PUNKARUAIVITE, LiTi2[Si4O11(OH)](OH)2{middle dot}H2O, A NEW MINERAL SPECIES FROM HYDROTHERMAL ASSEMBLAGES, KHIBINY AND LOVOZERO ALKALINE MASSIFS, KOLA PENINSULA, RUSSIA

Abstract

Punkaruaivite, ideally LiTi 2 [Si 4 O 11 (OH)](OH) 2 ·H 2 O, is a new species of Li titanosilicate [monoclinic, C 2/ c, a 26.688(2), b 8.7568(7), c 5.2188(5) A, β 91.189(2)°, V 1219.4(2) A 3 , Z = 4], chemically and structurally related to lintisite. The mineral is found in two different hydrothermal veins, within the Khibiny and Lovozero alkaline massifs, Kola Peninsula, Russia. In the Lovozero massif, it occurs within an ussingite – aegirine – microcline vein in nepheline syenite at Mt. Punkaruaiv in association with anglesite, belovite-(Ce), chkalovite, epididymite, epistolite, eudialyte, ferronordite-(Ce), karnasurtite-(Ce), lamprophyllite, lorenzenite, manganoneptunite, manganonordite-(Ce), monazite-(Ce), murmanite, natrolite, pectolite, rhabdophane-(Ce), steen-strupine-( Ce), tainiolite, tugtupite and vitusite-(Ce). In the Khibiny massif, it is found in a natrolite–microcline vein in foyaite at Mt. Eveslogchorr in association with aegirine, albite, belovite-(La), chivruaiite, epididymite, kuzmenkoite-Mn, lamprophyllite, monazite-(La), murmanite, palygorskite, pectolite, safflorite, sphalerite and tugtupite. In both veins, it forms small (up to 3 mm long) tabular elongate crystals, which grew on the walls of voids within leached ussingite (Lovozero massif) or natrolite (Khibiny massif). The mineral is transparent, yellowish brown to colorless, with a vitreous luster and a white streak. It has a perfect cleavage on {100}, and the pattern of fractures is step-like. The Mohs hardness is 4–5. In transmitted light, the mineral is brownish yellow, with a faint pleochroism: Y light brownish yellow, X brownish yellow; no dispersion is observed. Punkaruaivite is biaxial (−), α 1.658(2), β 1.696(2), γ 1.726(5) (for λ = 589 nm), 2 V meas 85(5)°, 2 V calc 82°. The optical orientation is X = b , Y Λ c = 12°. D calc = 2.55 g.cm −3, D meas = 2.60(5) g.cm −3 . The mean chemical composition determined with an electron microprobe is (wt.%): Li 2 O 3.22 (flame photometry), Na 2 O 0.29, K 2 O 0.14, CaO 0.01, MnO 0.31, FeO 0.21, Al 2 O 3 0.05, SiO 2 51.35, TiO 2 32.50, Nb 2 O 5 1.06, H 2 O 10.50 (Penfield method), total 99.64 wt.%. The empirical formula, calculated on the basis of Si + Ti + Nb + Fe + Mn + Al = 6 apfu and taking into account results of a single-crystal study, is (Li 1.02 Na 0.04 K 0.01 ) ∑1.07 {(Ti 1.92 Nb 0.04 Mn 0.02 Fe 3+ 0.01 ) ∑1.99 (OH) 2.00 [Si 4.03 O 11.03 (OH) 0.97 ]} ·1.26H 2 O. The simplified formula is LiTi 2 [Si 4 O 11 (OH)](OH) 2 ·H 2 O. The strongest six reflections in the powder-diffraction pattern [ d in A(I)( hkl )] are: 13.3(100)(200), 6.23(80)(310), 4.38(60)(020), 3.50(80)(710), 3.01(70)(42) and 2.81(70)(910). The structure of punkaruaivite was refined to R 1 = 0.084 on the basis of 1347 unique observed reflections. It is similar to the structures of vinogradovite, lintisite, and kukisvumite, and based upon a 3D framework consisting of parallel chains of corner-sharing SiO 4 tetrahedra linked into a framework by chains of corner-sharing LiO 4 tetrahedra and edge-sharing TiO 6 octahedra. The framework contains large channels occupied by H 2 O molecules. The mineral is named after its type locality at Mt. Punkaruaiv, in the Lovozero massif, Kola Peninsula, Russia. Punkaruaivite is a Na-free analogue of lintisite and related to it according to the substitution mechanism Na + + O 2− ↔ □ + (OH) − .