Occurrence

The Beltana deposit is a high-grade hypogene willemite deposit hosted in Lower Cambrian carbonate rocks in the northern Flinders Ranges (Groves et al., Reference Groves, Carman and Dunlap2003; Brugger et al., Reference Brugger, McPhail, Wallace and Waters2003). The willemite occurs as a replacement of dolomitised and hematitised Ajax Limestone of Lower Cambrian age. Mineralisation is structurally controlled and associated with brecciation and extensive hematite-rich hydrothermal zincian dolomitisation. The texture of the willemite is heterogeneous, resulting from various depositional mechanisms: direct replacement of carbonate host rock, open-space filling, internal sedimentation and brecciation. Acidic ore fluids corroded the host carbonate units and created open space by means of hydrothermal karsting and subsequent deposition of internal willemite sediment. Late-stage gangue minerals include Mn-rich calcite, dolomite and minor quartz. On the periphery of the deposit, smithsonite formed by weathering of willemite. Using numerical geochemical modelling, Brugger et al. (Reference Brugger, McPhail, Wallace and Waters2003) were able to show that willemite will precipitate at temperatures above 120°C as a result of water–rock interaction and fluid mixing processes. The mineralising fluids carried large quantities of oxidised arsenic, as demonstrated by the large amounts of hedyphane in the deposit. The presence of arsenate in the hydrothermal fluids is likely to have inhibited the oxidation of sulfate to sulfide and resulted in the stabilisation and precipitation of willemite rather than sphalerite and galena. Secondary arsenate minerals have formed in cavities in the willemite as a result of supergene alteration. A detailed description of the mineralogy is given by Elliott (Reference Elliott1991). The new mineral occurs in vugs in a matrix composed of willemite and hematite. Associated minerals are rhodochrosite, hedyphane and adamite.

Appearance and physical properties

Puttapaite occurs as diamond-shaped tablets in rosette-like aggregates to 50 μm across (Figs 1 and 2). Individual tablets are up to 45 μm in length and 5 μm in thickness. Crystals are flattened on {001} and the observed forms are {001} and {110} (Fig. 3). The colour is pale green with a pale-green streak and a vitreous lustre. The tenacity is brittle, no cleavage was observed, and the fracture is splintery. Due to the small size of the crystals, the Mohs hardness could not be measured. A density of 3.562 g/cm^–3 was calculated using the empirical chemical formula and unit-cell parameters from single-crystal data. Puttapaite is optically biaxial (–) with α = 1.700(5), β = 1.720(5) and γ = 1.730(5) (measured in white light). The 2V, measured on a spindle stage, using extinction data analysed with the program EXCALIBR (Gunter et al., Reference Gunter, Bandli, Bloss, Evans, Su and Weaver2004) is 67(1)°; the calculated 2V is 69.8°. Dispersion could not be observed. Crystals are pleochroic with X light blue grey, Y colourless and Z not observed; X > Y. The Gladstone–Dale compatibility, 1 – (K _P/K _C), (Mandarino, Reference Mandarino2007) is 0.047 (good) using the empirical formula and the unit-cell parameters determined from single-crystal data.

Figure 1. Pale-green crystals of puttapaite associated with smithsonite (orange brown) and willemite (white). The scale bar is 100 mm (specimen in private collection).

Figure 2. Scanning electron microscope photomicrograph showing crystals of puttapaite. The field of view is 0.3 mm (specimen in private collection).

Figure 3. Crystal drawing of puttapaite; clinographic projection in standard orientation.

Chemical composition

Chemical data (ten spot analyses) were obtained using a Cameca SXFive electron microprobe (wavelength dispersive spectroscopy mode, an acceleration voltage of 20 kV, a beam current of 20 nA and a 5 μm beam diameter). Raw X-ray intensities were corrected for matrix effects with a φ(ρZ) algorithm (Pouchou and Pichoir, Reference Pouchou, Pichoir, Quantitation, Heinrich and Newbury1991). Both the crystal structure and infrared spectroscopy data confirm the presence of H₂O. Analytical data are given in Table 1. The empirical formula for puttapaite, calculated on the basis of 36 oxygen atoms, is Pb_1.96(Mn²⁺_1.52Ca_0.28Sr_0.22)_Σ2.02(Zn_0.40Mg_0.39Cu_0.15)_Σ0.94(Cr³⁺_2.89Al_0.45Fe³⁺_0.40,Mn³⁺_0.26)_Σ4.00O₂[(AsO₄)_3.71(Cr⁶⁺O₄)_0.29]_Σ4.00(OH)_6.13·11.87H₂O. The ideal formula is Pb₂Mn²⁺₂ZnCr³⁺₄O₂(AsO₄)₄(OH)₆·12H₂O, which requires PbO 26.20, MnO 8.33, ZnO 4.78, Cr₂O₃ 17.84, As₂O₅ 26.98, H₂O 15.87, total 100 wt.%.

