Original paper

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

Christiansen, C. Claes; Gault, Robert A.; Grice, Joel D.; Johnsen, Ole

Abstract

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 lÃ¥venite. 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 2 V 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 (Na1.70Sr0.02)∑1.72(Na2.77 Ca1.23)∑4.00(Ca3.40Na0.60)∑4.00(Mn 1.06Ca0.82Y0.05Ce0.05La0.02)∑2.00 (Zr1.40Fe0.23Ca0.36Hf0.01)∑2.00(Ti1.61Nb0.21Zr0.10Al0.05V0.03)∑2.00(Si2O7)4F4(O2.39F1.48)∑3.87 D meas. = 3.32, D calc. = 3.31 g/cm3. Kochite is an Mn-, Ti-analogue of rosenbuschite.

Keywords

kochiterosenbuschite groupnew mineralcrystal structure