Hansesmarkite, Ca2Mn2Nb6O19·20H2O, a new hexaniobate from a syenite pegmatite in the Larvik Plutonic Complex, southern Norway

Abstract

AbstractThe new mineral hansesmarkite (IMA2015-067), Ca2Mn2Nb6O19·20H2O, 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 coatingsin patches on gonnardite. Hansesmarkite is biaxial (+) with refractive indices (white light): α = 1.683(2), β = 1.698(2) and γ = 1.745(3); 2V(meas.) = 60.7(6)° and 2V(calc.) = 60.3°. The mineral exhibits moderate dispersion (r>v)and is pleochroic withX(almost colourless) <Y( pale yellow) <<Z(orangey yellow). The optical orientation isX^c= 20°,Y^b= 16° andZ^a= 5°. The empirical formula based on five electron probemicroanalyses and calculated based on Nb = 6 apfu is (Ca1.93Na0.02K0.01)∑1.96(Mn1.79Fe0.11)∑1.90Nb6O18.84·20H2O, with H2O determined from the structure solution.The mineral is triclinic,P1, witha= 9.081(4),b= 9.982(8),c= 10.60(1) Å, α = 111.07(8), β = 101.15(6), γ = 99.39(5)°,V= 850.8(13) Å3 andZ= 1. The structure was solved at 120 K because of thermalinstability of the mineral and refined toR1= 2.50% forFo> 4σ. The strongest reflections in the x-ray diffraction diagram are: [dobs. in Å (I)(hkl)] 9.282(36)(001), 8.610(100)(100, 011), 3.257(30)(031, 131)and 3.058(18)(130, 212). 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.