**1. Introduction**

It is known that the speciation of chemical elements in meteoritic substance significantly differs from their speciation in contemporary terrestrial lithosphere [1]. Concerning phosphorus, the main geochemical factors governing the diversity of terrestrial phosphorus-bearing minerals are (1) highly oxidative conditions typical of the present Earth and (2) the aquatic environment, which dramatically multiplies the number of possible pathways for phosphate geosynthesis. Contrary to Earth, the reductive and (in general) water-free conditions that accompanied the formation and early evolution of celestial bodies determined the limited number of meteoritic phosphorus-bearing minerals [2].

The most common meteoritic phosphates are the minerals related to the join merrillite–ferromerrillite, Ca9NaMg(PO4)7–Ca9NaFe2<sup>+</sup>(PO4)7 [3,4]. They are the typical accessories of ordinary chondrites, lunar rocks, martian meteorites, iron and stony-iron meteorites [3–6]. Chlorapatite, Ca5(PO4)3Cl, is the second abundant phosphate in meteorites [2]. A series of Mg-rich phosphates are characteristic of stony-iron meteorites—pallasites and mesosiderites [6,7]. These minerals usually occur in association with schreibersite, (Fe,Ni)3P, and because of that, they are used for the assessment of phosphide–phosphate redox equilibria [8]. Stanfieldite, ~Ca4Mg5(PO4)6, is the most common phosphate in the given assemblages [6,7]. The mineral was discovered in the Estherville mesosiderite [9] and is recognized in all well-studied pallasites [6,7,9–13], several mesosiderites [14,15] and even in the Lunar samples [16]. Being one of a very few Ca-rich phases occurring in pallasites, stanfieldite acts as a carrier of rare-earth elements substituting for Ca, and thereby is used in the studies of *REE* distribution among these meteorites [10,17]. Based on the overall observations and experimental data, stanfieldite can be regarded a late-stage cumulate of silicate-phosphate melts [18]. It is noteworthy that, in spite of an ordinary, "rock-forming" set of elements in the chemical composition, stanfieldite has never been encountered in terrestrial rocks. However, the phosphate identifiable as stanfieldite was reported as a constituent of prehistoric slags in Tyrol, Austria [19], and phosphorus-doped basaltic melts [20]. Stanfieldite of technogenic origin was detected as a component of bone-repairing bioceramics [21–28] and incinerated phosphate-based fertilizers [29–32].

In view of the notable role of stanfieldite in the mineralogy of pallasites, it looks unusual that the data on its chemical composition are rather ambiguous. The formula was first reported as Ca4Mg5(PO4)6 [9], but later on, the variations in the Ca/Mg ratio were shown to exist [12], and in many cases, stanfieldite formula is oversimplified as Ca3Mg3(PO4)4, e.g., Reference [33]. Synthetic Ca3Mg3(PO4)4 was reported as a phase in the Ca3(PO4)2–Mg3(PO4)2 system [34], and nowadays, the compounds having the same inferred formula are widely explored as luminophores (e.g., References [35–39]). However, the powder XRD (X-ray diffraction) data given in these works [35–39] refer not to Ca3Mg3(PO4)4 but to the compound Ca7Mg9(Ca,Mg)2(PO4)12 [40], which was erroneously mislabeled as "Ca3Mg3(PO4)4" both in ICSD and ICDD databases. Synthetic Ca7Mg9(Ca,Mg)2(PO4)12 was shown to have the same cell metrics as natural stanfieldite but it crystallizes in a different space group [9,40]. The latter discrepancy was discussed by Steele and Olsen in the abstract devoted to a crystal structure of natural stanfieldite [41]. However, no further structural data were provided by these authors, and as a consequence, no crystal structure of natural stanfieldite is available so far. In the course of a research of phosphate–phosphide assemblages of iron and stony-iron meteorites, we have found well-crystallized stanfieldite in the Brahin meteorite and carried out the detailed study of this mineral. We herein present the results and try to resolve some ambiguities related to a crystal chemistry of natural stanfieldite and its synthetic analogues.
