Introduction

Nearly two hundred tellurium (Te) minerals have been described, around half of which contain oxygen. Though generally rare, the Te oxysalt minerals are remarkably chemically and structurally diverse (Christy et al. 2016; Missen et al. 2020a). Utahite was first described by Roberts et al. (1997a) with the formula Cu5Zn3(Te6+O4)4(OH)8·7H2O, one in a series of copper–tellurium oxysalts described from the Centennial Eureka mine in Juab County, Utah, USA (39°56’36’’N, 112°7’19’’W). The Centennial Eureka mine produced copper (Cu), silver (Ag) and gold (Au), resulting in the accumulation of large dumps during its main operation (most production ceased almost a century ago). It was not until these dumps were removed in 1991 to process them for their Au content decades later that the rich assemblage of Cu–Te6+–O minerals was exposed (Roberts et al. 1997a). Roberts and co-workers described six new Cu–Te6+–O minerals between 1994 and 1997, namely mcalpineite (Roberts et al. 1994), frankhawthorneite (Roberts et al. 1995; Grice and Roberts 1995), jensenite (Roberts et al. 1996a; Grice et al. 1996), leisingite (Roberts et al. 1996b; Margison et al. 1997), utahite (Roberts et al. 1997a) and juabite (Roberts et al. 1997b). Mcalpineite has a cotype locality, namely the eponymous McAlpine mine in Tuolomne County, Northern California, while the other five were described from the Centennial Eureka mine alone. Eurekadumpite was noted at the same time but was not described for over a decade (Pekov et al. 2010, 2011). Structure determinations exist for all of these minerals except for utahite and eurekadumpite. Here we present structural and spectroscopic data from an exceptionally well-crystallised specimen of utahite. This structural determination forms part of our broader study on the re-examination of all Te secondary minerals without known crystal structures.

Specimen descriptions

The portion of the type specimen of utahite (from the Centennial Eureka mine) examined in this study consists of an aggregate of utahite laths, sub-mm in size, embedded in a resin block, stored in the collections of the Natural History Museum, London, specimen number BM 1994,99. This was the portion of the utahite cotype specimens used in the original study (Roberts et al. 1997a) for the electron microprobe work and an attempted reflectance study, subsequently referred to as the NHM cotype in this paper. The remaining cotype material consists of three specimens and an SEM stub stored in the National Mineral Collection at the Geological Survey of Canada, Ottawa, catalogue number 67,415 (Roberts et al. 1997a).

An additional specimen of utahite was used for structural determination, and as such is now designated as an additional cotype specimen for utahite, from a new cotype locality. The specimen is from the Empire mine, Cochise County, Arizona, USA. It is stored in the collections of the Royal Ontario Museum (ROM), Toronto, with specimen number M59810 and was obtained by the ROM from Peter Megaw. Utahite forms several sprays of light blue crystals on this specimen along with very dark green (almost black) khinite and a variety of light and dark green crusts of poorly crystallised Cu–Te–O oxidation products (Fig. 1). Utahite crystals may reach 0.3 mm in length but are usually 0.1 mm long. Several spatially-separated crystal sprays cover almost 1 cm2 of the sample.

Fig. 1
figure 1

Optical images of utahite crystal clusters on an unknown green mineral, probably a poorly crystallised copper tellurite (ROM utahite specimen, M59810)

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Electron probe microanalysis (EPMA)

Chemical analytical data for the NHM cotype specimen (BM 1994,99) were collected on a Cameca SX100 Electron Microprobe (WDS mode, 20 kV, 20 nA, 1 μm beam diameter and PAP matrix correction) at the Imaging and Analysis Centre, Core Research Laboratories, Natural History Museum, London. The following elements were also analysed for but found to be below detection limit: Si, S, Ti, Mn, Fe, As, Se, Ag, Pb and Bi.

