The Crystal Structure of Mg-Al-CO₃ Layered Double Hydroxide

1. Introduction

Hydrotalcite supergroup minerals [Mills et al., 2012] are natural representatives of industrially applied Layered Double Hydroxides (LDHs) [Rives, 2001]. LDHs form a class of inorganic lamellar compounds with crystal structures consisting of alternating positively charged metal-hydroxide layers (octahedral sheets) and negatively charged interlayers [Duan and Evans, 2009; Aminoff and Broome, 1931]. LDHs can be represented by the general formula [Bukhtiyarova et al., 2019; Yu et al., 2017; Kameliya et al., 2023] [M²⁺_1-xM³⁺_x(OH)₂][A^n-]_x/n∙mH₂O, where M²⁺ = Mg²⁺, Ni²⁺, Fe²⁺, Mn²⁺, etc. and M³⁺ (e.g. Al³⁺, Fe³⁺, Cr³⁺ and other trivalent cations); x = 0.33 and 0.25 are the most common and correspond to the M²⁺:M³⁺ = 2:1 and 3:1 ratios, respectively, that are the most widespread among LDHs; A^n- is the anion of the n negative charge (the most common among mineral are CO₃^2-, Cl^-, SO₄^2-, Sb(OH)₆^-); m is the number of interlayer H₂O molecules. This general formula is further extended by minerals of (i) the wermlandite group [Rius and Allmann, 1984; Zhitova et al., 2021; Huminicki and Hawthorne, 2003; Cooper and Hawthorne, 1996] and their synthetic counterparts [Sotiles et al., 2019a,b; Sotiles and Wypych, 2020] that contain additional interlayera mono- or divalent cations (like Na, K, Ca etc.) and (ii) Li-Al (i.e. M⁺-M³⁺) LDHs, the structures of which are represented by gibbsite-like layers [Serna et al., 1982; Sissoko et al., 1985] with the general formula of such compounds given as [Al₄Li₂(OH)₂][A^n-]_x/n∙mH₂O.

Magnesium-aluminum LDHs with carbonate interlayer are among the longest known in the LDH family and applied as catalytic materials [Karcz et al., 2020,2021] and considered efficient and inexpensive oxygen evolution reaction electrocatalysts [Yin et al., 2023]. Mg-Al LDHs are often used as model systems for the LDH family of different (rare) chemical compositions (for example, the substitution of cations while maintaining stoichiometry). The Mg-Al-CO₃ LDHs are applied in pharmacology as a Talcid medicine. Recent pharmaceutical studies involving Mg-Al LDHs include hybrid materials with glibenclamide [Leao et al., 2019], nanocarriering material for antimicrobial chemotherapy [Francius et al., 2023] and porous nanocomposite of LDH with chitosan for cosmetic application [Kim et al., 2021]. LDHs, including Mg-Al varieties, are considered as sorbents of undesirable anions and cations; recent research include: methyl orange [Zaghloul et al., 2021; Gregoire et al., 2019], Pb^II [Zhao et al., 2011], Cu^II [Yang et al., 2021], Congo red dye [Nestroinaia et al., 2022] removal from aqueous solution; harmful anion (such as Cl^-, Br^-, I^-, AsO₄^3-, etc.) sorption from fertile soil [Guo et al., submitted]. The uptake of Cl^- and CO₃^2- anions by Mg-Al and Ca-Al LDHs from pore solutions was simulated for cementous materials [Ke et al., 2017]. The possibility of anion uptake by LDHs also makes them advanced materials for the CO₂ capture [Sakai et al., 2022]. High industrial and material science interest towards LDHs requires detailed understanding of the internal structure of these materials. Highly crystalline and extremely stable prototypes of such materials are natural compounds [Zhitova et al., 2023] – hydrotalcite supergroup minerals [Mills et al., 2012].

