Occurrence

The Kingsgate mines are situated in rugged, heavily wooded country at the eastern edge of the highly mineralised New England Tableland and comprise more than 60 workings, shafts, tunnels and small open cuts (England, Reference England1985). The deposit was first exploited for tin in the late 1800s, but grades were too low to be profitable. The discovery of native bismuth, bismuthinite and molybdenite resulted in intermittent mining until the 1950s for bismuth, molybdenum and piezo-electric quartz. The deposit has produced outstanding specimens of bismuth, molybdenite and quartz. Mineralisation occurs in quartz pipes that are contained in a coarse, mottled grey hornblende–biotite granite. More than 70 pipes have been mapped and show an unusually complex structure. The Old 25 Pipe is one of the larger pipes and was worked by opencut and stoping to a depth of 250 feet. Records are incomplete, but at least 350 tonnes of molybdenite and 200 tonnes bismuth are known to have been produced. The major primary minerals are native bismuth, bismuthinite and molybdenite (Lawrence and Markham, Reference Lawrence and Markham1962; Clissold et al., Reference Clissold, Leverett, Sharpe and Williams2008) with lesser amounts of other sulfides. A suite of secondary minerals, including molybdates, tungstates, phosphates and arsenates including such minerals as ferrimolybdite, gelosaite, wulfenite, powellite, scorodite and carminite, have been described by England (Reference England1985) and Sharpe and Williams (Reference Sharpe and Williams2004). During the present study, mambertiite and tancaite-(Ce) have also been identified. Secondary minerals have formed under supergene conditions at low pH and low temperature.

Kingsgateite occurs in cavities in a matrix comprised of quartz and minor muscovite. Associated minerals are molybdenite, gelosaite and mambertiite. Kingsgateite is the last mineral to be formed in the assemblage and is observed overgrowing molybdenite and gelosaite. Kingsgateite is the first mineral containing Zr to be recorded from Kingsgate. Accessory zircon in the host granite is probably the source of Zr, although it has not been recorded from the locality.

Physical and optical properties

Kingsgateite crystals are tabular in habit, with a maximum dimension of ~0.12 mm (Figs 1 and 2). Very small crystals are yellowish green in colour and transparent, larger crystals are bluish grey. Crystal forms are {001} and probably {100}. The lustre is vitreous, and the streak is white. The mineral is brittle; the hardness could not be determined because of the small size of the crystals. The fracture is uneven and we did not observe signs of a distinct cleavage. Density was not measured because of the scarcity of available material; the calculated density based on the empirical formula is 3.74 g/cm^–3. Kingsgateite exhibits anomalous biaxial (+) optics manifest as sector growth (Fig. 3). The sector growth, small crystal size and high indices of refraction prevented the measurement of the indices of refraction. The Gladstone–Dale relationship predicts an average index of refraction of 1.905 for the ideal formula. The indices of refraction based on this average and on birefringence measurements: (γ – α = 0.08 and β – α = 0.01) are α = 1.88, β = 1.89, γ = 1.96, providing 2V (calc.) = 42.6°. However, phases containing Mo⁶⁺ with short Mo=O bonds generally have higher average indices of refraction than those predicted by Gladstone–Dale, so the actual indices of refraction are likely to be significantly higher. Pleochroism is X and Y = colourless, Z = blue; X = Y << Z. Dispersion could not be observed.

Fig. 1. Tabular crystals of kingsgateite on quartz. The field of view is 1 mm across.

Fig. 2. Scanning electron microscopy photomicrograph showing a tabular crystal of kingsgateite 40 μm across on muscovite.

Fig. 3. Kingsgateite crystal showing anomalous birefringence. The image is looking down the c axis under crossed polars with the crystal 45° from the extinction position and the gypsum (first-order red) plate inserted with its slow direction oriented NE–SW.

Chemical composition

A small group of kingsgateite crystals was mounted in epoxy, polished, and carbon coated. Quantitative chemical analysis was obtained using a Cameca SXFive electron microprobe working in wavelength-dispersion mode with an accelerating voltage of 15 kV, beam current = 20 nA and beam diameter = 5 μm. Analytical data (12 analysis points) are given in Table 1. No other elements were detected by energy dispersive specroscopy. The raw data were reduced with the PAP routine of Pouchou and Pichoir (Reference Pouchou, Pichoir and Armstrong1985). Because insufficient material is available for a direct determination of H₂O, it has been calculated based upon the structure determination. The presence of H₂O was confirmed by infrared spectroscopy (see below). The empirical formula based on 11 anions per formula unit is

Table 1. Analytical data (wt.%) for kingsgateite.

