Original paper

Tsepinite-Ca, (Ca,K,Na,)2(Ti,Nb)2(Si4O12) (OH,O)24H2O, a new mineral of the labuntsovite group from the Khibiny alkaline massif, Kola Peninsula - Novel disordered sites in the vuoriyarvite-type structure

Pekov, Igor V.; Chukanov, Nikita V.; Ferraris, Giovanni; Gula, Angela; Pushcharovsky, Dmitry Y.u.; Zadov, Aleksandr E.


Tsepinite-Ca, ideally (Ca,K,Na,□)2(Ti,Nb)2(Si4O12)(OH,O)2·4H2O (Z = 4), is a new mineral species of the labuntsovite group (vuoriyarvite subgroup) found in the abandoned Lovchorrite mine, Hackman Valley, Yukspor Mountain, Khibiny massif, Kola Peninsula, Russia. It occurs in the hydrothermally altered zone of a large aegirine-nepheline-potassic feldspar pegmatite and is associated mainly with microcline, aegirine, natrolite, kentbrooksite, kupletskite, Mn-rich lamprophyllite, fluorapatite, catapleiite, ancylite-(Ce), ancylite-(La), fluorapophyllite, leucophanite, chabazite-Ca. The new mineral is late hydrothermal and occurs in cavities after leaching of rinkite, the probable source of Ti, Nb and Ca. Tsepinite-Ca forms coarse long-prismatic to needle-shaped and usually split crystals which are transparent, colourless to white and pale brownish, up to 0.2×0.6×5mm in size and form aggregates up to 1cm in size. The streak is white and the lustre is vitreous. Tsepinite-Ca is brittle, shows uneven fracture and no cleavage. Its Mohs' hardness is 5; Dmeas. = 2.73(1), Dcalc. = 2.72g/cm3. Biaxial (+), 2Vmeas.= 30 ± 10°, 2Vcalc. = 36°; α = 1.666(2), β = 1.676(2), γ = 1.780(4). Dispersion is very weak: r < ν. non-pleochroic; the orientation is y = b. The strongest IR bands (cm-1) are at 3260, 1103, 946, 673, 455. Electron microprobe analysis (H2O by TGA) gave: Na2O 1.32, K2O 2.23, CaO 5.29, SrO 3.01, BaO 3.52, MnO 0.16, FeO 0.04, ZnO 0.12, Al2O3 0.04, SiO2 41.06, TiO2 21.95, Nb2O5 9.08, H2O 13.30, total 101.12wt.%. The empirical formula, based on Si + Al = 8 and OH/O ratio from charge balance (Z = 2), is (Ca1.10K0.55Na0.50Sr0.34Ba0.27Mn0.03Zn0.02Fe0.01)Σ2.82(Ti3.21Nb0.80)Σ4.01(Si7.99Al0.01)Σ8O24[(OH)2.58O1.42]Σ4·7.35H2O. The strongest lines of the X-ray powder pattern [d in Å (I) (hkl)] are: 7.02(60)(020, 001), 6.38(40)(20-1, 200), 3.53(45) (131); 3.16(100)(041, 022), 2.62(45)(20-3, 202), 2.51(85)(44-1, 40-3, 042), 1.718 (50)(46-3, 461, 44-4, 442, 081). X-ray single crystal diffraction showed tsepinite-Ca is monoclinic, space group C2/m, with a = 14.484(4), b = 14.191(4), c = 7.907(2)Å, β = 117.26(2)°, V = 1445(1)Å3. The structure, refined to R 0.11 for 1559 observed reflections, is based on a framework consisting of corrugated chains of [(Ti,Nb)(O, OH)] octahedra connected by (Si4O12) rings. The larger cations are dominated by Ca and, together with the H2O molecules, are distributed, within zeolite-like cages, on several split sites, including a C′ site which is novel for this type of structure. According to the chemical analysis and the refined structure, the crystal-chemical formula is (Z = 1): {(Na1.40K1.11(H2O)0.351.14)Σ4[Ba0.35(H2O)3.65]Σ4} {[Ca0.52Sr0.40(H2O)2.600.48]Σ4[Ba0.19(H2O)3.81]Σ4}{[Sr0.28(H2O)3,72]Σ4[Ca0.843.16]Σ4} (Ca0.85Mn0.05Fe0.01Zn0.03tsepinite-ca • new mineral specieslabuntsovite groupvuoriyarvitesubgroupzeolite-like crystal structurestructural disorderkhibiny alkaline massifkola