• journal_title：Mineralogical Magazine
• Contributor：E. S. Zhitova ; V.N. Yakovenchuk ; S.V. Krivovichev ; A. A. Zolotarev ; Y.A. Pakhomovsky ; G. Yu. Ivanyuk
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2010.074.5.841
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：74
• issue：5
• firstpage：841
• section：Articles

Two crystals of Mg,Al-disordered quintinite-2H (Q1 and Q2), [Mg₄Al₂(OH)₁₂](CO₃)(H₂O)₃, from the Kovdor alkaline massif, Kola peninsula, Russia, have been characterized chemically and structurally. Both crystals have hexagonal symmetry, P6₃/mcm, a = 3.0455(10)/3.0446(9), c = 15.125(7)/15.178(5) Å, V = 121.49(8)/121.84(6) ų. The structures of the two crystals have been solved by direct methods and refined to R₁ = 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 Mg²⁺ and Al³⁺ cations. Average <M–OH> bond lengths for the metal site are 2.017 and 2.020 Å 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.