Crystal chemistry of natural layered double hydroxides. 2. Quintinite-1<i>M</i>: first evidence of a monoclinic polytype in <i>M</i>²⁺-<i>M</i>³⁺ layered double hydroxides

Abstract

Abstract Quintinite-1 M , [Mg 4 Al 2 (OH) 12 ](CO 3 )(H 2 O) 3 , is the first monoclinic representative of both synthetic and natural layered double hydroxides (LDHs) based on octahedrally coordinated di- and trivalent metal cations. It occurs in hydrothermal veins in the Kovdor alkaline massif, Kola peninsula, Russia. The structure was solved by direct methods and refined to R 1 = 0.031 on the basis of 304 unique reflections. It is monoclinic, space group C 2/ m , a = 5.266(2), b = 9.114(2), c = 7.766(3) Å, β = 103.17(3)°, V = 362.9(2) Å 3 . The diffraction pattern of quintinite-1 M contains sharp reflections corresponding to the layer stacking sequence characteristic of the 3 R rhombohedral polytype, and rows of weak superlattice reflections superimposed upon a background of streaks of modulated diffuse intensity parallel to c *. These superlattice reflections indicate the formation of a 2-D superstructure due to Mg-Al ordering. The structure consists of ordered metal hydroxide layers and a disordered interlayer. As the unit cell contains exactly one layer, the polytype nomenclature dictates that the mineral be called quintinite-1M. The complete layer stacking sequence can be described as …=Ac 1 B=Ba 1 C=Cb 1 A=… Quintinite-1 M is isostructural with the monoclinic polytype of [Li 2 Al 4 (OH) 12 ](CO 3 )(H 2 O) 3 .