Crystal chemistry of natural layered double hydroxides. 2. Quintinite-1M: first evidence of a monoclinic polytype in M²⁺-M³⁺ layered double hydroxides

Abstract

Quintinite-1M, [Mg4Al2(OH)12](CO3)(H2O)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 R1 = 0.031 on the basis of 304 unique reflections. It is monoclinic, space group C2/m, a = 5.266(2), b = 9.114(2), c = 7.766(3) Å, β = 103.17(3)°, V = 362.9(2) Å3. The diffraction pattern of quintinite-1M contains sharp reflections corresponding to the layer stacking sequence characteristic of the 3R 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 …=Ac1B=Ba1C=Cb1A=… Quintinite-1M is isostructural with the monoclinic polytype of [Li2Al4(OH)12](CO3)(H2O)3.

Crystal Chemistry of Chlormagaluminite, Mg4Al2(OH)12Cl2(H2O)2, a Natural Layered Double Hydroxide

“…Thus, the presence of the reflection with d = 4.57 Å in the powder pattern of pure LDHs having M 2+ :M 3+ = 2:1 is a reliable indicator of cation ordering similar to the Li 2 Al LDHs [47][48][49][50]. In contrast to Cl-bearing hydrotalcite-supergroup members, the carbonate species have been structurally characterized in more details including recent single-crystal or Rietveld structure refinements of hydrotalcite [31], pyroaurite [51][52][53], quintinite [34,[53][54][55][56][57][58], takovite [59], and zaccagnaite [60]. It has been found that C atoms of CO 3 groups occupy central parts of the interlayer prisms (Figure 7b), whereas O atoms of CO 3 groups and H 2 O molecules are disordered around the middle points of the prism edges.…”

Structural Synthon Approach to Predict the Possible Polytypes of Layered Double Hydroxides

“…A monoclinic LDH based on a metal hydroxide layer of a different symmetry is reported by Krivovichev and coworkers [44][45][46] in the mineral quintinite. Here the monoclinic distortion arises due to cation ordering in the hydroxide layer.…”

$$\hbox {NO}_{2}^{-}$$ NO 2 - and $$\hbox {SCN}^{-}$$ SCN -

“…For similar reasons, Taylor [1] proposed that the O-atom of the intercalated water molecule shares the same site as the O-atoms of the intercalated CO 2− 3 ion. This proposal is now widely accepted and adequately accounts for the observed diffraction data from both single crystalline [1,[5][6][7] and powder diffraction studies [3,4]. The same structure model is now extended to Cl − -intercalated LDHs [8], wherein the Cl − ion is shown to occupy the same site as that of the O-atom of carbonate ion.…”

“…The structure of the interlayer has to be determined by diffraction studies of laboratory-synthesized powder samples [3,4] or single crystalline mineral samples [1,[5][6][7]. Given the high affinity of the metal hydroxide layer for ambient CO 2 , a majority of the LDHs intercalate carbonate ions.…”