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

Kochite, ideally Na₂(Na,Ca)₄Ca₄(Mn,Ca)₂Zr₂Ti₂(Si₂O₇)₄(O,F)₄F₄, 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 × 1.0 mm. Associated minerals are nepheline, alkali feldspar and låvenite. 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 α 1.684 ± .002, β 1.695 ± .004, γ 1.718 ± .002 and 2V_meas. = 73°(2). The optical orientation is X = c and Z Λ [100] ∼ 20°. 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) Å, α = 90.192(4), β = 100.334(5), γ = 111.551(4) °, Z = 1. The 5 strongest X-ray lines are [d in Å (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 ≡ 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 (Na_1.70Sr_0.02)∑_1.72(Na_2.77Ca_1.23)∑_4.00(Ca_3.40Na_0.60)∑_4.00(Mn_1.06Ca_0.82Y_0.05Ce_0.05La_0.02)∑_2.00 (Zr_1.40Fe_0.23Ca_0.36Hf_0.01)∑_2.00(Ti_1.61Nb_0.21Zr_0.10Al_0.05V_0.03)∑_2.00(Si₂O₇)₄F₄(O_2.39F_1.48)∑_3.87. D_meas. = 3.32, D_calc. = 3.31 g/cm³. Kochite is an Mn⁻, Ti-analogue of rosenbuschite.