*4.2. General Features of Stanfieldite Structure and Its Formula*

The crystal structure of stanfieldite is a complex framework composed of 10 metal sites and 6 phosphate groups (Table 3, Figure 3, Supplementary Table S1). Dickens and Brown [40] gave the detailed description of each site in the structure of synthetic analogue of the mineral, and the present paragraph aims to overview stanfieldite structure and highlight its features. The most interesting one is a pseudo-hexagonal character of the framework which can be best viewed via the arrangement of [PO4] tetrahedra along the [10–2] axis (Figure 3A). In principle, stanfieldite, being presented by the oversimplified formula *M*3(PO4)2 (*M* = Mg, Ca; *Z* = 24), can be regarded as a derivative of the well-known glaserite structure type, K3Na(SO4)2 [49,50]. Dickens and Brown [40] discuss the relationships between stanfieldite and glaserite-related phosphates belonging to α- and β-Ca3(PO4)2 structural types. The latter is known as a basement of whitlockite-group mineral structures [51], two of which, merrillite and ferromerrillite, are of fundamental importance in the mineralogy of meteorites [3,4]. In view of the common and intimate association of stanfieldite and merrillite in pallasite meteorites (Figure 1), these relationships could be of particular interest. However, contrary to Dickens and Brown [40], we would not overestimate the similarity of stanfieldite and merrillite structures. The unusual face-sharing of adjacent [*M*O6] octahedron and [PO4] tetrahedron characteristic of merrillite [3] does not occur in stanfieldite structure.

The refinement of occupancies of four Ca-sites in the stanfieldite structure showed no evidence for either Mg or Fe substitution. However, refinement of four Mg sites using both single-crystal and Rietveld methods concordantly leads to a partial substitution of Mg for Fe, with iron being preferentially concentrated in Mg1 (tetrahedral) and Mg5 (octahedral) positions (Table 3). The tetrahedral coordination of Mg1 is highly unusual; however, it is sometimes encountered in mineral structures such as åkermanite, Ca2MgSi2O7 (melilite structure type), and spinel. The *M*5A site allows mixed occupancy by Ca, Mg and Fe, and thus will be discussed in the next section. Based on the structural data, the overall formula of stanfieldite can be written as Ca7*M*2Mg9(PO4)7, where *M* = Ca, Mg or Fe2<sup>+</sup>.

**Figure 3.** Crystal structure of stanfieldite. (**A**) Pseudo-hexagonal stacking of [PO4] tetrahedra (yellow), projection along the [10–2] axis. The cations were hidden for clarity. (**B**) A slice of the structure on {010}. Arrangement of Mg-polyhedra and [PO4] tetrahedra. Ca atoms reside in the spaces and have been hidden for clarity. (**C**) A view of a pair of distorted [*M5A*O6] octahedra corner-linked by [PO4] tetrahedra.
