Associated minerals

Wortupaite is intimately associated with nickel telluride mineral melonite (NiTe₂), upon which it is found growing. Calcite is the gangue mineral on the holotype specimen. Other minerals previously identified from the Wortupa mine include quartz, native gold, pyrite, chalcocite, malachite, azurite and goethite, as well as scapolite (Coats and Blissett, Reference Coats and Blissett1971), however none of these were observed on the holotype specimen.

Origin of the mineral

The first evidence of Te secondary minerals at the Wortupa mine was reported by Higgin (Reference Higgin1899) when he noted a green coating associated with melonite and suspected that it was “probably a tellurite of nickel”. Wortupaite formed after oxidation of melonite, from which its major component elements (Ni and Te) were derived, probably under near-surface conditions. The Mg²⁺ in wortupaite was most likely to have been leached from minerals such as magnesite or Mg-silicates in associated sedimentary rocks, which then mixed with the Ni- and Te-rich fluids produced on the surface of weathering melonite to form wortupaite.

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was conducted using a Nexsa Surface Analysis System at the Monash X-ray Platform (MXP) of Monash University, Clayton, Australia, and data were processed using Avantage software (both by Thermo Fisher Scientific). X-rays were generated with a Mg target producing Kα radiation with a beam diameter of 100 μm. The binding energy scale was referenced to amorphous C1s at 284.5 eV. A full-range spectrum was collected prior to collecting detailed scans in the Ni2p (845–885 eV) and Te3d regions (560–600 eV).

The XPS spectrum in the Te3d region shows two peaks at 575.3 and 585.9 eV corresponding to the 3d _5/2 and 3d _3/2 peaks of Te⁴⁺, based on comparison with the reference compound TeO₂ that displays Te⁴⁺ peaks at 576.3 and 586.7 eV (Thermo Fisher, 2013–2021).

The XPS spectrum in the Ni2p region shows a peak at 855.2 eV and a satellite at 861.4 eV, then a peak at 872.6 eV and a satellite at 878.7 eV which correspond to the 2p _3/2 and 2p _1/2 peaks of Ni²⁺, respectively, based on comparison with the reference compound NiO that shows a combined peak at 855.1 eV, satellite at 860.5 eV and then a second peak at 872.4 eV with satellite at 879.5 eV (Thermo Fisher, 2013–2021). No evidence of any Ni³⁺ was observed in the XPS spectra.

Powder X-ray diffraction

Powder X-ray diffraction (XRD) data were collected using an Oxford Xcalibur 2 system with Sapphire2 CCD and MoKα radiation (50 kV and 1 mA) at 293(1) K. A Gandolfi-type randomised crystal movement was achieved by rotations on the φ and ω axes. Data are presented in Table 2. The unit-cell parameters refined from the powder XRD pattern with whole pattern fitting using ChekCell software (Laugier and Bochu, Reference Laugier and Bochu2004) are a = 9.327(9) Å and c = 7.5946(3) Å.

Table 2. Powder XRD data (d in Å) for wortupaite with comparisons to zemannite and keystoneite shown for comparison.*

* The five strongest observed lines are highlighted in bold. Cutoff: averaged I >5.

Single-crystal X-ray diffraction

Single-crystal XRD data were collected on the micro-focus macromolecular MX2 beamline at the Australian Synchrotron, part of ANSTO. A 5 × 5 × 15 μm prism of wortupaite was selected from specimen M2021 by crushing an aggregate of crystals in oil then selecting a single-crystal fragment. We tested a number of crystal fragments from the type specimen, with the chosen crystal being the best diffractor despite providing only an average of 5 distinct spots per frame. Data were collected at 100 K by a Dectris EigerX 16M detector using monochromatic radiation with a wavelength of 0.71073 Å, collecting a full 360° of data with 1° step size and 1 second per step. The data were processed using XDS (Kabsch, Reference Kabsch2010), XPREP (Bruker, Reference Bruker2001) and SADABS (Bruker, Reference Bruker2001), finding 1830 reflections with R _int of 0.1031. Despite this low number of reflections, the crystal structure of the framework of wortupaite was readily refined, and a model for the arrangement of the channel species is presented below. Structure solution of the average structure of wortupaite was carried out by direct methods using SHELXT (Sheldrick, Reference Sheldrick2015a) followed by structure refinement in SHELXL (Sheldrick, Reference Sheldrick2015b). All atom positions and anisotropic displacement parameters for Ni and Te only were refined to final R ₁ and wR ₂ values (all data) of 0.0558 and 0.1393, respectively. Further details of data collection and structure refinement are provided in Table 3. Atomic coordinates and displacement parameters are shown in Table 4, selected bond-lengths in Table 5 and a bond-valence analysis in Table 6, using parameters of Gagné and Hawthorne (Reference Gagné and Hawthorne2015) for all atoms except Te (Mills and Christy, Reference Mills and Christy2013). The crystallographic information files have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).

