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

Abstract

Tsepinite- Ca, ideally (Ca,K,Na, h )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), fluorapo- phyllite, leucophanite, chabazite- Ca. The new mineral is late hydrothermal and oc- curs in cavities after leaching of rinkite, the probable source of Ti, Nb and Ca. Tse- pinite- 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/cm 3 . Biaxial (+), 2Vmeas. = 30 ± 10ƒ, 2Vcalc. = 36ƒ; a = 1.666(2), b = 1.676(2), g = 1.780(4). Dispersion is very weak: r < n. Non-pleochroic; the orientation is Y = b. The strongest IR bands (cm -1 ) are at 3260, 1103, 946, 673, 455. Electron microprobe analysis (H 2O by TGA) gave: Na 2O