Crystal chemistry of natural layered double hydroxides. 1. Quintinite-2H-3c from the Kovdor alkaline massif, Kola peninsula, Russia

摘要

The crystal structure of quintinite-2H-3c, [Mg_4Al_2(OH)_(12)](CO_3)(H_2O)_3, from the Kovdor alkaline massif, Kola peninsula, Russia, was solved by direct methods and refined to an agreement index (R_1) of 0.055 for 484 unique reflections with |F_o| ≥ 4σ_F. The mineral is rhombohedral, R32, a = 5.2745(7), c = 45.36(1) A . The diffraction pattern of the crystal has strong and sharp Bragg reflections having h-k = 3n and l = 3n and lines of weak superstructure reflections extended parallel to c~* and centred at h-k ≠ 3n. The structure contains six layers within the unit cell with the layer stacking sequence of ...AC=CA=AC=CA=AC=CA... The Mg and Al atoms are ordered in metal hydroxide layers to form a honeycomb superstructure. The full superstructure is formed by the combination of two-layer stacking sequence and Mg-Al ordering. This is the first time that a long-range superstructure in carbonatebearing layered double hydroxide (LDH) has been observed. Taking into account Mg-Al ordering, the unique layer sequence can be written as ...=Ab_1C=Cb_1A=Ab_2C=Cb_2A=Ab_3C=Cb_3A=... The use of an additional suffix is proposed in order to distinguish between LDH polytypes having the same general stacking sequence but with different c parameters compared with the 'standard' polytype. According to this notation, the quintinite studied here can be described as quintinite-2H-3c or quintinite-2H-3c[6R], indicating the real symmetry.