Occurrence

The White Rock No.2 quarry from which the type specimen was collected is located in the Bimbowrie Conservation Park, 24 km N of Olary, South Australia, Australia. Pegmatites and pegmatoids are ubiquitous throughout the region, and occur as sills, dykes, lenses and segregation bodies of ill-defined shape (Campana, Reference Campana1957). They have been intruded into rocks of the Archaean Willyama Complex. Willyama Supergroup rocks comprise upper greenschist- to amphibolite-grade metamorphosed and strongly deformed sedimentary and minor igneous rocks (Lottermoser and Lu, Reference Lottermoser and Lu1997), which are unconformably overlain by late Proterozoic Adelaidean metasediments. The White Rock pegmatite is one of more than 70 pegmatite bodies in the Olary Province of South Australia. It is a mineralogically zoned rare-element bearing pegmatite characterised by the occurrence of late-stage phosphate nodules between the quartz core and intermediate feldspar-rich zone and belongs to the beryl–columbite phosphate rare-element type in the classification of Černý (Reference Černý1991). Triplite–zwieselite was formed by metasomatic alteration of magmatic fluorapatite and has been transformed by hydrothermal alteration and weathering, in an oxidising, low-temperature, low-pH environment, to give a complex, microcrystalline intergrowth of secondary phosphate minerals (Lottermoser and Lu, Reference Lottermoser and Lu1997). At White Rock, three pegmatites with poor outcrops, up to 120 m long were mined for feldspar (both albite and microcline), muscovite and beryl over the period 1932–1973 (Olliver and Steveson, Reference Olliver and Steveson1982). Three quarries were excavated to a depth of 10 m with recorded production of 860 tonnes of feldspar and 8.1 tonnes of beryl. Triplite and associated secondary phosphate minerals have been exposed in only the No.2 quarry. Bimbowrieite occurs in seams in a matrix comprising triplite and fluorapatite. Associated minerals are ushkovite, bermanite, leucophosphite and sellaite.

Appearance and physical properties

Bimbowrieite occurs as aggregates of crystals to 0.4 mm across (Fig. 1). Crystals are dark olive green to greenish brown blades, up to 150 μm in length. The blades are flattened on {100} and exhibit the crystal forms {100}, {111} and {201} (Fig. 2). The streak is olive green, the lustre is vitreous, the tenacity is brittle and the fracture is irregular. There is one excellent cleavage on {100}. Optically, bimbowrieite is biaxial (+), α = 1.785(5), β = 1.795(5) and γ = 1.805(5) (measured in white light). The 2V_z measured on a spindle stage is 89.4(5)°; the calculated 2V_z is 90.5°. Dispersion is r < v, extreme. The optical orientation is Y = b, X ^ c ≈ 18° in obtuse β. The mineral is pleochroic with X = brown orange, Y = brown yellow, Z = blue green and Y < X < Z. The Gladstone–Dale compatibility index 1 – (K _P/K _C) for the empirical formula is 0.056 (good) (Mandarino, Reference Mandarino2007).

Figure 1. Greenish-brown crystals of bimbowrieite on fluorapatite, associated with ushkovite (orange) and sellaite (white). The field of view is 2.3 mm, South Australian Museum specimen G34762.

Figure 2. Crystal drawing of bimbowrieite (clinographic projection in standard orientation).

Infrared spectroscopy

The infrared spectrum (Fig. 3) of powdered bimbowrieite was recorded using a Nicolet 5700 FTIR spectrometer (range 4000 to 650 cm^–1, transmission mode) equipped with a Nicolet Continuum IR microscope and a diamond-anvil cell. The spectrum shows a broad absorption band due to OH stretching vibrations with maxima at 3568 cm^–1 and 3230 cm^–1. According to the correlation given by Libowitzky (Reference Libowitzky1999), the approximate O–H⋅⋅⋅O hydrogen bond-lengths range between 3.1 and 2.6 Å. A band found at 1575 cm^–1 is assigned to the ν₂ H–O–H bending vibration of water molecules. The bands at 1194 and 1028 cm^–1 may be assigned to the PO₄ ν₃ antisymmetric stretching vibrations and the band at 775 cm^–1 is assigned to the PO₄ ν₁ symmetric stretching vibration.

