Crystal chemistry of natural layered double hydroxides. I. Quintinite-2<i>H</i>-3<i>c</i> from the Kovdor alkaline massif, Kola peninsula, Russia

Abstract

Abstract The crystal structure of quintinite-2 H -3 c , [Mg 4 Al 2 (OH) 12 ](CO 3 )(H 2 O) 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, R 32, a = 5.2745(7), c = 45.36(1) Å. The diffraction pattern of the crystal has strong and sharp Bragg reflections having h – k = 3 n and l = 3 n and lines of weak superstructure reflections extended parallel to c * and centred at h–k ≠ 3 n . 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 carbonate-bearing layered double hydroxide (LDH) has been observed. Taking into account Mg-Al ordering, the unique layer sequence can be written as …=Ab 1 C=Cb 1 A=Ab 2 C=Cb 2 A=Ab 3 C=Cb 3 A=… 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-2 H -3 c or quintinite-2 H -3 c [6 R ], indicating the real symmetry.