Hansesmarkite, Ca2Mn2Nb6O19 center dot 20H(2)O, a new hexaniobate from a syenite pegmatite in the Larvik Plutonic Complex, southern Norway

摘要

The new mineral hansesmarkite (IMA2015-067), Ca2Mn2Nb6O19 center dot 20H(2)O, was discovered at the AS Granit larvikite quarry in Tvedalen, Larvik, Vestfold, Norway. Hansesmarkite forms faintly yellow crystals up to 0.3 mm or thin coatings in patches on gonnardite. Hansesmarkite is biaxial (+) with refractive indices (white light): alpha = 1.683(2), beta = 1.698(2) and gamma = 1.745(3); 2V((meas.)) = 60.7(6) and 2V((calc.)) = 60.3. The mineral exhibits moderate dispersion (r > v) and is pleochroic with X (almost colourless) < Y (pale yellow) Z (orangey yellow). The optical orientation is X boolean AND c = 20, Y boolean AND b = 16 and Z boolean AND a = 5. The empirical formula based on five electron probe microanalyses and calculated based on Nb = 6 apfu is (Ca1.93Na0.02K0.01)(Sigma 1.96)(Mn1.79Fe0.11)(Sigma 1.90)Nb6O18.84 center dot 20H(2)O, with H2O determined from the structure solution. The mineral is triclinic, PEmbedded Image, with a = 9.081(4), b = 9.982(8), c = 10.60(1) angstrom, alpha = 111.07(8), beta = 101.15(6), gamma = 99.39(5), V = 850.8(13) angstrom(3) and Z = 1. The structure was solved at 120 K because of thermal instability of the mineral and refined to R-1 = 2.50% for F-o > 4 sigma. The strongest reflections in the x-ray diffraction diagram are: [dobs. in angstrom (I)(hkl)] 9.282(36)(001), 8.610(100)(100, 01 (1) over bar), 3.257(30)(03 (1) over bar, 1 (3) over bar1) and 3.058(18)(1 (3) over bar0, 2 (1) over bar2). Hansesmarkite is the third naturally occurring hexaniobate in which six edge-sharing Nb-octahedra form the Lindqvist ion. These are linked via Mn-octahedra forming rods along [100] and Ca is located between the rods, creating a three dimensional structure via hydrogen bonds.