Mineralogical Magazine, May 2016, Vol. 80(3), pp. 415–545 REVIEW OPEN ACCESS A review of the structural architecture of tellurium oxycompounds 1 2,* 3 A. G. CHRISTY ,S.J.MILLS AND A. R. KAMPF 1 Research School of Earth Sciences and Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2601, Australia 2 Geosciences, Museum Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia 3 Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA [Received 24 November 2015; Accepted 23 February 2016; Associate Editor: Mark Welch] ABSTRACT Relative to its extremely low abundance in the Earth’s crust, tellurium is the most mineralogically diverse chemical element, with over 160 mineral species known that contain essential Te, many of them with unique crystal structures. We review the crystal structures of 703 tellurium oxysalts for which good refinements exist, including 55 that are known to occur as minerals. The dataset is restricted to compounds where oxygen is the only ligand that is strongly bound to Te, but most of the Periodic Table is represented in the compounds that are reviewed. The dataset contains 375 structures that contain only Te4+ cations and 302 with only Te6+, with 26 of the compounds containing Te in both valence states. Te6+ was almost exclusively in rather regular octahedral coordination by oxygen ligands, with only two instances each of 4- and 5-coordination. Conversely, the lone-pair cation Te4+ displayed irregular coordination, with a broad range of coordination numbers and bond distances. A threshold was applied for Te4+–O links of ∼2.45 Å or 0.3 valence units with some flexibility, as a criterion to define strongly bound Te–O polymers and larger structural units. Using this criterion, Te4+ cations display one-sided 3-, 4- or 5-coordination by oxygen (with rare examples of coordination numbers 2 and 6). For both valence states of Te, examples are known of TemOn complexes which are monomeric (m = 1; neso), noncyclic finite oligomers (soro), rings (cyclo), infinite chains (ino), layers (phyllo) and frameworks (tecto tellurates). There is a clear analogy to the polymerization classes that are known for silicate anions, but the behaviour of Te is much richer than that of Si for several reasons: (1) the existence of two cationic valence states for Te; (2) the occurrence of multiple coordination numbers; (3) the possibility of edge-sharing by TeOn polyhedra; (4) the possibility for oxygen ligands to be 3-coordinated by Te; and (5) the occurrence of TemOn polymers that are cationic, as well as neutral or anionic. While most compounds contain only one or two symmetrically distinct types of Te atom, Pauling’s Fifth Rule is frequently violated, and stoichiometrically simple compounds such as CaTeO3 can have polymorphs with up to 18 distinct Te sites. There is a tendency for local symmetry features such as the threefold axis of a TeO6 4+ octahedron or the acentric symmetry of a Te On polyhedron to be inherited by the host structure; the latter in particular can lead to useful physical properties such as nonlinear optical behaviour. We develop for the first time a hierarchical taxonomy of Te-oxysalt structures, based upon (1) valence state of Te; (2) polymerization state of TemOn complexes; (3) polymerization state of larger strongly-bound structural units that include non-Te cations. Structures are readily located and compared within this classification. K EYWORDS: tellurium, oxysalt, crystal chemistry, polymerization, crystal structure, structural heirarchy. Introduction *E-mail: [email protected] TELLURIUM (Te) is an unusual element in that its DOI: 10.1180/minmag.2016.080.093 cosmic abundance is greater than that of any other © 2016 The Mineralogical Society A. G. CHRISTY ET AL. element with an atomic number >40, as measured Housley et al., 2011; Pekov et al., 2010; Christy by relative number of atoms in C1 chondrite et al., 2016). This represents the greatest flurry of (Anders and Ebihara, 1982). Nevertheless, Te is activity in the study of Te secondary minerals since one of the rarest elements in the Earth’s crust (0.4‒ the 1970s. The majority of these new minerals are 10 ppb; Parker, 1967; Levinson, 1974; Govett, also compounds new to inorganic chemistry, and 1983; McDonough and Sun, 1995; Reimann and de possess new crystal-structure types. Crystal struc- Caritat, 1998) and also in seawater (up to 0.0009 ppb; tures are now known for 55 of the ∼80 Te Andreae, 1984; Lee and Edmond, 1985). It is thus oxyminerals. It is of particular interest that the Te 3 to 5 orders of magnitude less abundant than other oxyanions show a wide range of polymerization, even-number elements that are nearby in the somewhat analogous to silicates: examples range 4+ 2– 6+ 6– periodic table, such as tin and barium, and is in from isolated [Te O3] and [Te O6] anions to fact rarer than platinum or