TSEPINITE-Na (Na, H_3O, K, Sr, Ba)_2(Ti, Nb)_2(Si_4O_(12))(OH,O)_2 CENTRE DOT 3H_2O, A NEW MINERAL OF THE LABUNTSOVITE GROUP

摘要

Tsepinite-Na, a new mineral of the labuntsovite group, has been found in alkaline pegmatites at Khibinpakhchorr Mt., Khibiny massif, and Lepkhe-Nelm Mt., Lovozero massif (both at Kola Peninsula. Russia). It occurs as colourless, white and light-brown radial aggregates and separate prismatic crystals (up to 1 cm) together with microcline, aegirine, analcime, natrolite, catapleiite, apophyllite, labuntsovite-Mn, epididymite, fluorite, sphalerite, etc. (Khibinpakhchorr) and with microcline, aegirine, magnesio-arfvedsonite, natrolite, eudialyte, lamprophyllite, fluorapatite, neptunite, lorenzenite, titanite, vinogradovite, polylithionite, tainiolite, rancieite, tundrite-(Ce) (Lepkhe-Nelm). Crystal forms: {001}, {100}, {2-bar 01}, {010}, {012}, {2-bar 41}. Transparent to translucent. Lustre vitreous. Brittle, cleavage is not observed, Mohs' hardness 5. D_(meas) = 2.74(2), D_(calc) = 2.72(1) g/cm~3. Optically biaxial, positive, #alpha# 1.658(1), #beta# 1.668(1), #gamma# 1.770(5), 2V_(meas) = 19-3l deg, 2V_(calc) = 36 deg. The crystal structure is studied, R = 0.055. Monoclinic, Cm, a = 14.604(7), b = 14.274(8), c = 7.933(2) A, #beta# = 117.40(3), V=1468.2 A~3. Tsepinite-Na is a structural analogue of vuoriyarvite-K with Na > K and Ti > Nb. Na~+ in tsepinite-Na is partly substituted with H_3O~+. Presence of H_3O~+ is confirmed by IR spectral data. The average chemical composition for the structurally studied crystal is (electron probe, wt percent): Na_2O 5.48, K_2O 1.58, CaO 0.18, SrO 2.32, BaO 2.60, ZnO 0.04, Fe_2O_3 0.25, TiO_2 14.17, Nb_2O_5 20.69, SiO_2 40.38, total 87.69. H_2O content (by thermogravimetrical procedure in vacuum) is 13.18 percent. The empirical formula is (Z = 1): H_(34.83)(Na_(4.21)K_(0.80)Sr_(0.54)Ba_(0.41)Zn_(0.01))#SIGMA#6.05(Ti_(4.22)Nb_(3.71)Fe_(0.07)~(3+))#SIGMA#8.00 Si_(16)O_(70.78). Idealized formula is (Z=4): (Na, H_3O, K, Sr, Ba)_2(Ti, Nb)_2(Si_4O_(12))(OH,O)_2 centre dot 3H_2O. The strongest reflexes of the X-ray powder pattern are (d, A (I, percent; hkl)): 7.01(44; 001), 6.46(100; 200, 2-bar 01), 3.954(30; 201, 2-bar 02), 3.236(98; 400, 4-bar 02), 3.179(33;041), 3.160(38;022). The mineral is named after Anatoliy I. Tsepin (b. 1946) who firstly obtained electron microprobe data for labuntsovite group minerals. Type specimen is Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.