Hydrotalcite, ideally [Mg₆Al₂(OH)₁₆](CO₃)(H₂O)₄, was studied in samples from Dypingdal, Snarum, Norway (3R and 2H), Zelentsovskaya pit (2H) and Praskovie–Evgenievskaya pit (2H) (both Southern Urals, Russia), Talnakh, Siberia, Russia (3R), Khibiny, Kola, Russia (3R), and St. Lawrence, New York, USA (3R and 2H). Two polytypes, 3R and 2H (both ‘classical’), were confirmed on the basis of single-crystal and powder X-ray diffraction data. Their chemical composition was studied by electron-microprobe analysis, infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The crystal structure of hydrotalcite-3R was solved by direct methods in the space group R $ {\bar 3} $ m on three crystals (two data collections at 290 K and one at 120 K). The unit-cell parameters are as follows (290/290/120 K): a = 3.0728(9)/3.0626(3)/3.0617(4), c = 23.326(9)/23.313(3)/23.203(3) Å and V = 190.7(1)/189.37(4)/188.36(4) ų. The crystal structures were refined on the basis of 304/150/101 reflections to R₁ = 0.075/0.041/0.038. Hydrotalcite-2H crystallises in the P6₃/mmc space group; unit-cell parameters for two crystals are (data collection at 290 K and 93 K): a = 3.046(1)/3.0521(9), c = 15.447(6)/15.439(4) Å, V = 124.39(8)/124.55(8) ų. The crystal structures were refined on the basis of 160/142 reflections to R₁ = 0.077/0.059. This paper reports the first single-crystal structure data on hydrotalcite. Hydrotalcite distribution in Nature, diagnostic features, polytypism, interlayer topology and localisation of M²⁺–M³⁺ cations within metal hydroxide layers are discussed.

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 |F_o| ≥ 4σ_F. 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.

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) ų. 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.