Crystal chemistry of natural layered double hydroxides. 3. The crystal structure of Mg,Al-disordered quintinite-2<i>H</i>

Abstract

Abstract Two crystals of Mg, Al-disordered quintinite-2 H (Q1 and Q2), [Mg 4 Al 2 (OH) 12 ](CO 3 )(H 2 O) 3 , from the Kovdor alkaline massif, Kola peninsula, Russia, have been characterized chemically and structurally. Both crystals have hexagonal symmetry, P 6 3 / mcm , a = 3.0455(10)/3.0446(9), c = 15.125(7)/15.178(5) Å, V = 121.49(8)/121.84(6) Å 3 . The structures of the two crystals have been solved by direct methods and refined to R 1 = 0.046 and 0.035 on the basis of 76 and 82 unique observed reflections for Q1 and Q2, respectively. Diffraction patterns obtained using an image-plate area detector showed the almost complete absence of superstructure reflections which would be indicative of the Mg-Al ordering in metal hydroxide layers, as has been observed recently for other quintinite polytypes. The crystal structures are based on double hydroxide layers [ M (OH) 2 ] with an average disordered distribution of Mg 2+ and Al 3+ cations. Average &lt; M –OH&gt; bond lengths for the metal site are 2.017 and 2.020 Åfor Q1 and Q2, respectively, and are consistent with a highly Mg-Al disordered, average occupancy. The layer stacking sequence can be expressed as …=AC=CA=…, corresponding to a Mg-Al-disordered 2 H polytype of quintinite. The observed disorder is probably the result of a relatively high temperature of formation for the Q1 and Q2 crystals compared to ordered polytypes. This suggestion is in general agreement with the previous observations which demonstrated, for the Mg-Al system, a higher-temperature regime of formation of the hexagonal (or pseudo-hexagonal in the case of quintinite-2 H -3 c ) 2 H polytype in comparison to the rhombohedral (or pseudo-rhombohedral in the case of quintinite-1 M ) 3 R polytype.