Chemical analytical data on the additional cotype (ROM specimen M59810) were performed on a JXA-8530 F Field Emission Electron Microprobe (WDS mode, 15 kV, 5 nA, 20 μm beam diameter and PAP matrix correction), at the School of Earth Sciences, University of Melbourne, Australia. Fe was also analysed for but found to be below detection limit.

Note that although the totals for the two analyses differ by 10 %, the elemental ratios obtained are nearly identical. The difference in totals probably arises from the different conditions under which the two data sets were generated, a consequence of the analyses for the two specimens being performed on different instruments. Results, including the standards used for each EPMA data collection, are listed in Table 1. The empirical formulae for the NHM and ROM specimens (based on 24 O + OH anions pfu, with H calculated based on the crystal structure) are Mg0.96Na0.12Ca0.05Cu3.67Al0.04Zn2.19Te2.86Sb0.04P0.02O24H16.13 and Mg1.01Na0.12Cu3.78Zn2.05Te2.98Sb0.01O24H16.13 respectively. Substitutions are necessary to evenly fill the metal atom sites in the crystal structure, as the elemental totals from EPMA are not in integer ratios. These substitutions are Zn and Na for Cu (in different Cu-dominant sites), and Sb for Te in both specimens, along with Ca for Mg, and Al for Te in the NHM cotype specimen. It is worth noting that the calculated levels of Na in utahite may be affected by the Zn contents, with a peak overlap of emitted X-rays known for Na (Na Kα 1.041 keV) and Zn (Na Lα 1.012 keV). The grouped empirical formulae are (Mg0.96Ca0.05)Σ1.01(Cu3.67Zn0.20Na0.12)Σ3.99Zn1.99(Te2.86Sb0.04Al0.04P0.02)Σ2.96O13.87(OH)4.13·6H2O and Mg1.01(Cu3.78Na0.12Zn0.07)Σ3.97Zn1.98(Te2.98Sb0.01)Σ2.99O13.87 (OH)4.13·6H2O, respectively. The site-filling scheme was determined using the electron microprobe data in conjunction with structural considerations (see below). The ideal formula is MgCu2+4Zn2Te6+3O14(OH)4·6H2O, requiring MgO 3.38, CuO 26.69, ZnO 13.65, TeO3 44.19 and H2O 12.09, total 100 wt%.

Table 1 EPMA data for the two utahite specimens analysed, including Roberts et al. (1997a) for comparison
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Raman spectroscopy

Micro-Raman spectra of randomly oriented single crystals of utahite (Fig. 2) from the ROM additional cotype specimen were collected with a Horiba LabRAM ARAMIS micro-Raman spectrometer at the Royal Ontario Museum in Toronto, Canada. The 532 nm excitation line of a 50 mW Ar-ion laser was focused with a 50 LWD (0.5) objective lens created a 1.3 μm spot on the sample surface. Laser power was filtered to 25 % and a 400 μm slit, 100 μm hole, and 1200 lines/mm grating were used for data collection. An early data collection with laser power filtered to 50 % burned the sample. Data was collected for a total of 1200 s in the spectral range 4000 to 100 cm− 1, in 30 s intervals x 10 iterations, over 4 windows. The wavenumber accuracy was ± 1 cm− 1. The instrument was calibrated with a Si standard (521 cm− 1).

Fig. 2
figure 2

The Raman spectrum of utahite (from ROM M59810, additional cotype specimen)