* Calculated from the ideal formula.

S.D. – standard deviation

Zr_0.88U⁶⁺_0.02Th_0.01Fe²⁺_0.04Mo⁶⁺_1.94S_0.07P_0.02O_6.90Cl_0.02OH_2.08⋅2.00H₂O. The ideal formula is ZrMo⁶⁺₂O₇(OH)₂⋅2H₂O which requires ZrO₂ 26.49, MoO₃ 61.89, H₂O 11.62, Total 100 wt.%.

Infrared spectroscopy

An infrared-absorption spectrum of powdered kingsgateite (Fig. 4) was recorded using a Nicolet 5700 FTIR spectrometer equipped with a Nicolet Continuum IR microscope and a diamond-anvil cell. The spectrum shows a strong broad band centred on 3352 cm^–1 and with a shoulder at 3266 cm^–1 that is assigned to ν(OH) stretching vibrations. A medium strong band at 1647 cm^–1 with a shoulder at 1614 cm^–1 is attributed to the ν₂(δ)(H₂O) bending vibration and a band at 1417 cm^–1 may be due to Zr–OH bending vibrations (eg. Sarkar et al., Reference Sarkar, Mohapatra, Ray, Bhattacharyya, Adak and Mitra2007). Bands at 931 cm^–1, 835 cm^–1 and 721 cm^–1 are attributed to ν(Mo–O) stretching vibrations.

Fig. 4. Fourier-transform infrared spectrum of powdered kingsgateite.

Powder X-ray diffraction

Powder X-ray diffraction data (Table 2) for kingsgateite were recorded using a Rigaku R-Axis Rapid II curved imaging plate microdiffractometer with monochromatised MoKα radiation. A Gandolfi-like motion on the φ and ω axes was used to randomise the sample. Observed d values and intensities were derived by profile fitting using JADE 2010 software (Materials Data, Inc.). Unit-cell parameters refined from the powder data using JADE 2010 with whole pattern fitting are as follows: a = 11.489(4), c = 12.538(4) Å and V = 1655.0(13) Å³.

Table 2. Powder X-ray diffraction data (d in Å) for kingsgateite.*

*The strongest lines are given in bold

Crystal structure

A crystal fragment was attached to a MiTeGen polymer loop and single-crystal X-ray diffraction data were collected at the MX2 beamline of the Australian Synchrotron (Aragao et al., Reference Aragao, Aishima, Cherukuvada, Clarken, Clift, Cowieson, Ericsson, Gee, Macedo, Mudie, Panjikar, Price, Riboldi-Tunnicliffe, Rostan, Williamson and Caradoc-Davies2018). Data were collected using a Dectris EigerX 16M detector and monochromatic radiation with a wavelength of 0.71075 Å. The intensity data sets were processed using XDS (Kabsch, Reference Kabsch2010) without scaling and with absorption correction and scaling using SADABS (Bruker, Reference Bruker2001)

A structure solution was obtained in space group I4₁cd using SHELXT (Sheldrick, Reference Sheldrick2015a) within the WinGX program suite (Farrugia, Reference Farrugia2012). This structure is consistent with the structural model for ZrMo₂O₇(OH)₂(H₂O)₂ reported by Clearfield and Blessing (Reference Clearfield and Blessing1972). The structure was refined using SHELXL-2018 (Sheldrick, Reference Sheldrick2015b) to R ₁ = 0.0453 on the basis of 1159 unique reflections F _o > 4σ(F _o). H atoms could not be located in difference-Fourier maps. Details of the data collection and the structure refinement are provided in Table 3, refined atom coordinates and anisotropic-displacement parameters are listed in Table 4, selected interatomic distances are given in Table 5, and bond-valence values, calculated using the parameters of Gagné and Hawthorne (Reference Gagné and Hawthorne2015), are given in Table 6. 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 data, data collection and refinement details.

