Xenophyllite, Na 4 Fe 7 (PO 4 ) 6 , an Exotic Meteoritic Phosphate: New Mineral Description, Na-ions Mobility and Electrochemical Implications

: Xenophyllite, ideally Na 4 Fe 7 (PO 4 ) 6 , is a rare meteoritic phosphate found in phosphide-phosphate assemblages conﬁned to troilite nodules of the Augustinovka 4 Fe 7 (PO 4 ) 6 to almost Na 2 Fe 8 (PO 4 ) 6 are accounted for by Na-ions mobility. The latter property makes xenophyllite a promising prototype for cathode materials used in sodium-ion batteries.


Introduction
The discoveries of new functional materials have often been inspired by the study of their natural counterparts. Phosphate mineral triphylite, LiFePO 4 (the olivine structure type) [1] is among those prototypes whose structural motif has led to an emergence of a novel class of cathodes for lithium-ion batteries [2,3]. Meanwhile, the growing demand for inexpensive electrochemical energy sources has resulted in the development of Na-ion batteries which require novel electrode materials based on Several cm-sized fragments of troilite nodules from the Augustinovka were kindly provided for the present study by the Mining Museum, Saint Petersburg Mining University, Saint Petersburg, Russia. The fragments were shred into~1 mm thick slices which were used for a further preparation of the polished thin sections. The sections were first examined under a polarizing microscope in reflected and transmitted light. After that, selected sections containing phosphate grains were coated with conductive carbon film and subjected to the electron microprobe study. Because of instability (quick loss of sodium) of xenophyllite under the conditions of wavelength-dispersive (WDX) analysis, the chemical composition of the mineral and associated phases was studied in energy-dispersive mode (EDX) by means of an Oxford Instruments CamScan 4 scanning electron microscope (Abingdon, UK) equipped with a LINK AN1000 EDS detector, under 20 kV accelerating voltage, 1 nA beam current, 3 µm beam diameter and 30 s counting time per spot. Analytical standards used were chkalovite (for Na), orthoclase (K), diopside (Mg), metallic Mn and Co (Mn and Co), hematite (Fe), chromite (Cr), chlorapatite (P), and pyrite (S). The microprobe data were processed using the built-in Oxford Instruments software using a conventional ZAF procedure. The analyses of associated minerals were performed by use of an AzTec Energy X-Max 20 EDX spectrometer (AzTec Energy Group Inc., Brentwood, NY, USA) attached to a Hitachi S-3400N scanning electron microscope (SEM) (Hitachi, Ltd., Tokyo, Japan). EDX spectra were obtained using 20 kV accelerating voltage, 2 nA beam current, and 60 s acquisition time per spectrum, and were processed using ZAF. The density of xenophyllite was measured on the isolated grain by sink-float method in the diluted Clerici solutions. X-ray single-crystal data were obtained from the 0.05 × 0.05 × 0.05 mm grain extracted from a polished section, by means of a STOE IPDSII imaging plate diffractometer (STOE & Cie GmbH, Darmstadt, Germany) (MoKα-radiation, 50 kV, 40 mA, detector-to-crystal distance 100 mm). A set of 90 frames (0-180 • ) was collected using ω-scan (2 • frame width and 300 s exposure time per frame). Data processing and unit-cell parameters determination was performed by means of STOE X-area v.1.42 software (STOE & Cie GmbH, Darmstadt, Germany) [14]. X-ray powder diffraction pattern (Figures S1 and S2) was obtained using a Gandolfi method by means of a Rigaku RAXIS Rapid II diffractometer (Rigaku Corporation, Tokyo, Japan) equipped with a semi-cylindrical imaging plate and operated under the following setup: CoKα-radiation (rotating anode with microfocus optics), 40 kV acceleration voltage, 15 mA tube current, 127.4 mm detector-to-crystal distance and 60 min overall exposure time. Subsequent powder X-ray diffraction (XRD) data processing and unit-cell parameters refinement was performed using an osc2xrd and STOE WinXPOW v. 1.06 software [15,16]. The Raman spectrum of xenophyllite was recorded by means of a Horiba Jobin-Yvon LabRam HR 800 spectrometer (Horiba Scientific, Kyoto, Japan) equipped with a 514 nm Ar + laser and 50× confocal objective focused to 2 µm spot size. The output power of a laser was reduced by 30% to~13 mW (~1 mW on sample), in order to avoid radiation-induced damage of the mineral detected during the screening measurements.