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Between 1000 and 100 cm− 1, the spectrum confirms the presence of Te6+–O bonds. Two strong peaks, one centred on 774 cm− 1 and the other on 637 cm− 1, are attributed to the symmetric ν1 stretches of the two Te6+–O environments, namely Te6+O4(OH)2 octahedra and Te6+2O10 dimers. Additionally, the 774 cm−1 peak has a shoulder at 712 cm−1, attributable to the antisymmetric ν3 stretching mode of Te6+–O bonds. Compared to other minerals containing Te6+2O10 dimers, the mineral eckhardite (Kampf et al. 2013) has major Raman peaks at 729 and 692 cm−1, while thorneite (Kampf et al. 2012) has major peaks at 833 cm−1 and 710 cm−1 with a shoulder at 670 cm−1. The utahite peak at 1096 cm−1 may be attributable to minor phosphate (as detected in the NHM cotype specimen), probably occupying channel space. The region below 600 cm− 1 contains bands attributable to H2O libration modes, Te–O bending modes and vibrations of the Cu- and Zn-centred polyhedra, as well as lattice modes. The strongest of these are well-defined peaks at 547 cm−1 (weak, probably a H2O libration mode), 426 cm−1 (somewhat weak) and 233 cm−1 (moderately strong).

Between 4000 and 2000 cm− 1 (Fig. 2, inset), the Raman spectrum confirms the presence of O–H bonds, with a broad maximum formed from the overlap of at least two peaks, indicating two distinct average O–H bond lengths. The peak centred on 3148 cm− 1 is sharper, while the shoulder peak centred on 3516 cm− 1 and extending towards 3600 cm− 1 is broader. Using the correlation curve of Libowitzky (1999), the two mean H-bond donor–acceptor O···O distances in utahite are 2.92 Å and 2.69 Å. Above-background Raman intensity is observed between 2800 and 3600 cm− 1, thus a spread of O···O values may be expected. Utahite contains two OH and three H2O groups, meaning that many O···O distances (see also Table 4) are observed in the utahite crystal structure including OW1···O8 (2.79 Å) and OW2···O3 (3.09 Å). Further variation in O···O values is expected as the OW sites are more mobile than O or OH sites. Further discussion of a hydrogen-bonding network is provided below.

Single crystal X-ray diffraction (SCXRD)

Experimental

The single crystal X-ray diffraction experiment was carried out on a Synergy diffractometer (Rigaku Oxford Diffraction) at the School of Chemistry, University of Melbourne, Australia, using graphite-monochromated MoKα radiation (50 kV and 1 mA). The refined single-crystal X-ray diffraction pattern (details below) was used to generate a powder X-ray diffraction pattern, which was compared to the PXRD pattern reported for type utahite (Table 2) by Roberts et al. (1997a). The close match between the two PXRD patterns for both d spacings and measured intensities show that the studied sample of utahite is essentially identical in structure to type utahite.

Table 2 Comparison of calculated powder X-ray diffraction (PXRD) pattern of utahite (ROM M59810) from the Empire mine with the observed NHM cotype specimen (BM 1994,99) PXRD pattern. Cutoffs: averaged I > 5 and d > 2 Å
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A detailed search for the highest quality single crystals of utahite found that all crystals of utahite examined are twinned, with some overlapping reflections from the individual components. For this reason, reflections were placed into HKLF4 (reflections for one twin domain only) and HKLF5 (reflections for both twin domains) files. The refinement of the structure was performed using the HKLF4 dataset, with overlapping reflections excluded. Refinement using data from both twin domains (HKLF5) yielded a poorer quality dataset with an unacceptably high Rint value. In addition, refinement using the HKLF5 dataset yielded larger residual electron density peaks than those obtained using the HKLF4 data. Reflection intensities were integrated, corrected for Lorentz and polarisation effects and converted to structure factors using the program CrysalisPro® (Rigaku Oxford Diffraction), finding 5640 reflections with an Rint of 0.0959. Structure solution in P\(\stackrel{-}{1}\) was carried out using SHELXT (Sheldrick 2015a). Structure refinement by full-matrix least-squares was implemented by SHELXL (Sheldrick 2015b), using neutral atomic scattering factors. Four reflections with calculated F0/FC errors greater than 5.00 were excluded from the final refinement.