Despite wide chemical variability of LDHs, their crystal structures show a certain structural commonality in which one structural type extends to compounds of different compositions. For example, quintinite is isotypic to Li-Al gibbsite-based mineral akopovaite, Al₄Li₂(OH)₁₂CO₃∙3H₂O [Zhitova et al., 2019a], following the substitution scheme [Mg₄Al₂(OH)₁₂]²⁺ → [Al₄Li₂(OH)₁₂]²⁺. Both structures show cation (Mg/Al or Al/Li) ordering according to the $\sqrt{3}$ × $\sqrt{3}$ superstructures and identical scheme of interlayer species arrangement despite rather principal chemical variability. The same applies to their Cl-analogues: chlormagaluminite, Mg₄Al₂(OH)₂Cl₂∙3H₂O, [Zhitova et al., 2019b], and dritsite, Al₄Li₂(OH)₁₂Cl₂∙3H₂O [Zhitova et al., 2019c]. Comparison of the crystal structures of minerals shows that they are isotypic to their synthetic analogues, which is well shown by Li-Al LDHs [Britto et al., 2008; 2009 and 2011]. Therefore, the crystal structures obtained on minerals through single-crystal X-ray diffraction analysis (the technique that makes it possible to obtain models of crystal structures with greater accuracy) can be extended to industrially applied synthetic LDH materials.

In this study we provide crystal chemical characterization of quintinite from Jacupiranga (São Paulo, Brazil). Previously the crystal structure of quintinite from that locality has been described as stratified with two types of interlayers (see details below), which contradicts the more recent studies of quintinite from the Kovdor complex [Krivovichev et al., 2010a,b,2012; Zhitova et al., 2010,2018]. The goal of the present study is to resolve this contradiction, i.e. to check whether quintinites from different localities are isotypic to each other or not. In addition, we provide data that should be helpful for the identification of cation-ordered polytypes with hexagonal stacking sequences by powder X-ray diffraction method.

Previous crystal structure studies of quintinite

The first crystal structure study of quintinite (at that time – “manasseite” i.e. hydrotalcite) has been carried out on the sample from Jacupiranga [Arakcheeva et al., 1996]. The study demonstrated the presence of the Mg and Al ordering according to the $\sqrt{3}$ × $\sqrt{3}$ superstructure in the metal-hydroxide layer (or octahedral sheet); hexagonal layer stacking sequence and stratified interlayer that consisted of two types: (i) (H₂O) molecules and (ii) (CO₃)^2- that alternated along z direction [Arakcheeva et al., 1996] (Figure 1).

Almost simultaneously, quintinite has been approved as a separate (from hydrotalcite/manaseite) mineral species on the samples from alkaline complex of Mont Saint-Hilaire, Quebec [Chao and Gault, 1997]. The crystal structure has not been refined, but unit-cell parameters determined indicative of polytypes with the hexagonal and rhombohedral layer stacking sequences with doubled in comparison to previous data [Arakcheeva et al., 1996] lattice parameter a according to the 2$\sqrt{3}$ × 2$\sqrt{3}$ superstructure (which, however, has not been identified elsewhere else) (Figure 2).

We have studied polytypism of quintinite from the Kovdor alkaline complex (Kola peninsula, Russia), where the polytype character depends upon the following factors: (i) disorder or order of cations within metal-hydroxide layer and (ii) type of layer stacking [Krivovichev et al., 2010a,b; Zhitova et al., 2010; Krivovichev et al., 2012a; Zhitova et al., 2018a]. In total, five polytypes of quintinite have been described in Kovdor, among them, 2H, 2T, 3R and 1M polytypes appear commonly (Figure 3). In addition, it was shown that the sequence of polytype formation can be described as 2H → 2T → 1M. The polytype 2T-3c (or 2H-3c in the original version) can be characterized as a cell of 2T polytype shifted along z direction producing tripling of the number of layers within the unit cell; this polytype has been observed in one sample only. Later the study of “hydrotalcite” from two localities in Ural Mountains (Russia), Ural emerald mines (Malyshevskoe or former Mariinskoe deposit) [Zhitova et al., 2018b] and Bazhenovskoe Chrysotile–Asbestos deposit [Krivovichev et al., 2012b], demonstrated that Ural samples are in fact quintinite-1M (previously identified as hydrotalcite) isotypic to the Kovdor quintinite-1M. Recently quintinite-1M has been described from the Mount Mather Creek, British Columbia together with the 3T(?) polytype that possess some residual reflections [Piillonen et al., 2022].

2. Materials and Methods

2.1. Materials

Two samples from the Jacupiranga alkaline complex (Cajati, São Paulo, Brazil) labelled as “manasseite” have been investigated from the following sources: (a) the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Science (Moscow, Russia) stored under catalog number 91,002 and (b) from the collection of Smithsonian Institution (National Museum of Natural History in Washington, D.C., USA) stored under catalog number C7029.

