Elena S. Zhitova

Crystal chemistry of natural layered double hydroxides. I. Quintinite-2H-3c from the Kovdor alkaline massif, Kola peninsula, Russia

Abstract The crystal structure of quintinite-2H-3c, [Mg4Al2(OH)12](CO3)(H2O)3, from the Kovdor alkaline massif, Kola peninsula, Russia, was solved by direct methods and refined to an agreement index (R1) of 0.055 for 484 unique reflections with |Fo| ≥ 4σF. The mineral is rhombohedral, R32, a = 5.2745(7), c = 45.36(1) Å. The diffraction pattern of the crystal has strong and sharp Bragg reflections having h-k = 3n and l = 3n and lines of weak superstructure reflections extended parallel to c* and centred at h-k ≠ 3n. The structure contains six layers within the unit cell with the layer stacking sequence of ...AC=CA=AC=CA=AC=CA... The Mg and Al atoms are ordered in metal hydroxide layers to form a honeycomb superstructure. The full superstructure is formed by the combination of two-layer stacking sequence and Mg-Al ordering. This is the first time that a long-range superstructure in carbonate-bearing layered double hydroxide (LDH) has been observed. Taking into account Mg-Al ordering, the unique layer sequence can be written as ...=Ab1C=Cb1A=Ab2C=Cb2A=Ab3C=Cb3A=... The use of an additional suffix is proposed in order to distinguish between LDH polytypes having the same general stacking sequence but with different c parameters compared with the ‘standard’ polytype. According to this notation, the quintinite studied here can be described as quintinite-2H-3c or quintinite-2H-3c[6R], indicating the real symmetry.

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)º, 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 natural layered double hydroxides. 3. The crystal structure of Mg,Al-disordered quintinite-2H

Abstract 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) Å3. 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 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.

Natural Double Layered Hydroxides: Structure, Chemistry, and Information Storage Capacity

Layered double hydroxides (LDHs) constitute an important group of materials with many applications ranging from catalysis and absorption to carriers for drug delivery, DNA intercalation and carbon dioxide sequestration (Rives 2001; Duan and Evans 2006). The structures of LDHs are based upon double brucite-like hydroxide layers [M n 2+ M m 3+ (OH)2(m+n)]m+, where M2+ = Mg2+, Fe2+, Mn2+, Zn2+, etc.; M3+ = Al3+, Fe3+, Cr3+, Mn3+, etc. The positive charge of the layer is compensated by interlayer species that may consist of anions (CO 3 2− , Cl−, SO 4 2− , etc.) or both anions and cations (Na+, Ca2+, Sr2+, etc.).

Crystal Chemistry of Pyroaurite from the Kovdor Pluton, Kola Peninsula, Russia, and the Långban Fe–Mn deposit, Värmland, Sweden

Pyroaurite [Mg6Fe23+ (OH)16][(CO3)(H2O)] from the Kovdor Pluton on the Kola Peninsula, Russia, and the Långban deposit in Filipstad, Värmland, Sweden were studied with single crystal and powder X-ray diffraction, an electron microprobe, and Raman spectroscopy. Both samples are rhombohedral, space group R3̅m, a = 3.126(3), c = 23.52(2) Å (Kovdor), and a = 3.1007(9), c = 23.34(1) (Långban). The powder XRD revealed only the 3R polytype. The ratio of di- and trivalent cations M2+: M3+ was determined as ~3.1–3.2 (Kovdor) and ~3.0 (Långban). The Raman spectroscopy of the Kovdor sample verified hydroxyl groups and/or water molecules in the mineral (absorption bands in the region of 3600–3500 cm–1) and carbonate groups (absorption bands in the region of 1346–1058 cm–1). Based on the data obtained, the studied samples should be identified as pyroaurite-3R (hydrotalcite group).

Quintinite-1 M from the Mariinsky Deposit, Ural Emerald Mines, Central Urals, Russia

The paper describes the first finding of quintinite [Mg4Al2(OH)12][(CO3)(H2O)3] at the Mariinsky deposit in the Central Urals, Russia. The mineral occurs as white tabular crystals in cavities within altered gabbro in association with prehnite, calcite, and a chlorite-group mineral. Quintinite is the probable result of late hydrothermal alteration of primary mafic and ultramafic rocks hosting emerald-bearing glimmerite. According to electron microprobe data, the Mg: Al ratio is ~2: 1. IR spectroscopy has revealed hydroxyl and carbonate groups and H2O molecules in the mineral. According to single crystal XRD data, quintinite is monoclinic, space group C2/m, a =5.233(1), b = 9.051(2), c = 7.711(2) Å, β = 103.09(3)°, V = 355.7(2) Å3. Based on structure refinement, the polytype of quintinite should be denoted as 1M. This is the third approved occurrence of quintinite-1M in the world after the Kovdor complex and Bazhenovsky chrysotile–asbestos deposit.