Crystal structure

Many detailed studies of the crystal structure exist (e.g. Obradors et al., Reference Obradors, Collomb, Pernet, Samaras and Joubert1985; Utsunomiya et al., Reference Utsunomiya, Tanaka, Morikawa, Marumo and Kojima1988; Moore et al., Reference Moore, Sen Gupta and Le Page1989; Wagner Reference Wagner1998). It is based on an essentially closest-packed arrangement of oxygen (O) and A atoms, with B metals occupying voids. One fundamental building block, S, forms a CCP two-layer sequence, ⋅cc⋅. A fraction of the interstitial sites is occupied by metal atoms in the same fashion as in the spinel structure, which gives an overall composition {B ₆O₈}²⁺ of the block. A different block, denoted R, is built up of a three-layer HCP sequence, ·hhh·. A quarter of the O atoms of the intermediate h layer is replaced by a large cation A (usually Ba²⁺, Pb²⁺, Ca²⁺ or K⁺ in minerals). Taking the interstitial B atoms into consideration, R is equal to {AB ₆O₁₁}^2– in composition. By stacking of the blocks along the hexagonal c axis in the sequence ⋅RSR*S*⋅, with a repeat of 22–23 Å, the magnetoplumbite unit cell with Z = 2 is obtained (Fig. 1). Starred blocks are rotated 180° in accordance with the space-group symmetry of the crystal structure, P6₃/mmc. The a unit-cell dimension is ~5.6 Å (= 4 × the radius of O^2–).

Fig. 1. Polyhedral representation of the ideal magnetoplumbite-type structure viewed approximately along [310]. The M1 octahedra (yellow) and the M3 tetrahedra (orange) are in the central section of the S block. The trigonal bipyramidal M2 positions in (green), face-sharing M4 octahedra (blue) and the large A atoms (grey spheres) belong to the central part of the R block. Layers of edge-sharing M5 octahedra (red) are sandwiched between the cores of blocks.

In the structure, the large A cation is ideally 12-coordinated to O, forming a triangular orthobicupola, at (⅔, ⅓, ¼). The interstitial B atoms occupy five unique sites with designations M1–M5. (Table 2). The five-fold coordinated M2 atom, ideally located at the centre of a trigonal bipyramid (2b), is in reality slightly displaced (split) into two statistically half-occupied, pseudotetrahedral 4e sites (Obradors et al., Reference Obradors, Collomb, Pernet, Samaras and Joubert1985). This kind of disorder is dynamic in most situations, i.e. a rapid diffusion of the metal atom takes place through the mirror plane of the bipyramid (Kimura et al., Reference Kimura, Ohgaki, Tanaka, Morikawa and Marumo1990; Kreisel et al., Reference Kreisel, Lucazeau and Vincent1998; Du and Stebbins, Reference Du and Stebbins2004; Krzątała et al., Reference Krzątała, Panikorovskii, Galuskina and Galuskin2018). The M4 coordination polyhedra are trigonally distorted octahedra that occur in pairs sharing a common face in a hematite-like arrangement, i.e. forming B ₂O₉ dimers. The total unit-cell contents for an AB ₁₂O₁₉ compound can thus be expressed as A ₂[^{6}(M1)₂ ^{5}(M2)₂ ^{4}(M3)₄ ^{6}(M4)₄ ^{6}(M5)₁₂]_Σ24O₃₈.

Table 2. Properties of crystallographic sites for A and B metal atoms in magnetoplumbite-group minerals.

CN – coordination number

The magnetic structure of magnetoplumbite can be described by the Néel model of ferrimagnetism. The spin orientation of Fe³⁺ at each site (Table 2) is a result of superexchange interaction through the O^2– ions. As the cation has a spin-only magnetic moment of 5 μ_B (Bohr magnetons), the total magnetisation per formula unit would be (6–2–2 + 1+1) × 5 μ_B = 20 μ_B at absolute temperature, which is in good agreement with experimental results (Kojima, Reference Kojima and Wohlfarth1982). Magnetoplumbite possesses a large magnetocrystalline anisotropy, which is related to a strong preference of the magnetic moments of the ions to align along c.

β-alumina (diaoyudaoite), ideally Na[Al₁₁]O₁₇, is a structural derivative of magnetoplumbite (Felsche, Reference Felsche1968) and a common solid-state ion conductor and catalyst. The three O3 atoms at 6h (x, –x, ¼) in the middle h layer of the R block have collapsed to a single point 2c (⅓, ⅔, ¼), compensating for the total lower charge of the metal atoms in this compound. Consequently, R encompasses {AB ₅O₉}^2– and does not contain the nominally 5-coordinated M2 site. The mirror planes at z = ¼ and ¾ correspond to the ion conduction layer in β-alumina.

Nomenclature

Subdivision

The nomenclature is devised to be simple and flexible at the same time. The group is divided into subgroups based on composition, specifically the dominant A-type cation (Table 3). The rationale for this scheme is that variations in A atom composition tend to be less complex compared to that of B atoms, and information on the precise stoichiometry, including any structural vacancies at cation or anion sites that might be present, is not necessary to determine the position at the highest hierarchal level in the group.

Table 3. Classification of the magnetoplumbite group.

