Crystal chemistry of natural layered double hydroxides; I, Quintinite-2H-3c from the Kovdor alkaline massif, Kola Peninsula, Russia

The crystal structure of quintinite-2H-3c, [Mg (sub 4) Al (sub 2) (OH) (sub 12) ](CO (sub 3) )(H (sub 2) O) (sub 3) , from the Kovdor alkaline massif, Kola peninsula, Russia, was solved by direct methods and refined to an agreement index (R (sub 1) ) of 0.055 for 484 unique reflections with F (sub o) > or =4sigma (sub F) . The mineral is rhombohedral, R32, a=5.2745(7), c=45.36(1) Aa. 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 (super *) and centred at h-knot =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 carbonate-bearing layered double hydroxide (LDH) has been observed. Taking into account Mg-Al ordering, the unique layer sequence can be written as... =Ab (sub 1) C=Cb (sub 1) A=Ab (sub 2) C=Cb (sub 2) A=Ab (sub 3) C=Cb (sub 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-2H-3c or quintinite-2H-3c[6R], indicating the real symmetry.