* R ₁ = Σ||F _o|–|F _c||/Σ|F _o|

^† wR ₂ = {Σ[w(F _o²–F _c²)²]/Σ[w(F _o²)²]}^½. w = 1/[σ²(F _o²) + (aP)² + bP] where a is 0. 0.0911, b is 0. 4.20 and P is [2F _c² + Max(F _o²,0)]/3

Table 4. Fractional atomic coordinates and displacement parameters (in Å²) for kingsgateite.

Table 5. Selected interatomic distances (Å) for kingsgateite.

Table 6. Bond-valence analysis for kingsgateite.

Bond valence parameters are from Gagné and Hawthorne (Reference Gagné and Hawthorne2015).

In the crystal structure of kingsgateite, there are two cation sites. The Zr site is [7]-coordinated by four O^2– atoms and two OH groups forming squat pentagonal bipyramids and with <Zr–O> = 2.150 Å. The most common coordination number for Zr in mineral structures is [6]. Seven-coordinated Zr sites (pentagonal bipyramids) are found in the structures of several minerals, including baddeleyite, calzirtite, laachite, stefanweissite, voggite and zirconolite. The Mo site is [6]-coordinated by four O^2– atoms, one OH group and one H₂O group forming a distorted octahedron. The coordination is characterised by a short and a long apical bond distance (1.733 and 2.301 Å) and equatorial bond distances that range from 1.778 to 2.127 Å. This coordination geometry for Mo⁶⁺ cations has been observed in the structures of many in natural and synthetic molybdates, including the Bi molybdates gelosaite, koechlinite, sardignaite and mambertiite.

Zr[O₅(OH)₂] polyhedra share equatorial edges with two Mo[O₄(OH)(H₂O)] octahedra to form a ZrMo[O₁₁(OH)₂)(H₂O)₂] cluster (Fig. 5). Clusters link by corner-sharing via O2 anions to form chains in the c direction. Chains are linked into a framework by corner-sharing via O3 anions along a1 and a2 (Fig. 6).

Fig. 5. The crystal structure of kingsgateite viewed along the a ₂ axis. Zr[O₅(OH)₂] polyhedra are orange; Mo[O₄(OH)(H₂O)] octahedra are blue. The unit cell is outlined.

Fig. 6. The crystal structure of kingsgateite viewed along the c axis. Legend as in Fig. 5.

Although H atoms could not be located, four possible hydrogen bonds were identified based on O⋅⋅⋅O distances: OH5⋅⋅⋅O1 (2.774 Å), OH5⋅⋅⋅OW6 (2.761 Å), OW6⋅⋅⋅O1 (2.700 Å) and OW6⋅⋅⋅OH5 (2.761 Å). The observed O_D⋅⋅⋅O_A distances are consistent with H-bond valences of 0.19. Adding these respective H bond valence contributions results in satisfactory bond-valence sums at the anions.

Structural relations

Among minerals, kingsgateite is unique in terms of structure and chemistry. As noted, its synthetic analogue ZrMo₂O₇(OH)₂⋅2H₂O, has been reported by Clearfield and Blessing (Reference Clearfield and Blessing1972). The compound is a representative of a large group of synthetic compounds, including Zr molybdates and tungstates, that have been studied for their promising cation-exchange properties. Most are amorphous, non-stoichiometric and non-equilibrium salts.

Zirconium tungstates in a crystalline form that are isostructural with ZrMo₂O₇(OH)₂⋅2H₂O have also been described. Clearfield and Blessing (Reference Clearfield and Blessing1974) prepared Zr tungstate which was shown by powder X-ray diffraction to be isomorphous with ZrW₂O₇(OH)₂⋅2H₂O. ZrW₂O₇(OH,Cl)₂⋅2H₂O was synthesised by Dadachov and Lambrecht (Reference Dadachov and Lambrecht1997) and ZrW₂O₇(OH_0.984,Cl_0.016)₂⋅2H₂O by Cao et al., (Reference Cao, Deng, Ma, Wang and Zhao2006). The structures were refined using Rietveld refinement. The coordination (bond distances and angles) of the Zr and (Mo,W) sites in kingsgateite and in these compounds are not significantly different.