THE CRYSTAL STRUCTURE OF MCKELVEYITE-(Y)-2M, A NEW MONOCLINIC POLYTYPE FROM VAL MALENCO, ITALIAN ALPS

A new monoclinic (Cc) polytype, mckelveyite-(Y)-2M, NaBa3CaY(CO3)6•3H2O, was found in “Cava Fabi”, a talc quarry near Lanzada in Val Malenco, Lombardy, Italy, associated with anatase, brookite, brucite, calcite, dolomite, ilmenite, lindsleyite, magnesite, magnetite, pyrite, quartz, redledgeite and rutile, in hydrothermal carbonate veins intercalated in ultrabasic rocks and dolomitic marble in contact with serpentinites. The origin of the mineral is related to circulation of CO2-rich fluids in serpentinites, during the latest stages of Alpine metamorphism. The monoclinic mineral, space group Cc, has cell parameters a 15.8213(7), b 9.1364(4), c 13.7522(6) Å, b 112.44(1)°, V 1837.35(15) Å3, with marked pseudorhombohedral symmetry. The crystals are pseudopyramidal, up to 4 mm in length, with distinct trigonal character derived from intergrowth of three individuals twinned about a pseudo-threefold axis, in some cases showing re-entrant angles. The chemical analysis obtained by wavelengthdispersion electron-microprobe analysis, supplemented by structural data for CO2 and H2O, gave (wt%): Na2O 3.50, CaO 5.68, SrO 3.45, BaO 45.33, Y2O3 8.82, Sm2O3 0.05, Gd2O3 0.45, Dy2O3 1.10, Er2O3 0.85, Yb2O3 0.08, CO2 27.13, H2O 5.55, total 101.99, corresponding to the empirical formula (calculated on the basis of six metal sites in the asymmetric unit, as shown in the structure solution): Na1.00(Ca0.83Na0.07Sr0.11)S1.01(Ba2.79Sr0.20)S2.99(Y0.74REE0.13Ca0.12)S0.99[CO3]6•3H2O. The crystal structure has been refined to a final R = 0.0283 for 4454 observed reflections [I > 2s(I)], and corresponds to the first accurate determination of the structure of mckelveyite-(Y). The unit cell contains three independent atomic sites occupied by Ba plus minor Sr, and three sites occupied almost exclusively by Na+, Ca2+, and Y3+ (plus the REE), respectively; there are alternate double layers of pseudohexagonal rings perpendicular to the c* axis containing either the Ba atoms or the other metal atoms, with no substantial disorder; each double layer is the mirror image on {010} of the adjacent ones. The strongest six lines in the X-ray powder-diffraction pattern [dobs in Å(I)hkl] are: 3.106(100)(312), 4.299(48)(310), 6.369(36)(002), 2.029(29)(602), 3.180(28)(004), 2.635(27)(602). The pattern shows significant differences with respect to type triclinic mckelveyite-(Y).