**5. Ca3Mg3(PO4)4 Phosphors: Do They Exist?**

In this section, we would like to clarify the mistake caused by the incorrect database assertion of primary structural data on the synthetic analogue of stanfieldite reported by Dickens and Brown [40]. In the title of their article, Dickens and Brown report the formula Ca7Mg9(Ca,Mg)2(PO4)12, with Ca = Mg in the Ca5 site, leading to a bulk one Ca4Mg5(PO4)6. It looks obvious that the mistake was introduced in the stage of structural data transfer from the article tables to the ICSD database. The Ca5 site, equally occupied by Ca and Mg [40], was erroneously assigned to be fully occupied by Ca. The latter had led to a wrong formula, Ca3Mg3(PO4)4, which still appears in the ICSD database [53] (ICSD code 23642). Moreover, the calculated X-ray powder diffraction pattern has been further included into the ICDD (JCPDS) database under the reference number JCPDS-ICDD 73-1182 (Figure 4). There is a substantial interest to the family of luminophores (phosphors) based on the stanfieldite structure [35–39]. It is erroneous that the mistake in the chemical formula caused by the incorrect primary data transfer has passed first to the ICDD database and then to the papers devoted to a study of these phosphor materials [35–39]. Unfortunately, neither of the published articles does contain quantitative chemical data on synthesized phosphors. Thus, in lieu of any evidence supporting the existence of Ca3Mg3(PO4)4, one can state that these phosphors are in fact stanfieldite-based, Ca4Mg5(PO4)4 compounds.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/10/6/464/s1: Supplementary crystallographic data for stanfieldite from the Brahin meteorite in Crystallographic Information File (CIF) format. Alternatively, CCDC reference number 1998335 contains the same data, and it can be obtained free of charge from the Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk. S2, supplementary crystallographic Tables S1–S3.

**Author Contributions:** Conceptualization, writing—original draft preparation, S.N.B.; investigation, S.N.B., M.G.K. and V.N.B.; data curation, E.V.O. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the Russian Science Foundation, grant number 18-17-00079.

**Acknowledgments:** The authors gratefully acknowledge the curators of the Mining Museum, Saint Petersburg Mining University, for providing the sample of the Brahin meteorite used in this study. We are thankful to anonymous reviewers for their comments and suggestions. This research was supported by the resource Centre for X-ray diffraction studies and the Geomodel resource centre of Saint Petersburg State University.

**Conflicts of Interest:** The authors declare no conflict of interest.
