Crystal chemistry of natural layered double hydroxides; 3, The crystal structure of Mg,Al-disordered quintinite-2H

Two crystals of Mg,Al-disordered quintinite-2H (Q1 and Q2), [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, have been characterized chemically and structurally. Both crystals have hexagonal symmetry, P6 (sub 3) /mcm, a=3.0455(10)/3.0446(9), c=15.125(7)/15.178(5) Aa, V=121.49(8)/121.84(6) Aa (super 3) . The structures of the two crystals have been solved by direct methods and refined to R (sub 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) (sub 2) ] with an average disordered distribution of Mg (super 2+) and Al (super 3+) cations. Average <M-OH> bond lengths for the metal site are 2.017 and 2.020 Aa for 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 2H 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-2H-3c) 2H polytype in comparison to the rhombohedral (or pseudo-rhombohedral in the case of quintinite-1M) 3R polytype.