*4.1. Stanfieldite: A Complete Structure-Composition Dataset*

The X-ray examination of Brahin stanfieldite was carried out by two different methods. Both single-crystal and Rietveld refinements of the unit cell had led to almost identical parameters (deviation between the unit-cell volumes is 0.04%, Table 1). The refined Mg/Fe occupancies were also well converged (Table 3). The chemical composition of studied stanfieldite (wt.%, average of 3 points): CaO 26.21, MgO 21.66, FeO 3.02, MnO 0.27, P2O5 49.38, total 100.54 can be recalculated to the formula Ca8.04Mg9.25Fe0.72Mn0.07P11.97O48. The latter is close to the composition determined by the structure refinement, Ca7.97Mg9.47Fe0.56P12O48. Based on these results, one can state that the bond lengths and derivative bond-valence sums (Table 3) are herein calculated with a good confidence.

Fuchs [9], in 1969, could reliably determine the unit-cell metrics, but misrecognized the space group of the mineral (Table 4), perhaps due to the same pseudo-twinning of the crystals [40,41] which we observed on our studied stanfieldite.


**Table 3.** Selected structural parameters of cation sites in natural and synthetic stanfieldite 1.

<sup>1</sup> Complete listings of atomic coordinates, thermal displacement parameters and bond lengths are given in Supplementary Tables S1–S3. <sup>2</sup> CN, coordination number. <sup>3</sup> Average cation-oxygen bond lengths (Å). <sup>4</sup> BVS, bond-valence sums (in valence units), calculated by summation of individual element contributions based on parameters reported by Brese and O'Keeffe [48]. <sup>5</sup> Refined Mg occupancies for Mg1–Mg5 sites. Data for Brahin include both single-crystal and Rietveld refinement results separated by slash.


**Table 4.** Unit cell parameters of natural and synthetic stanfieldite refined from single-crystal data.

<sup>1</sup> Space group assignment and cell axes permutation are discussed in References [40,41].

Dickens and Brown [40] have synthesized the synthetic, Fe-free analogue of stanfieldite and thoroughly described its crystal structure. However, the latter authors did not perform independent determination of the chemical composition of synthesized material—as one will see, this is an essential requirement in view of the widely varying composition of at least one structural site of stanfieldite. Steele and Olsen [41] have reported the preliminary results of structural examination of natural stanfieldite from the Imilac pallasite. They gave the analytical chemical formula of the mineral, but did not provide full structural data, confining the results to unit cell metrics, average bond lengths and selected site occupancies (Tables 3 and 4). As a consequence, no complete structure-composition dataset for stanfieldite is available so far, and the data provided herein are the first report of that type.