Figure 3. The FTIR spectrum of powdered bimbowrieite.

Chemical composition

Quantitative chemical data were collected on two polished crystal aggregates using a Cameca SXFive electron microprobe (WDS mode, 20 kV, 20 nA, 5 μm beam diameter). Data were reduced using the ϕ(ρZ) method of Pouchou and Pichoir (Reference Pouchou, Pichoir, Heinrich and Newbury1991). Twenty-eight points were analysed (Table 2). The small amount of material available did not allow for the direct determination of H₂O, so it was calculated give 10 H atoms per formula unit. The empirical formula, based on 24 O atoms, is (Na_0.81Ca_0.19)_Σ1.00(Mg_0.75Mn²⁺_0.19Fe²⁺_0.05)_Σ0.99(Fe³⁺_4.99Al_0.01)_Σ5.00(PO₄)_3.97(OH)_5.88⋅2.05 H₂O.

Table 2. Analytical data for bimbowrieite.

*Fe₂O₃ and FeO calculated to give full occupancy of the M1, M3, and M4 sites by Fe³⁺+Al.

**H₂O calculated from the crystal structure analysis.

S.D. = standard deviation

The ideal formula is NaMgFe³⁺₅(PO₄)₄(OH)₆⋅2H₂O which requires Na₂O 3.67, MgO 4.77, Fe₂O₃ 47.27, P₂O₅ 33.62, H₂O 10.67, total 100 wt.%.

X-ray crystallography and crystal-structure determination

Powder X-ray diffraction data (Table 3) 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 Pro software (Materials Data, Inc.). The unit-cell parameters refined from the powder data using JADE Pro with whole-pattern fitting are: a = 26.07(2), b = 5.17(2), c = 13.95(2) Å, β = 111.56(2)° and V = 1749(7) ų, which are in good agreement with the single-crystal study below.

Table 3. Powder X-ray data for bimbowrieite. Only calculated lines with I ≥ 6 are listed.

A crystal was attached to a MiTeGen polymer loop and X-ray diffraction data was collected at the micro-focus macromolecular MX2 beamline at 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, 2001). Structure solution in space group C2/c was carried out using SHELXT (Sheldrick, Reference Sheldrick2015a) as implemented in the WinGX suite (Farrugia, Reference Farrugia2012). The atom coordinates were then transformed to correspond to those in the structure of dufrénite (Moore, Reference Moore1970). SHELXL-2018 (Sheldrick, Reference Sheldrick2015b) was used for the refinement of the structure. All H atom sites were located in difference-Fourier maps and were refined with soft restraints of 0.82(3) Å on the O–H distances. The site occupancies at the X site and the M2 site were fixed to (Na_0.81Ca_0.19) and (Mg_0.75Mn²⁺_0.19Fe²⁺_0.05), respectively, in accordance with the electron microprobe data. The final refinement converged to an agreement index of R ₁ = 1.97% for 1060 observed reflections with F _o > 4σ(F _o). Data collection and refinement details are given in Table 4, atom coordinates and displacement parameters in Table 5, selected bond distances in Table 6 and a bond valence analysis in Table 7. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 4. Crystal data, data collection and refinement details.

* R ₁ = Σ||Fo|-|Fc|| / Σ|Fo|

^† wR ₂ = Σw(|Fo|²–|Fc|²) ² / Σw|Fo|²)^½; w = 1/[σ²(Fo²) + (0.0177 P)² + 7.86 P];

P = ([max of (0 or F ₀²)] + 2Fc²)/ 3

Table 5. Fractional coordinates and displacement parameters (Ų) for atoms for bimbowrieite.