Only cation sites were refined anisotropically, except for the channel Mg site which was left isotropic. Thus, the Cu1 site was refined as 95 % Cu and 5 % Zn, the Cu3 site was refined as 88 % Cu and 12 % Na and the Te2 site was refined as 99 % Te and 1 % Sb. The joint-occupancies described here are based on the common substitution of Zn2+ for Cu2+, Na+ for Jahn-Teller distorted Cu2+ and the joint-occupancy of Sb5+ and Te6+ in joëlbruggerite (Mills et al. 2009a). The refinement converged to final R1 and wR2 (all data) values of 0.126 and 0.296, respectively. Details of data collection and structure refinement are provided in Table 3. Despite these values being relatively high, all fractional atom coordinates and atom displacement parameters (Uij) as shown in Table 4 result in a chemically sensible structure. Selected bond lengths are provided in Table 5. A bond valence table is provided in Table 6.

Table 3 Crystallographic information relating to data collection and refinement of utahite
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Table 4 Fractional atom coordinates, occupancies and displacement parameters for the atomic sites of utahite
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Table 5 Bond lengths (Å) table for utahite*
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Table 6 Bond valence sums (in valence units, vu) for utahite
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Structure description

Utahite has a three-dimensional framework structure (Fig. 3). Jahn-Teller distorted Cu1O4(OH)2 octahedra and Cu2O6 octahedra (in a 2:1 ratio of Cu1- to Cu2-centred octahedra) form ribbons of average composition [CuO2.67(OH)1.33]n via edge-sharing along b. Two Te1O6 octahedra share edges to form Te16+2O10 dimers, which cross-link the [CuO2.67(OH)1.33]n ribbons along the a direction, with each Te16+2O10 dimer forming eight edge-sharing links to the [CuO2.67(OH)1.33]n ribbons. In vacant sites (one sixth of the octahedral sites in the overall anionic [Cu3Te12O8(OH)4]2− layers are vacant), ZnO3(OH) tetrahedra sit above and below the plane of the layers, with three of the tetrahedral vertices to the layers, and one (O4) pointing into the interlayer. This O4 site is an important vertex for both isolated Te26+O4(OH)2 octahedra and Cu3O4(H2O)2 octahedra, which edge-share with each other to form [Cu3Te2O4(OH)2(H2O)2]2− chains in the interlayer along a, with linkage to the [Cu3Te12O8(OH)4]2− layers through the ZnO3(OH) tetrahedra. The Cu3O4(H2O)2 octahedra display the greatest Jahn-Teller distortion of the Cu sites.

Fig. 3
figure 3

Crystal structure of utahite in two orientations. Cuφ6 polyhedra are in shades of blue, Teφ6 octahedra in dark green, ZnO3(OH) tetrahedra in grey and Mg(H2O)6 octahedra in orange. a Viewed down a, showing layers parallel to the ab plane. Note that the Cu2, Cu1, Cu1 pattern is repeated ad infinitum. Right-hand Cu3 not shown so that Te2 is visible. b Layer of the framework viewed down c, with ZnO4 tetrahedra found above and below the octahedral vacancies

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Between these chains are the Mg(H2O)6 octahedra, connected to the framework by hydrogen bonds and through the long Cu3–OW3 bonds. Due to the twinned nature of the crystal and high thermal motion of the OW sites, H-atom positions could not be located; however, the O, OH or H2O character of each O site can be inferred using bond-valence and charge-balance considerations (see Table 6). The OW sites bonded to the Mg site of the Mg(H2O)6 octahedra have an average bond-valence of 0.34 vu, with the remaining valence provided by two O–H bonds. Underbonded O sites around the edge of the channels containing Mg(H2O)6 octahedra gain additional valence from accepting H-bonds donated by the channel H2O groups.

Additionally, framework O sites with a bond-valence of approximately 1 vu are expected to be OH sites. The O3 site is bonded only to the Te2 site with a Te2–O3 bond-valence of 0.90 vu. O3 points into the interlayer space, providing ample room for the associated H atom. The O1 site, one of the three vertices where the ZnO4 tetrahedra are connected to the main layers, is the other fully-occupied OH site. These two OH sites (O1 and O3) provide the required 4OH pfu required for charge balance. A small percentage of the other O sites may also be OH sites < 5 % of the time, adding extra charge to compensate for the 12 % Na in the Cu3 site and 1 % Sb in the Te2 site.

