**5. Conclusions**

The refinement of the demesmaekerite crystal structure makes it possible to determine the H atoms positions belonging to the interstitial H2O molecules. The refinement of the guilleminite crystal structure allows the determination of one additional site arranged within the void of the interlayer space and occupied by the H2O molecule, which suggests the formula of guilleminite to be written as Ba[(UO2)3(SeO3)2O2](H2O)4 instead of Ba[(UO2)3(SeO3)2O2](H2O)3.

Our numerous attempts to determine the crystal structure of haynesite, [(UO2)3(SeO3)2(OH)2](H2O)5, were unsuccessful. However, several assumptions could be made. First, the presence of (OH)− groups within such a dense layer is doubtful. At least, there is no other evidence for O atoms' protonation in the structures of all the other natural and synthetic uranyl selenites, whose structures are built upon layers of the same topology. Moreover, the structure of haynesite has to be electroneutral, and H2O molecules from the interlayer space should be replaced by H3O<sup>+</sup> cations, which points to the similarity and, probably, to the common genesis or even the closest relationships between the haynesite and larisaite, Na(H3O)[(UO2)3(SeO3)2O2](H2O)4.

Comparison of the isotypic natural and synthetic uranyl-bearing compounds suggests that uranyl selenite mineral formation requires heating, which most likely, keeping in mind their surface or near-surface occurrence conditions, can be attributed to the radioactive decay.

Structural complexity studies revealed an interesting tendency in that the majority of synthetic compounds have the topological symmetry of uranyl selenite building blocks equal to the structural symmetry, which means that the highest symmetry of uranyl complexes is preserved regardless of the interstitial filling of the structures. Whereas the real symmetry of chained and layered complexes in the structures of uranyl selenite minerals is lower than their topological symmetry, which means that interstitial cations and H2O molecules significantly affect the structural architecture of natural compounds. At the same time, structural complexity parameters for the whole structure are usually higher for the minerals than that for synthetic compounds of a similar or close organization, which probably indicates the preferred existence of such natural-born architectures.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/9/12/639/s1: Cif files for guilleminite and demesmaekerite.

**Author Contributions:** Conceptualization, V.V.G. and J.P.; Methodology, V.V.G., V.M.K. and I.V.K.; Investigation, V.V.G., I.V.K., V.M.K., M.N.M., A.V.K. and J.P.; Writing-Original Draft Preparation, V.V.G., I.V.K., V.M.K., M.N.M., A.V.K. and J.P.; Writing-Review & Editing, V.V.G. and J.P.; Visualization, V.V.G. and I.V.K.

**Funding:** This research was funded by the Russian Science Foundation (grant 18-17-00018 to V.V.G. and I.V.K.) and through the project of the Ministry of Education, Youth and Sports National sustainability program I of the Czech Republic (project No. LO1603 to J.P.).

**Acknowledgments:** The XRD measurements of guilleminite have been performed at the X-ray Diffraction Centre of the St. Petersburg State University. We are grateful to reviewers for useful comments.

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