Table 3. Crystal structure refinement details for wortupaite.

Table 4. The atom coordinates and displacement parameters from the crystal structure refinement for wortupaite.

Table 5. Selected bond lengths for wortupaite.

M1: Ni_0.93Fe_0.07 and M2: Mg_0.42Ni_0.08. Bond-lengths for the M3 site are not shown as it is probably coordinated by a different OW site(s), not detectable in this crystal structure refinement Fourier map.

Table 6. Bond-valence table for wortupaite.

Bond-valence parameters for all elements taken from Gagné and Hawthorne (Reference Gagné and Hawthorne2015) except for Te (Mills and Christy, Reference Mills and Christy2013). M1: Ni_0.93Fe_0.07 and M2: Mg_0.42Ni_0.08. Bond valence for the M3 site is not calculable as it is probably coordinated by a different OW site(s), not detectable in this crystal structure refinement Fourier map.

Space group for wortupaite

The space-group in which zemannite-like minerals and compounds crystallise in has generally focused on two hexagonal space-groups: centrosymmetric P6₃/m and non-centrosymmetric P6₃. Recent work has proposed a trigonal P3 symmetry for zemannite due to a different ordering scheme of the M ²⁺ and M ³⁺ cations within the framework of the zeolitic structure (Effenberger et al., Reference Effenberger, Ende and Miletich2023). The influence of framework cation ordering and also the H-bonding network leads to a non-centrosymmetric refinement in zemannite (Cametti et al., Reference Cametti, Churakov and Armbruster2017, Missen et al., Reference Missen, Mills, Spratt, Birch and Brugger2019a). However, synthetic zemannite-like compounds Na₂[Co₂(TeO₃)₃]⋅3H₂O and Na₂[Zn₂(TeO₃)₃]⋅3H₂O (Miletich, Reference Miletich1995b), which have only one framework transition metal cation crystallise in centrosymmetric P6₃/m. Refinements in both space groups were explored for wortupaite, with the centrosymmetric refinement preferred for the final refinement as the non-centrosymmetric setting did not give a stable structural refinement. The issue with the non-centrosymmetric refinement included a Flack parameter close to ½, and the presence of non-positive definite atoms such as the channel Mg²⁺ and framework O sites). Additionally, lower symmetry space groups are also possible for zemannite, with recent examples of lower symmetry found in monoclinic twin domains in zemannite-structured K[(Cu²⁺,Mn²⁺,Mn³⁺)₂(TeO₃)₃]⋅2H₂O (Eder et al., Reference Eder, Weil and Miletich2023b). Refinements in P2₁/m and P $\bar{6}$ were also undertaken but did not result in improved statistics for wortupaite.

Crystal structure description: framework

The crystal structure of wortupaite (Fig. 4) is similar to that of zemannite, with a characteristic host-guest structure (Cametti et al., Reference Cametti, Churakov and Armbruster2017; Missen et al., Reference Missen, Mills, Spratt, Birch and Brugger2019a; Effenberger et al., Reference Effenberger, Ende and Miletich2023). Like zemannite, wortupaite is a zeolitic tellurite mineral containing isolated neso Te⁴⁺O₃^2– pyramids (Christy et al., Reference Christy, Mills and Kampf2016), linked into a three-dimensional microporous framework by face-sharing Ni₂O₉ octahedra. The Te⁴⁺O₃^2– pyramids feature a typically bimodal bond-length distribution, with three short strong bonds forming the trigonal pyramid with an average Te–O distance of 1.85 Å, and four (2×2) long, weak secondary Te–O bonds with an average length of 3.03 Å (Christy and Mills, Reference Christy and Mills2013) resulting in a total bond-valence sum of 4.26 valence units (vu). Nickel (with minor substitution by Fe³⁺, represented in the crystal structure as M1) is coordinated by six O anions with a bimodal distribution: three shorter Ni1–O2 bonds with length 2.02(2) Å and three longer Ni1–O1 bonds with length 2.10(2) Å.