Occurrence and Associated Minerals
Xenophyllite is a rare mineral hosted within millimeter-sized phosphate-phosphide assemblages confined to centimeter-sized ovoidal troilite nodules common in the Augustiovka meteorite. Troilite has a fine-grained lenticular texture revealed under crossed polars ( Figure 1a). The dominant phase among phosphates is sarcopside, which forms pseudo-euhedral grains reaching up to 3 mm disseminated in troilite ( Figure 1). Macroscopically, sarcopside is translucent; the fresh mineral has a grayish color but it is frequently dissected by the submicroscopic web of secondary vivianite veinlets which impart the pale-to deep-blue color to sarcopside grains. Although sarcopside often exhibits very sharp euhedral faces (Figure 1a), examination in transmitted light under crossed polars (Figure 1b) always reveals very complex mosaic structure; thus the observed faces do not correspond to sarcopside single crystals in its present state. The mineral grains commonly consist of complex polysynthetic twins ( Figure 1c). Sometimes, sarcopside grains are severely fragmented ( Figure 1d) and even dissected by the late troilite veinlets (Figure 1e). Sarcopside is frequently enveloped by the rims composed of fine-grained intergrowths of chromite and schreibersite (Figure 1e). In the course of powder X-ray examination of phosphate assemblages of Augustinovka, we only detected sarcopside itself, with no evidence of graftonite previously mentioned by Olsen and Fredriksson [11]. The unit-cell parameters of sarcopside (space group P2 1 /c) are: a 10.554(9), b 4.748(5), c 6.054(3) Å, β 91.09 (8) • . The mineral is very uniform in chemical composition and has no compositional zoning; it contains 1.7 to 2.6 wt.% MnO (Table 1).

Chemical Composition
Examination of several xenophyllite grains by means of an electron microprobe (Table 5) had revealed some distinctive features related to sodium contents in its composition. It was found that sodium is readily burned off from xenophyllite grains even under relatively soft analytical conditions. In particular, a 30 s exposure at 2 nA beam current and 1 µm beam diameter led to a loss of about one third of total sodium. In order to eliminate this effect, we used 1 nA beam current and 3 µm beam diameter. At these conditions, the loss of sodium was negligible; however, the accuracy of element determination was intentionally reduced to the first decimal ( Table 5). The chemical composition of the type xenophyllite can be represented by the following empirical formula (based on 24 oxygen atoms per formula unit): (Na 3.67 K 0.09 ) Σ3.76 (Fe 2+ 6.12 Mn 2+ 0.85 Cr 0.11 ) Σ7.08 P 5.99 O 24.00 . The latter one corresponds to the Na 4 Fe 7 (PO 4 ) 6 end-member. However, it was found that sodium contents in xenophyllite may considerably vary from grain to grain, ranging from Na 4 Fe 7 (PO 4 ) 6 to nearly Na 2 Fe 8 (PO 4 ) 6 ≡ NaFe 4 (PO 4 ) 3 . The latter formula, in due course, corresponds to a composition of galileiite [18], the mineral mentioned in the preceding section. Moreover, plotting the (Fe, MnCr)/(Na, K, Ca) ratios determined for a series of xenophyllite grains reveals a remarkably straight, linear dependence between Na-and Fe-group elements which inspires the existence of continuous solid solutions between Na 4 Fe 7 (PO 4 ) 6 and Na 2 Fe 8 (PO 4 ) 6 end-members (Figure 3). The minerals of similar intermediate composition were reported from the euthectic chondritic melts described in Yanzhuang (H6) and Chelyabinsk (LL5) ordinary chondrites [19][20][21].   Table 5.

Raman Spectroscopy
The Raman spectrum of xenophyllite was recorded at a reduced (to~1 mW on sample) laser power and is depicted in Figure 4. The quality of the spectrum was degraded due to a strong concurrent fluorescence effect, hence only four main Raman bands could be reliably measured. The bands at 975 and 1114 cm −1 lie in the region typical of stretching modes of [PO 4 ] 3− tetrahedra; their frequencies are close to the frequencies of galileiite-like phosphates reported from the ordinary chondrites [20]. The bands at 600 and 641 cm −1 are attributed to bending vibrations of orthophosphate anion and are distinguished from the bands reported for galileiite-like minerals from chondrites [20].  Table 5).

Relationships between Xenophyllite, Synthetic Na 4 Fe 7 (PO 4 ) 6 and ANa 3 M 7 (PO 4 ) 6 Phosphates
The recently reported characterization of a synthetic analogue of xenophyllite [5] had confirmed our previous data [6] on the symmetry ( Table 4) and composition of the mineral. In spite of that, the structural relationships between xenophyllite and other phosphates were not yet defined. Herein we provide a comparison of xenophyllite, Na 4 Fe 7 (PO 4 ) 6 , with a series of synthetic phosphates ANa 3 M 7 (PO 4 ) 6 (A = K, Rb, Cs; M = Mg, Fe 2+ , Mn 2+ , Zn, Co 2+ ) ( Table 6). The first member of the family, CsNa 3 Zn 7 (PO 4 ) 6 , was described by Yakubovich with co-authors [22]. Later on, nine compounds of Fe, Mn and Co were synthesized and studied with respect to their unusual magnetic properties [23,24]. The last reported phosphate of a series, RbNa 3 Mg 7 (PO 4 ) 6 , was found to be a promising host matrix for the development of luminophore materials [25,26]. A series of structurally similar phosphates and arsenates was also reported, many of them being Na-ion conductors [27][28][29][30][31][32][33]. Table 6. Comparison of a monoclinic subcell of xenophyllite with the unit cells of representative ANa 3 M 7 (PO 4 ) 6 phosphates.