Originally quintinite has been described in Jacupiranga as “manasseite” (hexagonal form of hydrotalcite that is currently discredited) [Menezes and Martins, 1984]. Quintinite and manasseite/hydrotalcite have a confusing history {see hydrotalcite vs. quintinite description in Snarum (Norway) by [Mills et al., 2016; Raade, 2013; Zhitova et al., 2019a] of their description because both are Mg-Al-CO₃ minerals with the different Mg:Al ratios: 2:1 (quintinite) and 3:1 (hydrotalcite and formerly “manasseite”). Then the crystal structure of quintinite from the Jacupiranga mine was studied [Arakcheeva et al., 1996], it was stated that the mineral is a potentially new mineral species from the ‘hydrotalcite-manasseite group’ that has previously been described as “manasseite”. Paying attention to the synchronism of this work on structure refinement (1996) [Arakcheeva et al., 1996] with the description of a new mineral – quintinite (1997) [Chao and Gault, 1997] and the subsequent citation of this structural work in the first description of quintinite, it is clear that the mineral studied by Arakcheeva et al. [1996] was in fact quintinite. At the time of the crystal structure description, the name of the quintinite has not yet been approved by the Commission on New Minerals, Nomenclature and Classification therefore not mentioned in the text. In the most recent study of the Jacupiranga minerals [Oliveira and Sant`Agostino, 2020], this mineral is described as quintinite. In Jacupiranga, quintinite was found in carbonatite rocks [Menezes and Martins, 1984] and considered as late-stage hydrothermal mineral.

2.2. Single-crystal X-ray diffraction

Single-crystal X-ray diffraction analysis was carried out for the quintinite samples 91,002 and C7029 using a Bruker Smart Apex diffractometer (X-ray diffraction Resource Center, St. Petersburg State University), MoKα radiation, operated at 50 kV and 40 mA, equipped with a CCD area detector, more details on data collection are given in Table 1. The unit-cell parameters were refined by least-squares methods. The data were processed using the P-3c1 space group. During data process we first successfully processed single-crystal X-ray diffraction data and refined crystal structure in the P6₃/mcm space group as the most probable option suggested by CrysAlis Software [Rigaku Oxford Diffraction, 2015] (this space group was used for the refinement of chlormagaluminite structure [Zhitova et al., 2019]). However, this led to a large number of unaccounted for reflections, as a result of which we decided to use the P-3c1 space group previously used for quintinite [Zhitova et al., 2018].

The structure was solved and refined using the ShelX program package [Sheldrick, 2015] incorporated into the Olex2 software shell [Dolomanov et al., 2009], to R₁ = 0.053/0.066 for 320/424 independent reflections with I ≥ 2σ(I) for samples 91,002 and C7029.

The positions of Mg, Al and oxygen of the metal-hydroxide layer were located by direct methods and refined anisotropically (Table S2). The interlayer species (O and C) and hydrogen atoms of metal-hydroxide layer were located from the inspection of difference-Fourier maps and refined isotropically. Site occupancies of the Mg, Al and O sites of the metal-hydroxide layer were found close to 100 % and fixed. Site occupancies of the interlayer atoms were refined for O atoms and fixed for C atoms. The main crystallographic characteristics and structure refinement parameters are listed in Table 1.

2.3. Powder X-ray diffraction

Powder X-ray diffraction (XRD) data were collected for sample 91,002 by means of a Rigaku R-Axis Rapid II single-crystal diffractometer using Debye-Scherrer geometry (d = 127.4 mm). The diffractometer is equipped with a rotating anode (CoKα, λ = 1.79026, voltage = 40 kV and current = 15 mA), microfocus optics and a cylindrical image plate detector. The data were converted using osc2xrd program [Britvin et al., 2017].

The unit-cell parameters were refined by the Pawley method using Topas 4.2 [Bruker, 2009], with the hexagonal structure model, space group P-3c1, with the starting unit-cell parameters of the sample 91,002 reported herein. The refinement was based on the reflections in the 2θ region 10–60º. The indexing of the pattern and refinement of the unit-cell parameters were done with the fixed atom coordinates, site scattering and isotropic displacement parameters. Neutral scattering factors were used for all atoms. The background was modelled using a Chebyshev polynomial approximation of the 14th order. The peak profile was described using the fundamental parameters approach. Refinement of preferred orientation parameters confirmed the presence of significant preferred orientation along the [001] direction.