Figure 4. The structure of wortupaite viewed down c. Te⁴⁺O₃ trigonal pyramids in dark green, Ni²⁺O₆ octahedra in light blue and MgO₆ trigonal prisms (M2 shown only) in yellow. Drawn with Vesta (Momma and Izumi, Reference Momma and Izumi2011).

The structures of zemannite and wortupaite differ with regards to the valence of the framework cations, geometry of M(H₂O)₆ clusters in the channels and also by the presence of two (rather than one) extra-framework cation site within the channels for charge balance. In zemannite (and all other zemannite-framework minerals described to date, see below), half of the framework cations are divalent and the other half are trivalent, leading to an ordered arrangement (Missen et al., Reference Missen, Mills, Spratt, Birch and Brugger2019a; Effenberger et al., Reference Effenberger, Ende and Miletich2023). In wortupaite, all the framework cations are divalent and Ni-dominant, with no ordering by definition. The wortupaite framework has a higher negative charge (–2 in ideal wortupaite) than zemannite (–1), leading to a different arrangement of channel species than the half-occupied octahedral Mg(H₂O)₆ complexes typical of zemannite, kinichilite, ilirneyite and keystoneite.

Crystal structure description: channels

Due to the low levels of diffraction from wortupaite crystals and correspondingly low number of reflections collected in the refinement, the refinement of channel species is not unambiguous. The data are subject to Fourier truncation effects which means that the highest peaks in the difference-Fourier map (i.e. those in the channel) may originate from not having sufficient Fourier coefficients. In this section, we present the best model we refined: the model with EPMA fitted across three metal sites (one framework site, two channel sites). The crystallographic information file of the unrefined model (M1 as Ni, M2 as Mg and M3 as Ni) is also provided in Supplementary material. Once the framework species are refined, the largest remaining peak in the difference-Fourier map is at (0 0 ¾) and with a site-scattering value of ~8.6 e ^–, indicating less than full occupation by the M2 ²⁺ cation, modelled initially as a partially occupied Mg²⁺ site (again noting that the high value of z i.e. ¾ is potentially due to Fourier truncation effects). The largest peak following the refining of the first peak is at (0 0 0.86), with site scattering of ~5 e ^–, modelled initially as a partially occupied Ni²⁺ site (M3 ²⁺ cation). Both of these sites were refined with fixed U _iso values of 0.05. Refinement allowing the site occupancies to vary results in an initial tunnel composition of M2 _0.60M3_0.43, close to the ideal cation requirement of 1.0 cations pfu. Thus, the required M ²⁺ content of the channels, (Mg_0.57Ni_0.39Mn_0.04)_Σ1.00 pfu, as indicated by the EPMA data, can be reasonably attributed to two sites, i.e. M2(2a) (Mg_0.42Ni_0.08)_Σ0.5 pfu at (0 0 ¾) and M3(4e) (Mg_0.15Ni_0.31Mn_0.04)_Σ0.5 pfu at (0 0 0.846(6)) based on the site-scattering factors. Fitting occupancies with the EPMA data changes the R indices by less than 0.05%.

