Kochite, a new member of the rosenbuschite group from the Werner Bjerge alkaline complex, East Greenland

Kochite, ideally Na-2(Na,Ca)(4)Ca-4(Mn,Ca)(2)Zr2Ti2(Si2O7)(4)(O,F)(4)F-4, is a new member of the rosenbuschite group from the alkaline complex of Werner Bjerge, East Greenland. It occurs in nepheline syenite as lath-shaped to acicular crystals in aggregates up to 0.3 x 1.0 mm. Associated minerals are nepheline, alkali feldspar and lAvenite. It is transparent with a brownish colour and has a vitreous lustre. It is brittle with a perfect {100} cleavage and an uneven fracture. The mineral is biaxial positive with alpha 1. 684 +/- .002, beta 1.695 +/- .004, gamma 1.718 +/- .002 and 2V(meas.) = 73degrees(2). The optical orientation is X = c and Z Lambda [100] similar to 20degrees. The pleochroism is weak with X colourless and Z = pale yellow. The mineral is triclinic with a = 10.032(2), b = 11.333(2), c = 7.202(1) Angstrom, alpha = 90.192(4), beta = 100.334(5), gamma = 111.551(4) degrees, Z = 1. The 5 strongest X-ray lines are [d in Angstrom (Int.)] 2.908(10), 2.600(8), 3.028(6), 1.868(6) and 1.670(5). An average of four electron microprobe analyses of kochite gave SiO2 31.55, Al2O3 0.05, V2O3 0.03, TiO2 8.44, ZrO2 12.12, HfO2 0.09, Nb2O5 1.86, Ta2O5 0.02, MgO 0.01, MnO 4.92, FeO 1.08, CaO 21.39, SrO 0. 12, Na2O 10.33, Y2O3 0.39, La2O3 0.25, Ce2O3 0.57, F 6.83, O drop F 2.88, total 97.71 wt.%. The empirical formula, based on 8 Si atoms in accordance with the structure refinement (R(F) = 4.0 %), is (Na1.70Sr0.02)(Sigma1.72)(Na2.77Ca1.23)(Sigma4.00)(Ca3.40Na0.60)(Sigma4.00)(Mn1.06Ca0.82Y0.05Ce0.05La0.02)(Sigma2.00) (Zr1.40Fe0.23Ca0.36Hf0.01)(Sigma2.00)(Ti1.61Nb0.21Zr0.10Al0.05V0.03)(Sigma2.00)(Si2O7)(4)F-4(O2.39F1.48)(Sigma3.87). D-meas. = 3.32, D-calc. = 3.31 g/cm(3). Kochite is an Mn-, Ti-analogue of rosenbuschite.