The crystal structure of quintinite-2H-3c, [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 vertical bar F(o)vertical bar >= 4 sigma(F). The mineral is rhombohedral, R32, a = 5.2745(7), c = 45.36(1) angstrom. The diffraction pattern of the crystal has strong and sharp Bragg reflections having h-k not equal 3n and l = 3n and lines of weak superstructure reflections extended parallel to c* and centred at h-k 0 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(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-2H-3c or quintinite-2H-3c[6R], indicating the real symmetry.
|Состояние||Опубликовано - окт 2010|