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
Lykova, Inna S.; Pekov, Igor V.; Chukanov, Nikita V.; Belakovskiy, Dmitry I.; Yapaskurt, Vasiliy O.; Zubkova, Natalia V.; Britvin, Sergey N.; Giester, Gerald
European Journal of Mineralogy Volume 28 Number 4 (2016), p. 835 - 845
published: Nov 1, 2016
ArtNo. ESP147052804009, Price: 29.00 €
The new mineral calciomurmanite, (Na,)2 Ca(Ti, Mg, Nb)4 [Si2 O7]2 O2 (OH, O)2 (H2 O)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. Dmeas = 2.70(3), Dcalc = 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)º. The IR spectrum is reported. The chemical composition (wt%, electron-microprobe data, H2 O by the Alimarin method) is: Na2 O 5.39, K2 O 0.30, CaO 7.61, MgO 2.54, MnO 2.65, FeO 1.93, Al2 O3 0.85, SiO2 30.27, TiO2 29.69, Nb2 O5 6.14, P2 O5 0.27, H2 O 11.59, total 99.23. The empirical formula of the holotype sample, calculated on the basis of Si + Al = 4 apfu, is: Na1.34 Ca1.04 K0.05 Mg0.49 Mn0.29 Fe0.21 Nb0.36 Ti2.85 (Si3.87 Al0.13)Σ4 O16.40 (OH)1.60 (PO4)0.03 (H2 O)4.94. Calciomurmanite is triclinic, P -1, a = 5.3470(6), b = 7.0774(7), c = 12.146(1) Å, α = 91.827(4), β = 107.527(4), γ = 90.155(4)º, V = 438.03(8) Å3 and Z = 1. The strongest reflections of the X-ray powder pattern [d, Å(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 Si2 O7 groups, TiO6 octahedra and CaO8 polyhedra. H2 O molecules occupy two sites in the interlayer space.