Calciomurmanite, (Na, [])2 Ca(Ti, Mg, Nb)4 [Si2 O7 ]2 O2 (OH, O)2 (H2 O)4, a new mineral from the Lovozero and Khibiny alkaline complexes, Kola Peninsula, Russia

Abstract

The new mineral calciomurmanite, (Na,□)2Ca(Ti,Mg,Nb)4[Si2O7]2O2(OH,O)2(H2O)4, a Na-Ca ordered analogue of murmanite, was found in three localities at Kola Peninsula, Russia: at Mt. Flora in the Lovozero alkaline complex (the holotype) and at Mts. Eveslogchorr (the cotype) and Koashva, both in the Khibiny alkaline complex. Calciomurmanite is a hydrothermal mineral formed as a result of late-stage, low-temperature alteration (hydration combined with natural cation exchange) of a high-temperature, anhydrous phosphate-bearing titanosilicate, most likely lomonosovite and/or betalomonosovite, in the peralkaline (hyperagpaitic) rocks. The holotype sample is associated with microcline, aegirine, lorenzenite and fluorapatite, whereas the cotype sample occurs with microcline, aegirine, lamprophyllite, tsepinite-Ca and tsepinite-K. The mineral occurs as lamellae up to 0.1 × 0.4 × 0.6 cm, sometimes combined in fan-shaped aggregates up to 3.5 cm. Calciomurmanite is pale brownish or purple; the streak is white. The lustre is nacreous on cleavage surface and greasy on broken surface across cleavage. The (001) cleavage is perfect, mica-like; the fracture is stepped. D meas = 2.70(3), D calc = 2.85 g cm−3. The mineral is optically biaxial (–), α = 1.680(4), β = 1.728(4), γ = 1.743(4), 2 V meas = 58(5)°. The IR spectrum is reported. The chemical composition (wt%, electron-microprobe data, H2O by the Alimarin method) is: Na2O 5.39, K2O 0.30, CaO 7.61, MgO 2.54, MnO 2.65, FeO 1.93, Al2O3 0.85, SiO2 30.27, TiO2 29.69, Nb2O5 6.14, P2O5 0.27, H2O 11.59, total 99.23. The empirical formula of the holotype sample, calculated on the basis of Si + Al = 4 apfu , is: Na1.34Ca1.04K0.05Mg0.49Mn0.29Fe0.21Nb0.36Ti2.85(Si3.87Al0.13)Σ4O16.40(OH)1.60(PO4)0.03(H2O)4.94. Calciomurmanite is triclinic, P -1, a = 5.3470(6), b = 7.0774(7), c = 12.146(1) A, α = 91.827(4), β = 107.527(4), γ = 90.155(4)°, V = 438.03(8) A3 and Z = 1. The strongest reflections of the X-ray powder pattern [ d ,A( I )( hkl )] are: 11.69(100)(001), 5.87(68)(011, 002), 4.25(89) (-1-11, -111), 3.825(44)(-1-12, 003, -112), 2.940(100)(-1-21, -121), 2.900(79)(004, 120). The crystal structure was solved by direct methods from single-crystal low-temperature (200 K) X-ray diffraction data and refined to R = 0.0656 for the holotype and 0.0663 for the cotype. The structure is based on a three-sheet HOH block: an octahedral ( O ) sheet containing alternating chains of NaO6 and TiO6 octahedra and two heteropolyhedral ( H ) sheets consisting of Si2O7 groups, TiO6 octahedra and CaO8 polyhedra. H2O molecules occupy two sites in the interlayer space.