Comparison with other te oxysalt structures

The majority of Te6+ minerals contain neso Te6+O6 octahedra, with a minority (including utahite) having soro non-cyclic clusters or higher connectivity of Te6+O6 octahedra (Christy et al. 2016). Schieffelinite (Kampf et al. 2012) and chromschieffelinite (Kampf et al. 2012) contain [Te6 + 2O8(OH)3]7− dimers formed from corner-sharing of two Te6+φn octahedra, where φ = O or OH. Schieffelinite and chromschieffelinite also contain an additional neso Te6+φn isolated octahedral unit, [Te6+O4(OH)2]4− in utahite and [Te6+O2(OH)4]2− in schieffelinite and chromschieffelinite (Kampf et al. 2012). Thorneite (Kampf et al. 2010) and eckhardite (Kampf et al. 2013) both have Te6+2φ10 dimers formed from the edge-sharing of two Te6+O6 octahedra in their structures, but no additional neso Te6+φn octahedra.

Utahite also has some notable structural similarities to other Cu–Te6+–O minerals from the Centennial Eureka mine. Utahite has been observed growing directly with other Te-oxysalts, especially with leisingite (Fig. 4). The utahite and leisingite (Fig. 5) structures are rather similar, in that both have layers composed of Cuφ6 and Teφ6 octahedra with Mg(H2O)6 octahedra found between the layers (Margison et al. 1997). However, utahite also has additional Te- and Cu-centred polyhedra between the main layers and ZnO3(OH) tetrahedra providing linkage to additional interlayer sites, compared to the low-Zn (0.45 wt%) leisingite, which has no Zn-dominant sites. Minerals which grow nearby or overlie each other may have similar crystal structure motifs due to the phenomenon of topotactic growth (Glasser et al. 1962; Marler and Gies 2012; Génin et al. 2012). Topotactic growth has been explored for a variety of minerals and related synthetic compounds, such as silver tellurides (Baumer and Nilges 2017), the complex silicates of the chevkinite group (Galanciak et al. 2020) and most pertinently, in secondary minerals such as arsenates like jeankempite, hypothesised to grow topotactically following the dehydration of underlying guérinite (Olds et al. 2020).

Fig. 4
figure 4

Utahite (blue) growing in direct association with leisingite (yellow). Both specimens are from Centennial Eureka Mine. a Field of view 1 mm, available from https://www.mindat.org/photo-797227.html, photo credit Douglas Merson. b Field of view 2 mm, available from https://www.mindat.org/photo-482940.html, photo credit Brent Thorne

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Fig. 5
figure 5

The crystal structure of leisingite in two orientations. Colour scheme is identical to that used for utahite. a Viewed down a showing layers parallel to the ab plane. b Viewed down c. Interlayer sites not shown for clarity. Note similarities to utahite

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There are likely to be many more Mg–Cu–Zn–Te–O compounds which are yet to be synthesised, or potentially found in Nature. The incorporation of Mg2+ into the more commonly studied Cu–Zn–Te–O system would provide a further layer of chemical and structural complexity. For instance, in the synthetic Cu–Zn–Te6+–O system, Cu5Zn4(TeO6)3 and Cu3Zn3(TeO6)2 have been characterised (Wulff and Müller-Buschbaum 1998) but no compounds with additional Mg2+ are known. Three compounds in the Cu–Zn–Te4+–O system with additional anions (SxO42− or Cl−) have also recently been described, showing the versatility of adding new elements to a system, whether anions or cations (Missen et al. 2020b). This study again shows that there are many more diverse Te–O crystal structures to be discovered both in Nature and in the laboratory.