3. Results

3.1. Crystal structure solution and refinement

The collected singe-crystal X-ray diffraction data were processed in the P-3c1 space group, a = 5.246/5.298 Å, c = 15.110/15.199 Å for the samples 91002/C7029. Eight atomic positions were located from the structure solution and refinement for each sample (sites labelled): Mg, Al, O1 and H1 are part of metal-hydroxide layer (or octahedral sheet) and fully occupied (Table 2).

The Mg and Al sites are ordered according to the $\sqrt{3}$ × $\sqrt{3}$ superstructure; each of the metal sites is coordinated by six protonated oxygen atoms with the average <Mg–O> and <Al–O> bond distances being in the ranges 2.022–2.053 Å and 1.974–1.978 Å, respectively (Table 3). The other four sites C1, C2, O2 and O3 are at the interlayer level and correspond to the low-occupied sites of statistically disordered (CO₃)^2- groups and (H₂O)⁰ molecules. All interlayers are identical and consist of both (CO₃)^2- groups and (H₂O)⁰ molecules in one layer.

The O1–H1 bond is nearly perpendicular to the plane of the octahedral sheet; the bond distance is 0.82/0.91 Å for the samples 91002/C7029 (Table 4). The O1 atom acts as a donor (D), while the interlayer O2 and O3 atoms are acceptors (A) of hydrogen bonding. The H^…A bond distance is almost two times longer than the D–H distance (Table 4).

3.2. Powder X-ray diffraction

The main set of reflections of the powder X-ray diffraction pattern (Figure 1) corresponds to the classic (for LDHs) 2H polytype (Table S1). The low-intensity superstructure reflection indicative of the Mg and Al ordering according to the $\sqrt{3}$ × $\sqrt{3}$ superstructure is observed at d = 4.57 Å and indexed (hkl) as 010. Another characteristic feature of quintinite powder X-ray diffraction pattern is the d-value of 7.56 Å that agrees well with the previous crystal chemical studies of quintinite [Zhitova et al., 2016].

Figure 4. Indexed powder X-ray diffraction pattern of quintinite from Jacupiranga, sample 91,002 {space group P-3c1; lattice parameters from powder X-ray diffraction data: a = 5.2783(3) Å, c = 15.1171(16) Å, V = 364.74(6) ų}.

Figure 4. Indexed powder X-ray diffraction pattern of quintinite from Jacupiranga, sample 91,002 {space group P-3c1; lattice parameters from powder X-ray diffraction data: a = 5.2783(3) Å, c = 15.1171(16) Å, V = 364.74(6) ų}.

4. Discussion

In this work, using X-ray diffraction analysis, it is shown that quintinite from Jacupiranga is characterized by a hexagonal type of layer stacking, a superstructure according to the Mg and Al ordering by $\sqrt{3}$ × $\sqrt{3}$ pattern, and identical interlayers composed of disordered (H₂O)⁰ molecules and (CO₃)^2- groups at the same level in one gallery. This polytype is designated as 2T according to the Ramsdell notation [Guinier et al., 1984]. Topologically, quintinite-2T from Jacupiranga is identical to quintinite-2T from Kovdor alkaline complex [Zhitova et al., 2018] and is the second confirmed finding of quintinite-2T in the world. It should be noted that, due to the high degree of disorder of the interlayer components, the positions of localized interlayer C and O atoms may differ from sample to sample, but the electron density distribution maps at the interlayer level are identical. The same refer to the space group selection, i.e. potential polytypes with structures refined in space groups P-3c1 or P6₃/mcm with identical crystal chemical features should not be taken as separate polytypes (see details in section 2.2).

