CHIVRUAIITE, CA4(TI,NB)5[(SI6 O17)2[(OH,O)5]·13-14 H2O, A NEW MINERAL FROM HYDROTHERMAL VEINS OF KHIBINY AND LOVOZERO ALKALINE MASSIFS

Abstract

Chivruaiite is a new Ca titanosilicate [orthorhombic, Cmmm, a = 7.17(2), b = 22.98(9), c = 6.94(2) Å, V = 1144.4 Å3, Z = 1], chemically and structurally related to zorite. The mineral is found in three different hydrothermal veins within the Khibiny and Lovozero alkaline massifs, Kola Peninsula, Russia. It is associated with microcline, eudialyte, natrolite, astrophyllite, aegirine, etc. Chivruaiite occurs as elongate-prismatic crystals (up to 3 mm long) with {100}, {010}, {001}, {101}, and {011} as dominant faces, as well as radiating aggregates. The mineral is transparent, pale-pink to colorless, with vitreous luster and white streak. Cleavage is distinct on {100} and {010}; fracture is step-like. Mohs hardness is about 3. In transmitted light, the mineral is pale-pink, with a faint pleochroism: Z= pale-pink, on X and Y = colorless; dispersion r < v. Chivruaiite is biaxial (+): α = 1.705(5), β = 1.627(2), γ = 1.612(2) (for λ = 589 nm), 2Vmens.= 40 ± 5°, 2Vcalc = 31.7°. Optical orientation: X = b, Y = a, Z = c, Dcalc, = 2.42 g/cm3, Dmeas = 2.40-2.42 g/cm3. The mean chemical composition determined by electron microprobe is (wt%): SiO2 45.14; TiO2 20.63; Al2O3 0.07; Fe2O3 0.18; MnO 0.02; MgO 0.01; CaO 10.53; Na2O 0.10; K2O 1.30; SrO 0.28; Nb2O5 3.63; H2O 17.30; sum. 99.19. Empirical formula calculated on the basis of Si = 12 is (Ca3.00K0.44Na0.05 Sr0.04Mn0.01)∑=3.54 (Ti4.13Nb0.44Fe0.043+ Al0.02)∑=4.63[Si12O34 I(OH)4.51O0.49]·13.08H2O. Simplified formula is Ca4(Ti,Nb)5[(Si6 O17)2 (OH,O)5]·13-14 H2O. The strongest X-ray powder-diffraction lines [d in Å, (I), (hkl)] are 11.6 (100) (020), 6.91 (90) (110, 001), 5.23 (50) (130), 3.41 (50) (220), 3.35 (50) (061, 151), 3.04 (80) (221, 240). The structure of chivruaiite was refined to R1 = 0.038 on the basis of 687 unique observed reflections. It is based upon an open framework of SiO4 tetrahedra, TiO6 octahedra, and TiO5 pyramids. Framework cavities are occupied by Ca2+ and K+ cations, and H2O molecules. The mineral is named after its type locality in the Chivruai River valley (the Lovozero massif, Kola Peninsula, Russia). Chivruaiite is a Ca-analog of zorite and ETS-4 and is closely related to haineaultite.