*4.3. The M5A Site, A Key to A Flexibility of Stanfieldite Composition*

This cation site (which was previously referred to as Ca5 [40]) deserves a special discussion as it determines the variability of stanfieldite composition which, in turn, has led to misinterpretations of the chemical formula of the mineral and its synthetic analogues. The central atom resides in the general 8*f* position and coordinates to six oxygen atoms to form a highly distorted octahedron (Figure 3C), with the bond distances varying from 2.1 to almost 2.5 Å (Table 5).


**Table 5.** Bond lengths (Å), bond-valence sums (BVS, valence units) and occupancy factors for the *M*5A site of stanfieldite and its synthetic analogue 1.

<sup>1</sup> Bond-valence sums were calculated by summation of individual contributions for each element, based on parameters reported by Brese and O'Keeffe [48].

The *M*5A octahedra form paired clusters in the structure via corner-linking by phosphate tetrahedra (Figure 3C). Based on previous reports [40,41,52] and our data (Table 5), *M*5A may accommodate Ca, Mg, Fe2<sup>+</sup> and Ni in different proportions, with the total occupancy equal to unity. Natural stanfieldite is a Mg-dominant mineral, and the refinement of *M*5A occupancy leads to a dominance of Mg over Fe2<sup>+</sup> as well (Table 5). The latter is supported by calculation of the bond-valence sum, which is almost identical to that of synthetic analogue of stanfieldite (Table 3). The variability of *M*5A occupancy substantiates the existence of solid solution between hypothetical Mg and Ca end members.

The former would have the composition corresponding to Ca7Mg2Mg9(PO4)12. The latter member would correspond to Ca7Ca2Mg9(PO4)12, that is equal to Ca3Mg3(PO4)4. The intermediate composition having Ca = Mg in *M*5A results in a formula Ca7(CaMg)Mg9(PO4)12, or, in a simplified form, Ca4Mg5(PO4)4. One can see that the latter perfectly fulfils the ideal composition of stanfieldite proposed by Fuchs [9]. It is noteworthy that stanfieldite from Brahin described herein, the previously reported mineral from Imilac [41] and the synthetic analogue of stanfieldite [40] have *M*5A occupancies almost equally shared between Ca and (Mg + Fe) (Table 5). This could lead to the assumption that the ordering between Ca and Σ(Mg, Fe) might exist in the *M*5A site. However, neither our observations nor previously reported data reveal superstructure reflections which would evidence the Ca/Mg ordering. In this respect, an overview of reported compositions of stanfieldite-like minerals and compounds would be of special interest. We have collected the chemically relevant data which are gathered in Table 6 and plotted in Figure 4. It can be seen that the overwhelming majority of stanfieldite compositions fall within the range corresponding to Ca ≈ (Mg + Fe) in the *M*5A site. Therefore, the above assumption on the possible Ca/Mg ordering, albeit speculative, has a statistically substantiated basis.

**Figure 4.** Plot of ΣCa versus ΣMg group element contents in natural, technogenic and synthetic stanfieldite. Left and bottom scale: formula amounts recalculated on the basis of 48 oxygen atoms per formula unit. Right and upper scale: expected occupancy factors for the *M*5A site. The grey straight line shows the linear fit for the depicted analytical data. The red dots denote theoretical (calculated) compositions corresponding to (1) Ca7Mg2Mg9(PO4)12, (2) Ca7(CaMg)Mg9(PO4)12 ≡ Ca4Mg5(PO4)6, (3) Ca7Ca2Mg9(PO4)12 ≡ Ca3Mg3(PO4)4. The blue dots and labels mark the compositions of particular interest which are discussed in the paper. References and source data are given in Table 6.


**Table 6.** Formula amounts of cations in stanfieldite and its analogues grouped by elements 1.

<sup>1</sup> Atoms per formula unit calculated on the basis of 48 oxygen atoms. <sup>2</sup> Meteorite names. Non-meteoritic sources are shown in italic type. <sup>3</sup> ΣCa includes (Ca, Na, K). <sup>4</sup> ΣMg includes (Mg, Fe, Mn, Al, Cr, Ti). <sup>5</sup> Present work.

The next interesting point is a significant departure of total cationic sums of many analyses from the ideal value requiring 18 cations per formula unit. These departures are readily revealed by the shifts of corresponding analytical points from the linear fit in Figure 4. At present, we have no explanation for the observed departures. They could imply the existence of analytical errors in the reported microprobe data. On the other hand, these shifts might mean the occurrence of vacancies in cationic sites of stanfieldite structure, and then they deserve a special investigation.

Although the majority of reported data fall within the central area of the plot in Figure 4, there are a few points showing significant prevalence of (Mg + Fe) sum over total Ca. These include one analysis from the Eagle Station pallasite [7] and the mineral found in the Lunar sample 66095 returned by the Apollo 16 mission [16]. These two analyses approach the Ca7Mg2Mg9(PO4)12 end-member of the *M*5A solid solution. At the opposite extreme of the plot, there is a single point approaching hypothetical Ca3*M*3(PO4)4 composition. This analysis, along with two more listed in Table 6, relate to a stanfieldite-like phosphate described from the ancient slags found in Tyrol [19]. The main feature of this compound is wide variations both in Ca/(Fe + Mg) and Fe/Mg ratios, up to nearly Fe-dominant compositions. Schneider, with co-authors [19], has provided Raman spectrum for this phosphate, but in the absence of Raman spectra for genuine stanfieldite, the comparison was not possible. We herein provide the Raman spectrum of stanfieldite from the Brahin meteorite (Figure 5). A comparison of this spectrum with that reported by Schneider with co-authors [19] shows that the latter can represent a poorly crystallized Fe-dominant analogue of stanfieldite.

**Figure 5.** The Raman spectrum of stanfieldite from the Brahin meteorite. The bands between 900 and 1150 cm−<sup>1</sup> correspond to stretching vibrations in [PO4] tetrahedra. Bands in the range 400–650 cm<sup>−</sup><sup>1</sup> relate to bending modes of phosphate anion.