Definition of species

Individual species of the magnetoplumbite group are further defined from their composition and distribution of cations over the B-type positions (Table 4). Monovalent, divalent, tetravalent and pentavalent cations are incorporated in the magnetoplumbite structure by charge-coupled substitutions of A ²⁺ or B ³⁺ ions (Table 5). It is evident that a large number of theoretically possible combinations of cation arrangements exist. However, studies on both minerals and synthetic materials show that most cations exhibit preferential ordering depending on their ionic size, charge and electronic configuration (Grey et al., Reference Grey, Madsen and Haggerty1987; Wagner and O'Keefe, Reference Wagner and O'Keeffe1988; Xie and Cormack, Reference Xie and Cormack1990; Bermanec et al., Reference Bermanec, Holtstam, Sturman, Criddle, Back and Scavnicar1996; Holtstam, Reference Holtstam1996; Nagashima et al., Reference Nagashima, Armbruster and Hainschwang2010). An important trend observed is that divalent B-type ions strongly prefer the tetrahedrally coordinated M3 sites (Batlle et al., Reference Batlle, Obradors, Rodriguez-Carvajal, Pernet, Cabañas and Vallet1991), whereas highly charged species, like Ti⁴⁺, Mn⁴⁺ and Sb⁵⁺, become enriched in the M4 octahedra (Doyle et al., Reference Doyle, Schofield, Berry, Walker and Knight2014; Nemrava et al., Reference Nemrava, Vinnik, Hu, Valldor, Kuo, Zherebtsov, Gudkova, Chen, Tjeng and Niewa2017). For compositions with a high degree of replacement of trivalent ions, divalent species also become concentrated at octahedrally coordinated sites, preferentially M5 (Cabañas et al., Reference Cabañas, Gonzlez-Calbet, Rodríguez-Carvajal and Vallet-Regi1994). Some trivalent d cations (Cr³⁺ and Mn³⁺) are ordered at the distorted M5 octahedra (e.g. Katlakunta et al., Reference Katlakunta, Meena, Srinath, Bououdina, Sandhya and Praveena2015; Shlyk et al., Reference Shlyk, Vinnik, Zherebtsov, Hu, Kuo, Chang, Lin, Yang, Semisalovaeh, Perov and Langer2015; Nemrava et al., Reference Nemrava, Vinnik, Hu, Valldor, Kuo, Zherebtsov, Gudkova, Chen, Tjeng and Niewa2017). This behaviour is explained largely by crystal-field effects. The Fe³⁺ cation, in cases when diluted in the compound and less abundant among B positions, e.g. in hibonite, is accumulated at M2 and M3 (Holtstam, Reference Holtstam1996; Medina and Subramanian, Reference Medina, Li and Subramanian2017). Al³⁺ in turn, when competing with other trivalent species, tends to be concentrated at M1 (Bermanec et al., Reference Bermanec, Holtstam, Sturman, Criddle, Back and Scavnicar1996), with the smallest octahedral volume.

Table 4. Major components at the cation sites of magnetoplumbite-group minerals. Species-defining elements are given in bold.

Table 5. Significant types of substitution in the magnetoplumbite group.

* Describes the relation between magnetoplumbite and plumboferrite, and explains non-stoichiometry in some synthetic magnetoplumbite samples (Holtstam, Reference Holtstam2003).

New or unaccredited mineral compositions

In the literature, analytical data are available that suggest the existence of new, yet officially unrecognised members of the group. Titanium-rich analogues of yimengite and hawthorneite (~5 Ti atoms per formula unit) were analysed by Lu and Chou (Reference Lu and Chou1994). Lu et al. (Reference Lu, Liu and Xiao2007) have described a “Ca analogue to yimengite” or rather a Ca analogue to hawthorneite, which would fit into the hibonite subgroup. Rezvukhin et al. (Reference Rezvukhin, Alifirova, Korsakov and Golovin2019) recently found yimengite with high Al (>1 atom per formula unit) contents. Sandiford and Santosh (Reference Sandiford and Santosh1991) described zoned ‘hibonite’ grains with REE-rich cores (ΣREE > 0.6 atoms per formula unit). Holtstam (Reference Holtstam1994) reported a Ti-rich magnetoplumbite sample for which Ti > Fe³⁺ at M4 could be inferred (a possible Pb analogue to batiferrite). A Mn³⁺-analogue to plumboferrite was detected by Chukanov et al. (Reference Chukanov, Aksenov, Jančev, Pekov, Göttlicher, Polekhovsky, Rusakov, Nelyubina and Van2016). Furthermore, Chukanov et al. (Reference Chukanov, Vorobei, Ermolaeva, Varlamov, Plechov, Jančev and Bovkun2019) recently published analyses of a Ba-dominant analogue to nežilovite and of an Al analogue to yimengite.

From a vast amount of studies of synthetic compounds, it can be speculated that many new natural members exist with, for example: A = Sr²⁺, REE (Ce³⁺, La³⁺ etc.), Mg²⁺, Rb⁺, Cs⁺ or Ag⁺ along with enrichment in the B positions (non-exhaustive list) of: Si⁴⁺, Sc³⁺, Ti³⁺, V²⁺, V³⁺, V⁴⁺, Co²⁺, Ni²⁺, Cu²⁺, Ga³⁺, Ge⁴⁺, Zr⁴⁺, Nb⁵⁺, In³⁺, Sn⁴⁺, Te⁴⁺, Ta⁵⁺ or Bi³⁺ (e.g. Coutellier et al., Reference Coutellier, Ferrand, Daval, Grange and Joubert1984; Morgan and Miles, Reference Morgan and Miles1986; Li et al., Reference Li, Medinam, Stalick, Sleight and Subramanian2016). The range of possible cation valences seem to be limited to 1–3 for A and 2–5 for B sites, which has implications when casting formulae of uncharacterised members of the group. Particular caution is needed for samples containing some of the divalent ionic species, as Fe²⁺, Mg²⁺ and Pb²⁺ have been shown to enter both kinds of sites. Substitutions at anion sites seem to be limited for this structure type.