Compound
Space Group The cell metrics of a monoclinic subcell of xenophyllite is obviously related to the unit cells of ANa 3 M 7 (PO 4 ) 6 phosphates (Table 6), distinguishing from them just by a doubly reduced c-axis. That is, together with the same stoichiometry, it strongly suggests structural relationships between xenophyllite and these phosphates. It should be noted that the above compounds possess unique channel-like structures not encountered among natural phosphates. That is, in due course, this largely eliminates speculation about the possible relation of xenophyllite to fillowite-group minerals [34].

Xenophyllite and Galileiite
Galileiite, NaFe 4 (PO 4 ) 3 , was described from several iron meteorites [18] (Table 5) which, like the Augustinovka, belong to a structural group of medium octahedrites and a chemical group IIIAB. The mineral assemblages of galileiite [12,18] are almost identical to those of xenophyllite from the Augustinovka. The analytical abilities did not allow the X-ray single-crystal data for galileiite to be obtained. The powder diffraction pattern of the mineral (Gandolfi method) was reported and was indexed assuming the possible relationship of galileiite to fillowite-group minerals [18]. However, the authors note that the indexing in rhombohedral (fillowite) cell [34] was not entirely satisfactory, and the assertion of galileiite as a mineral belonging to the fillowite group "is yet to be demonstrated" [18]. What is more interesting is that the cell metrics of an I-centered subcell of xenophyllite almost exactly reproduces the unit-cells of a wide series of synthetic phosphates AM 3-4 (PO 4 ) 3 having the stoichiometry of galileiite, NaFe 4 (PO 4 ) 3 (Table 7). In particular, the "potassium analog of galileiite" [18] has a direct synthetic KFe 4 (PO 4 ) 3 counterpart [34]. The crystal structures of these phosphates are very similar to each other and do not belong to a fillowite structure [35]. Instead, they have tunnel-like architectures derivative of the α-CrPO 4 structure type [36]. Many of those phosphates have ion-conductive properties [37]. Taking into account the evidence available to date, one can state that there are many similarities between these two minerals. First, note the numerous overlaps between the strong X-ray powder diffraction lines of xenophyllite (Table 4) and galileiite [18]. Second, our electron microprobe data evidence for the occurrence of a continuous series of compounds between Na 4 Fe 7 (PO 4 ) 6 and Na 2 Fe 8 (PO 4 ) 6 [≡NaFe 4 (PO 4 ) 3 ] (Table 5, Figure 3). Note that Olsen & Steele [18] have used relatively harsh analytical conditions during the microprobe examination of galileiite (WDX system, 15 kV, 25 nA) that could result in a considerable loss of sodium during their analyses. However, our measurements were carried out with a great care under the soft conditions (cf. Materials and Methods section), therefore we can confirm that the galileiite-type NaFe 4 (PO 4 ) 3 is a mineral rather than an artifact produced by the overburning of Na-rich xenophyllite under the hard electron beam. The occurrence of phosphates having intermediate xenophyllite-galileiite composition was also reported later on from a series of ordinary chondrites [19][20][21]45,46], anomalous meteorite Sahara 03505 [47] and from burned coal piles [48,49].

Conclusions
Xenophyllite, being discovered as a rare mineralogical curiosity [6], has became a natural prototype of synthetic Na 4 Fe 7 (PO 4 ) 6 , a new material with promising electrochemical properties [5]. The sodium ions mobility first revealed during electron microprobe measurements of the mineral has met its electrochemical implications. The close relationships between xenophyllite and a series of synthetic phosphates (Tables 6 and 7) might suggest that the latter compounds bearing redox-sensitive cations Fe 2+ , Mn 2+ and Co 2+ have to be explored for their applicability as materials for Na-ion batteries. Further structural studies are needed to reveal the exact mechanisms of Na-ions mobility in xenophyllite and the obvious relationships with its nearest mineralogical counterpart, galileiite.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/10/4/300/s1, Figure S1: As-recorded imaging plate pattern of xenophyllite. Rigaku RAXIS Rapid II diffractometer, semi-cylindrical imaging plate, CoKα-radiation (rotating anode with microfocus optics), 40 kV acceleration voltage, 15 mA tube current, 127.4 mm detector-to-crystal distance, 60 min exposure time. The black rectangle outlines the area used for the integration of imaging plate data into an XRD profile., Figure S2: Powder XRD profile of xenophyllite obtained by integration of imaging plate data shown in Figure S1.