**5. Conclusions**

Summarizing our crystal's chemical observations, we could suggest that the majority of the discovered natural uranyl sulfates were grown from heated solutions, and the temperature range could be assumed from the manner of interpolyhedral linkage. The presence of edge-sharing uranyl bipyramids (phosphuranylite anion topology [22]), most likely pointed to the temperatures of higher than 100 ◦C, and the crystal growth should apparently occur directly in hydrothermal conditions. The linkage of sulfate tetrahedra with *Ur* through the common edges also involved elevated temperatures, but of less values (~70–100 ◦C), which could be achieved by cooling the system. Moreover, in the second case, crystallization might start much later at environmental conditions but from the initially heated solutions. The enriched solution might pass some way along the cracks in the bedrock, transferring klaprothite-like clusters in the dissolved form. It is of interest that complexity parameters of the synthetic compounds were generally lower than that of minerals, whose structures

were based on the complexes with the same or genetically similar topologies. Furthermore, the topological complexity of the uranyl sulfate structural units contributed the major portion to the overall complexity of the synthesized compounds, while the complexity of the respective minerals was largely governed by the interstitial structure and H-bonding system.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/9/12/660/s1: Cif files for 1, 3, and 4.

**Author Contributions:** Conceptualization, O.S.T. and V.V.G.; Methodology, O.S.T. and I.V.K.; Investigation, O.S.T., I.V.K., S.N.B., and A.A.Z.; Writing-Original Draft Preparation, O.S.T. and V.V.G.; Writing-Review and Editing, V.V.G.; Visualization, V.V.G. and I.V.K.

**Funding:** This research was funded by the Russian Science Foundation (grant 18-17-00018).

**Acknowledgments:** The XRD and EDX measurements have been performed at the X-ray Diffraction Center and Center for Microscopy and Microanalysis of the St. Petersburg State University.

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