The highest remaining difference-Fourier peak (site-scattering factor ~2 e ^–) is at an appropriate distance from the channel cation sites and from framework O1 and O2 sites to be an OW site, defined as 50% occupied to avoid unreasonably short OW–OW distances and again positionally refined using a fixed U _iso value of 0.05. The clear preference for the OW sites in wortupaite is for an arrangement around M2 showing significant distortion away from octahedral coordination (Fig. 5). The addition of the half-occupied OW site to the model lowers the R ₁(all) to 0.054 from 0.076. Given the limited nature of the dataset, it is possible that this site is related to disorder or Fourier truncation effects, but we have chosen to present the final model including the O sites for completeness of the M2 coordination sphere (M2–OW: 2.13(5) Å × 6) (Fig. 5). It is worth noting that the quarter-occupied M3 (Ni-dominant) channel site is not expected to be coordinated by the half-occupied OW site in its refined position (which with M3 would display a one-sided coordination featuring three M3–OW bonds at 1.77(5) Å and three at 2.36(6) Å). Both the one-sided coordination and bonds to OW with distance <1.80 Å are essentially impossible for O ligand geometry around the M3 transition metal-dominant cation. Instead, M3 may be expected to be coordinated by two additional 12i sites (‘OW2’ and ‘OW3’) for 6-fold coordination, but such low-scattering sites were not locatable in the Fourier map of the available refinement data. This possibility also suggests that wortupaite may have a hydration > 3H₂O groups per formula unit (up to 6H₂O groups pfu possible as a maximum). We also explored the possibility for the OW sites (isotropic displacement parameter of the single position, if refined, is > 0.05 Ų) to be disordered over multiple sites – which would allow for a more symmetric stereochemical arrangement of OW sites around M3, as well as a reasonable geometry and bond-valence sum, however this attempted refinement was unstable. In the synthetic zemannite-like compound of Eder et al. (Reference Eder, Marsollier and Weil2023a) with the formula K₂[Co₂(TeO₃)₃]⋅2.5H₂O, channel cation positions were found in positions similar to those of the OW sites in wortupaite. We tried to refine a model with the channel cation sites away from the centre of the channel, but the refinement was also unstable.

Figure 5. M2 geometry in wortupaite, with the Mg-dominant site showing symmetrical MgO₆ trigonal prisms.

Relationship to other species

Wortupaite is structurally most closely related to the two synthetic zemannite-like compounds, Na₂[Co₂(TeO₃)₃]⋅3H₂O and Na₂[Zn₂(TeO₃)₃]⋅3H₂O (Miletich, Reference Miletich1995b), which also have frameworks with a formal –2 charge. Other related compounds include a selenite analogue of the zemannite structure type, K₂[Ni₂(SeO₃)₃]⋅2H₂O (Wildner, Reference Wildner1993), and an unpublished Sr-dominant Co zemannite, Sr_x[Co₂(SeO₃)₃]⋅yH₂O (Wildner, Reference Wildner1993), the latter featuring a fully occupied divalent channel cation (Sr²⁺). Ni–O bond lengths in the Ni²⁺O₆ octahedra of K₂[Ni₂(SeO₃)₃]⋅2H₂O are two triplets of 2.057(3) Å and 2.137(4)Å (average 2.097 Å). The Na⁺-containing compounds such as Na₂[Co₂(TeO₃)₃]⋅3H₂O usually have Na⁺ cations situated near the edges of the microporous channels, rather than in the centre as is typical for zemannite. A series of mainly cryptocrystalline zemannite-like compounds with Na⁺ in the centre of the microporous channels (compositions ranging from Na_1.25Fe³⁺_0.75Zn_1.25(TeO₃)₃⋅3H₂O to Na_0.55Fe³⁺_1.44Zn_0.56(TeO₃)₃⋅3H₂O) have also been synthesised (Missen et al., Reference Missen, Mills and Spratt2019b). In the studied sample of wortupaite, two partially-occupied channel cation sites were located [at different positions with coordinates in the form (0 0 z)]. The only previously described naturally occurring Ni-bearing Te oxysalt keystoneite, ideally Mg_0.5Ni²⁺Fe³⁺(TeO₃)₃⋅4H₂O, has Ni–O bonds with an average length of 2.085 Å. In addition, several new zemannite-like compounds have recently been reported for the first time, including Ni-bearing Na₂[Ni₂(TeO₃)₃]⋅2.5H₂O and K₂[Ni₂(TeO₃)₃]⋅H₂O (Eder et al., Reference Eder, Marsollier and Weil2023a) and the first Jahn-Teller distorted framework in a zemannite-like structure observed in K[(Cu²⁺,Mn²⁺,Mn³⁺)₂(TeO₃)₃]⋅2H₂O (Eder et al., Reference Eder, Weil and Miletich2023b). The Ni-bearing compounds have Ni–O distances of 2.064 and 2.069 Å, respectively – close to the average 2.06 Å bond length in the wortupaite framework. There are likely to be many more zemannite-structured compounds yet to be discovered as minerals or to be synthesised in the laboratory.