Table 1. Analytical data for puttapaite

* MnO and Mn₂O₃ calculated to give Cr³⁺ + Al + Fe³⁺ + Mn³⁺ = 4.00

** Cr₂O₃ and CrO₃ calculated to give AsO₄ + Cr⁶⁺O₄ = 4.00

^§ calculated from the refined formula; S.D. – standard deviation

Infrared spectroscopy

The infrared-absorption spectrum of powdered puttapaite, recorded using a Nicolet 5700 FTIR spectrometer equipped with a Nicolet Continuμm IR microscope and a diamond-anvil cell in the range 650–4000 cm^–1, is shown in Fig. 4. A broad band in the O–H stretching region, with three sharp peaks at 3506, 3359 and 3149 cm^–1, can be attributed to the presence of H₂O and OH groups in the structure. Using the correlation of Libowitzky (Reference Libowitzky1999), the inferred O···O (donor–acceptor) distances are 2.90, 2.77 and 2.69 Å, which correspond to weak- to medium-strength hydrogen bonds. A band at 1651 cm^–1 is assigned to the ν₂ (δ) H₂O bending mode. Bands at 850, 812 and 791cm^–1 may be assigned to ν₃ vibrations of the AsO₄ tetrahedra.

Figure 4. The Fourier-transform infrared spectrum of powdered puttapaite.

Crystallography

Powder X-ray diffraction

Powder X-ray diffraction data for puttapaite were obtained using a Rigaku R-AXIS Rapid II curved-imaging-plate microdiffractometer, with monochromatised MoKα radiation (50 kV, 40 mA). 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 Pro software (Materials Data, Inc.). Data (in Å for MoKα) are given in Table 2. Unit-cell parameters refined from the powder data using JADE Pro with whole-pattern fitting are a = 12.480(5), b = 10.588(5), c = 12.297(5) Å, β = 106.434(14)°, V = 1558.5(12) ų and Z = 2.

Table 2. Powder X-ray diffraction data for puttapaite

Single-crystal X-ray diffraction

A single-crystal data collection was made at the macromolecular beam line MX2 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 0.710760 Å. The data set was processed using XDS (Kabsch, Reference Kabsch2010) without scaling, and with absorption correction and scaling using SADABS (Bruker, Reference Bruker2001). Data collection details are given in Table 3.

Table 3. Crystal data, data collection and refinement details

^† wR2 = Σw(|F _o|²–|F _c|²) ²/Σw|F _o|²)^1/2; w = 1/[σ²(F _o²) + (0.02P)²]; P = ([max of (0 or F _o²)] + 2F _c²)/ 3

Structure determination

Structure solution in space group C2/m was carried out using SHELXT (Sheldrick, Reference Sheldrick2015a) and the structure was refined using SHELXL-2018 (Sheldrick, Reference Sheldrick2015b) as implemented in the WinGX suite (Farrugia, Reference Farrugia2012). It was impossible to separate a high-quality individual from the rosette-like intergrowths. The best fragment found exhibited high mosaicity and multiple diffraction spots. As a result, relatively high R _int and final R values were obtained, 17.3% and 11.89%, respectively. We were unable to locate the H atom positions in difference-Fourier maps. Final atom coordinates and anisotropic-displacement parameters are listed in Table 4, selected interatomic distances and are given in Table 5, and bond-valence values, calculated using the parameters of Gagné and Hawthorne (Reference Gagné and F.C2015) are given in Table 6. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 4. Fractional atomic coordinates and displacement parameters (in Ų) for puttapaite