^a Refined occupancy Na_0.81Ca_0.19

^b Refined occupancy Mg_0.75Mn²⁺_0.19Fe²⁺_0.05

Table 6. Selected interatomic distances (Å), angles (°) and hydrogen bonds for bimbowrieite.

Note: BLD = bond-length distortions (Renner and Lehmann, Reference Renner and Lehmann1986); OAV = octahedral angle variance (Robinson et al., Reference Robinson, Gibbs and Ribbe1971).

Table 7. Bond valence* (vu) sums for bimbowrieite.

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

Bond valences for the X and M(2) sites are based on the refined occupancy.

The main feature of the structure is a trimer of face-sharing octahedra, the “h-cluster” described by Moore (Reference Moore1970), which is also a feature in the structures of a number of other basic iron-phosphate minerals. A central M2ϕ₆ octahedron shares two trans faces with two M4ϕ₆ octahedra, via the OH5, O6 and O7 anions, to form a trimer of the form [M ₃ϕ₁₂]. Linkage in the a-direction is via corner-sharing M1 octahedra and T2 tetrahedra. Trimers link in both the b-direction and the c-direction by sharing corners with M3 octahedra and with T1 and T2 tetrahedra (Fig. 4).

Figure 4. The crystal structure of bimbowrieite viewed along [010]. Hydrogen atoms are small grey spheres. The unit cell is outlined.

The X site occupies channels that run along [010] and is coordinated by six O atoms and two H₂O molecules to form a square antiprism. The refinement yields an X site occupied by Na_0.84Ca_0.16 (12.43 epfu), in good agreement with the chemical analysis that shows Na_0.81Ca_0.19 (12.71 epfu). The bond-valence sum at the site of 1.34 is also in agreement with a mixed (Na,Ca) site population. Each of the M sites is coordinated by six anions in an octahedral arrangement. The M2 site is occupied by Mg plus minor Mn²⁺ and Fe²⁺ and is coordinated by four O anions and two OH groups. The site was refined with joint occupancy by Mg and Mn, yielding a site-scattering value of 16.32 e ^–. This is greater than the site-scattering value of 15.17 e ^– based on the site occupancy indicated by the empirical formula. The most likely explanation is that the crystal used for the structure determination was higher in Mn and lower in Mg than the crystal used for electron probe microanalysis (EPMA). The large variations in these elements noted during the EPMA further support this explanation. Fe³⁺ occurs at three symmetrically distinct sites, M1, M3 and M4. The M1 site is coordinated by two O atoms, two OH groups and two H₂O groups. The M3 site is coordinated by three O atoms and three OH groups and the M4 site is coordinated by four O atoms and two OH groups. The observed mean M–ϕ bond-lengths for the M1, M3 and M4 sites (2.018, 2.015 and 2.026 Å respectively) and bond-valence sums (Table 7) support the occupancy of the M1 site by Fe³⁺ plus minor Al. This is in agreement with the structure refinements of other members of the dufrénite group in which the smaller M1, M3 and M4 octahedral sites are dominated by either Fe³⁺ or Al. Of the M sites, M3 and M4 are more distorted in terms of bond-length distortion (BLD) and M2 and M4 are more distorted in terms of octahedral angle variance (OAV) (Table 6). Two symmetrically distinct sites, P1 and P2 in the structure are fully occupied by P. The PO₄ tetrahedra show similar <P–O> distances and degrees of geometrical distortion.

There are three OH groups and one H₂O group in the structure. The hydrogen bonding scheme (Table 6) for bimbowrieite is the same as that reported in previous studies on the dufrénite-group minerals burangaite (Selway et al., Reference Selway, Cooper and Hawthorne1997) and matioliite (Atencio et al., Reference Atencio, Coutinho, Mascarenhas and Ellena2006). The OH5 and OH8 groups provide hydrogen bonds accepted by O4 and O9, respectively. OW12 provides three hydrogen bonds accepted by O1, O3 and OH8. The hydrogen bonds are of weak to medium strength with O–O distances in the range 2.580 to 3.065 Å.