gold. complex three-dimensional frameworks. The The extreme depletion of Te in the Earth’s crust is analogy to rock-forming silicates is strengthened probably due to its strongly siderophile character at by the observation that, in a locality with an high pressure, which resulted in much primeval Te unusually large number of tellurate species, there being sequestered in the core, and the small appears to be a correlation between polymerization amounts of Te in the outer layers of the Earth state and both the early or late position of a mineral arriving after core formation in a “late veneer” in the local paragenetic sequence, and the (Wang and Becker, 2013). The extreme scarcity of abundance of ‘network-modifying’ species such Te makes it all the more remarkable that there are as Cu2+ (Christy et al., 2016). ∼160 Te minerals described from Nature: ∼3% of A search of the Inorganic Crystal Structure all known species. Christy (2016) showed that most Database (ICSD) and recent literature has found chemical elements show a power-law dependence good-quality crystal structures for 703 compounds, between their abundance in the Earth’s crust and the in all. The number of structures referenced per year number of mineral species in which they are for the present study suggests that the rate of essential constituents. Other elements that are synthesis and structure refinement has been major constituents of 150–200 species are much increasing through time, and that 40 new more abundant, such as Ce and Ni, present in the compounds and structures per year may now be crust at 33 and 105 ppm, respectively, according to typical (Fig. 1). Thus, the current interest in both Taylor and McLennan (1985). Conversely, if Te synthetic and natural Te oxycompounds, along with followed the typical trend, there would be only the anomalously large number of the latter, justifies seven Te minerals. Tellurium is, in fact, the most a review of their structural chemistry. It should be extreme example of an element that forms an noted that new compounds appear in the literature anomalously large number of distinct species in the constantly, but we had to stop updating our working Earth’s crust. Telluride minerals, containing Te as an list in mid-2015, in order to avoid repeated anion, are probably best known, and are well studied shuffling of the database and the associated risk due to their association with gold in epithermal Au– of introducing errors. Te deposits (cf. Cook and Ciobanu, 2005; Ciobanu Examination of the known structures of Te et al., 2006), often related to alkaline magmatism oxycompounds reveals extraordinary diversity (e.g. Jensen and Barton, 2000). Rare sulfosalts are due to a combination of factors, namely: (1) Te also known in which cationic Te4+ plays a role may occur as Te4+ or Te6+, which are of comparable analogous to As3+, such as the tetrahedrite-group stability under atmospheric conditions, so mineral goldfieldite, ideally Cu102(TeS3)4S(Trudu compounds also occur with both oxidation states and Knittel, 1998). Hence, Te can adopt either coexisting. (2) Te6+ is almost invariably octahed- anionic or cationic roles as a chalcophile element, rally coordinated by oxygen (Christy and Mills, 6+ like As and Sb. Also, like those elements, it oxidizes 2013). The Te O6 group is strongly bound, in that readily to form secondary oxycompounds under the average Te–O bond valence is unity. In contrast, near-surface conditions. About half of the known Te Te 4+ has a stereoactive lone electron pair, and may minerals are such tellurites and tellurates. adopt a wide range of coordination geometries. The recent explosion of new secondary mineral Usually, three to four oxygens are strongly bound to species, particularly from Otto Mountain, form an asymmetric coordination polyhedron, but California, has seen publication of descriptions there may also be several other neighbours at longer for 14 new Te minerals from 2010 up to September, distances (Christy and Mills, 2013). (3) As noted 2015 (Kampf et al., 2010a;Backet al., 2011; above, TeOn polyhedra polymerize readily to form 416 THE STRUCTURAL ARCHITECTURE OF TELLURIUM OXYCOMPOUNDS FIG. 1. Number of crystal structure references cited per year for the current study. oligomers, chains, layers and frameworks. These repulsion, although edge-sharing of non-silicon units also link readily to other strongly-bonding tetrahedra and 3-coordination of oxygen atoms are cations to form heteropoly structural building units. seen in beryllosilicates and zincosilicates, where (4) The geometries of TeOn polymers are even more the lower cation valence decreases repulsion, and flexible than those of silicates, in that the polymers gives a small effective non-bonded radius relative may contain Te with various coordination numbers, to bond distances (O’Keeffe and Hyde, 1981).