Previously, we assumed [Zhitova et al., 2018] that the structure obtained for quintinite from the Jacupiranga [Arakcheeva et al., 1996] with stratified interlayer is not entirely correct from the crystal-chemical point of view, since, probably, the "carbonate" and "water" interlayers should have had different heights of the interlayer gallery, which was not observed. It should also be noted here that such a division into two types of interlayers in the previous work was most likely due to the fact that not all electron density peaks at the interlayer level were detected, since their intensity can be very low, on the order of 1 ē. A similar problem is typical for earlier refinements of many LDH minerals, and is a consequence of the (apparently) insufficient accuracy of the equipment of that time. Although we had no opportunity to examine the same quintinite crystal as earlier researchers [Arakcheeva et al., 1996], we suggest that quintinite does not form structures with stratified interlayers. We also suggest that quintinite is characterized by four main polytypes (Figure 3), one of which is 2T that is described in this work. We believe that this is a rather important assumption, since experimentally obtained crystal structures often become the basis for the Rietveld refinements or computer simulation. Unfortunately, sometimes in the literature it is possible to observe crystal structures that are unrealistic from a crystal chemical point of view, in which, for example, H₂O molecules and carbonate groups are located in the same interlayer gallery at different levels.

The presence of M²⁺ and M³⁺ cation ordering within metal-hydroxide layers that form a $\sqrt{3}$ × $\sqrt{3}$ superstructure is manifested in the powder X-ray diffraction pattern by the presence of the superstructure reflection with d ~ 4.57 Å (indexed as 010 in case of 2T polytype). The same superstructure reflection was observed for chlormagaluminite [Zhitova et al., 2019b]. The powder X-ray diffraction patterns for both quintinite and chlormagaluminite were recorded using Gandolfi technique reducing the effect of the sample preferred orientation along (001). The presence of such a reflection is indicative of the M²⁺ and M³⁺ cation ordering that should further be combined with the layer stacking sequence. However, the absence of this reflection, especially taking into account its low intensity (i.e. difficulty of detection), should not been taken as an indication of the absence of cation ordering.

An indirect criterion for the topological identity of quintinites from different natural environments and places can be the value of the interlayer distance (d-value) of quintinite, which is ~ 7.56 Å (Table 5) (or slightly above if the Mg:Al ratio is higher than the ideal of 2:1 [Zhitova et al., 2016]). Quintinite findings from different localities around the globe and different genetic types are characterized by the steady d_00n-value, despite some obvious difference in physicochemical and thermodynamic conditions of their formation (Table 5). Thus, we believe that the internal structure of natural samples indicates certain principles for the construction of metal-hydroxyl layers and interlayers, as well as their interactions of the crystal structure, and which can be transferred to artificially obtained compounds. In particular, we emphasize that, under room conditions, we do not observe fundamental changes in the content of interlayer H₂O molecules and related structural transformations of quintinite.

Finally, we would like to emphasize that, according to the available structural data, for quintinites from different findings and differing in geological position, that is, the conditions of formation, the crystal structures are built in the same way: (i) the structure of the interlayers is the same, (ii) the structure of the octahedral sheet differs only in the registered ordering of Mg and Al or without registered ordering, i.e. "disordered" (although the disorder may be apparent due to the incorporation of impurities) and (iii) different types of stacking of layers.

5. Conclusions

In this work, we have refined the crystal structure of Jacupiranga quintinite and have shown that, from a structural point of view, it is identical to the 2T polytype of Kovdor quintinite and consists of metal hydroxide layers [Mg₄Al₂(OH)₁₂]²⁺ and the [(CO₃)(H₂O)₃]^2- interlayers. The Mg and Al ordering according to the $\sqrt{3}$ × $\sqrt{3}$ superstructure is detected by single-crystal X-ray diffraction and powder X-ray diffraction by the appearance of additional (to 2H pattern) low-intensity superstructure reflections, including the reflection with d₀₁₀ ~ 4.57 Å in the powder pattern. Combining our data and literature data, as well as the information on the d_00n-values of quintinites (d_00n ~ 7.56 Å) of various genetic types and localities, we conclude that the structure of quintinite obeys certain crystal chemical principles and is sustained in the light of layer-interlayer interactions and construction of octahedral sheets and interlayers. We consider it highly probable that quintinite polytypes already known from the Kovdor alkaline complex can be found in other localities, as shown in the example of quintinite from the Jacupiranga alkaline complex. In addition, we also hope that a more detailed history of the description of quintinite from Jacupiranga will lead to the correct definition of this mineral, which will help streamline the literature data on the description of hydrotalcite and quintinite.