Table 5. Selected interatomic distances (Å) and possible hydrogen bonds (Å) for puttapaite

Table 6. Bond-valence analysis for puttapaite

Assignment of cation site-populations to the M sites and T site was completed based on observed mean bond lengths (Table 7). Mn²⁺ plus minor Ca and Sr were assigned to the larger M1 site, Zn, Mg plus minor Cu were assigned to the medium-sized M2 site and Cr³⁺ plus minor Al, Fe³⁺ and Mn³⁺ were assigned to the smaller M3 and M4 sites. Chemical analysis shows more Cr than is required to fill the M3 and M4 sites and insufficient As to fill the T site, hence 0.29 atoms per formula unit Cr⁶⁺ was assigned to the T site. This assignment is supported by the <T–O> distance of 1.671 Å (Table 4), which is less than the distance expected for full occupancy by As⁵⁺ of 1.687 Å (Gagné and Hawthorne, Reference Gagné and F.C2018).

Table 7. Refined site-scattering values (epfu) and assigned site-populations for puttapaite

epfu – electrons per formula unit

Structure description

The Pb site is [10]-coordinated (Fig. 5) with three short bonds to O2 and O5, three medium length bonds to OH6 and OW10 and four very long bonds to O4. The site exhibits one-sided coordination typical of Pb²⁺ with a stereochemically active 6s ² lone-electron-pair. Chemical analysis shows that the site is fully occupied by Pb. The M sites are each [6]-coordinated by O^2– anions, OH groups and H₂O groups in regular octahedral arrangements. The As site is coordinated as a regular tetrahedron by four O^2– anions and shows only slight distortions [angular range 107.1(8)–111.6(10) Å].

Figure 5. The Pb²⁺ coordination in puttapaite showing Pb–O bond lengths in angstroms, Å. All crystal structure drawings were done with ATOMS (Dowty, Reference Dowty1999).

Two M3 and two M4 octahedra share edges to form a M ₄O₁₆ tetrameric cluster. Four TO₄ tetrahedra share O3 and O4 anions with each M ₄O₁₆ cluster to form a M ₄T ₄O₂₄ cluster. M1 octahedra link to TO₄ tetrahedra via corner sharing to link clusters in the [010] direction and M2 octahedra link to M4 octahedra via corner sharing to link clusters in the [001] direction to form sheets parallel to {100}. Sheets link in the [100] direction by Pb–O bonds. The structure is shown in Fig. 6.

Figure 6. The crystal structure of puttapaite viewed (left) along [100] and (right) along [010]. The unit cell is outlined.

The structure contains four H₂O molecules and two OH groups. Although the H atoms could not be located during the refinement, a possible hydrogen bonding scheme based on O···O bond distances is summarised in Table 5. Under this scheme, the underbonded O1, O2, O3 and O4 anions are receptors of hydrogen bonds as are each of the H₂O molecules. The observed O⋯O distances range from ∼2.7 to ∼3.3 Å indicating strong to very weak hydrogen bonds.

Relationship to other minerals

Puttapaite represents a unique chemistry and structure type for minerals and inorganic compounds. There are many arsenate minerals that contain Pb and Zn and five arsenate minerals that contain Pb and Mn. Besides puttapaite, there is only one other arsenate mineral that contains Pb and Cr, fornacite. In fornacite, Cr is hexavalent whereas in puttapaite Cr is trivalent. The crystal structures of anthoinite, AlWO₃(OH)₃ (Grey et al., Reference Grey, Madsen, Mills, Hatert, Paterson and Bastow2010), and bamfordite, Fe³⁺Mo₂O₆(OH)₃⋅H₂O, (Birch et al., Reference Birch, Pring, McBriar, Gatehouse and McCammon1998) contain M ₄O₁₆ tetrameric clusters of edge-sharing octahedra and they have a similar layer structure. In the anthoinite structure there are two types of tetramer that share octahedral vertices with four adjacent tetramers to form stepped layers parallel to (001). Connectivity between layers is by hydrogen bonding. In the bamfordite structure, tetramers interconnect via octahedral dimers, Fe³⁺₂(O,OH,H₂O)₁₀, to form stepped layers parallel to (100). The layers are linked by hydrogen bonding.