Eliseevite, Na1.5Li[Ti2Si4O12.5(OH)1.5]{middle dot}2H2O, a new microporous titanosilicate from the Lovozero alkaline massif (Kola Peninsula, Russia)

Abstract

Eliseevite, Na 1.5 Li[Ti 2 Si 4 O 12.5 (OH) 1.5 ]∙2H 2 O, is a new microporous titanosilicate of the lintisitekukisvumite 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) Å 3 , 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 longprismatic 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. D calc = 2.706 g/cm 3 , D meas = 2.68(4) g/cm 3 . The mean chemical composition (n = 7) determined by the Penfield method (water), ICP-MS (Li), and electron microprobe (other elements) is (wt%): H 2 O 10.50, Li 2 O 2.85, Na 2 O 9.15, K 2 O 0.08, CaO 0.05, Fe 2 O 3 0.21, Al 2 O 3 0.08, SiO 2 46.87, TiO 2 29.40, Nb 2 O 5 0.72, total 99.91. The empirical formula calculated on the basis of Si = 4 apfu is: (Na 1.51 K 0.01 Ca 0.01 ) Σ1.53 Li 0.98 [(Ti 1.89 Nb 0.03 Fe 3+ 0.01 Al 0.01 ) Σ1.94 Si 4 O 12.26 (OH) 1.74 ]∙2.12H 2 O. The simplified formula taking into account the results of a single-crystal study is Na 1.5 Li{Ti 2 O 2 [Si 4 O 10.5 (OH) 1.5] }∙2H 2 O. 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)(621). 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.