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

Abstract

Kochite, ideally Na 2 (Na,Ca) 4 Ca 4 (Mn,Ca) 2 Zr 2 Ti 2 (Si 2 O 7 ) 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 × 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 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) A, α = 90.192(4), β = 100.334(5), γ = 111.551(4) °, Z = 1. The 5 strongest X-ray lines are [ d in A (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 SiO 2 31.55, Al 2 O 3 0.05, V 2 O 3 0.03, TiO 2 8.44, ZrO 2 12.12, HfO 2 0.09, Nb 2 O 5 1.86, Ta 2 O 5 0.02, MgO 0.01, MnO 4.92, FeO 1.08, CaO 21.39, SrO 0.12, Na 2 O 10.33, Y 2 O 3 0.39, La 2 O 3 0.25, Ce 2 O 3 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 1.70 Sr 0.02 )∑ 1.72 (Na 2.77 Ca 1.23 )∑ 4.00 (Ca 3.40 Na 0.60 )∑ 4.00 (Mn 1.06 Ca 0.82 Y 0.05 Ce 0.05 La 0.02 )∑ 2.00 (Zr 1.40 Fe 0.23 Ca 0.36 Hf 0.01 )∑ 2.00 (Ti 1.61 Nb 0.21 Zr 0.10 Al 0.05 V 0.03 )∑ 2.00 (Si 2 O 7 ) 4 F 4 (O 2.39 F 1.48 )∑ 3.87 . D meas. = 3.32, D calc. = 3.31 g/cm 3 . Kochite is an Mn − , Ti-analogue of rosenbuschite.