ray powder-diffraction pattern [d in Å, (I), (hkl)] are: 13.76(100)(200), 6.296(60)(310), 3.577(80)(710), 3.005(70)(421), 2.881(70)(910), 2.710(50)(621̅). The mineral is named in honor of Nikolai Aleksandrovich Eliseev (1897–1966), a remarkable Russian geologist and petrologist, Professor at Leningrad State University, for his contributions to the geology and petrology of metamorphic and alkaline complexes.

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