**1. Introduction**

All natural compounds of U(VI) and selenium are selenites. Uranyl selenites can be justifiably attributed to rare mineral species. Nowadays, there are only seven uranyl selenite mineral species approved by the International Mineralogical Association as of 20 October 2019 (for comparison, there are >40 uranyl sulfates and ~50 uranyl phosphates): Guilleminite, Ba[(UO2)3(SeO3)2O2](H2O)3 [1], demesmaekerite, Pb2Cu5[(UO2)2(SeO3)6(OH)6](H2O)2 [2], marthozite, Cu[(UO2)3(SeO3)2O2](H2O)8 [3], derriksite, Cu4[(UO2)(SeO3)2](OH)6 [4], haynesite, [(UO2)3(SeO3)2(OH)2](H2O)5 [5], piretite, Ca(UO2)3(SeO3)2(OH)4·4H2O [6], and larisaite, Na(H3O)[(UO2)3(SeO3)2O2](H2O)4 [7]. Their occurrence is limited to just a few localities. First, these are Musonoi and Shinkolobwe mines in DR Congo [6], two of the minerals were only found in the Repete mine (San Juan County, Utah, USA) [5], and a few more occurrences in Europe could be mentioned (small uranium deposit Zálesí in the Czech Republic, Liauzun in France, and La Creusaz U prospect in Switzerland) [8]. Nevertheless, apart from mineralogy, uranyl selenites are of great interest from the geochemical and radiochemical points of view. It is known that fission products contain 53 g per ton [9] of long-lived 79Se isotope with a half-life of 1.1 <sup>×</sup> 106 years [10] after three years of nuclear fuel irradiation in the reactor. Thus, an understanding of the processes of mineral formation in nature and their synthetic analogs in laboratories can help in the processing of nuclear wastes. Crystal chemical and structural investigations are key points in such a material's scientific studies due to the essential knowledge of how the variation in the chemical composition and growth conditions affects the crystal structure formation.

Herein, we review the topological diversity and growth conditions of natural and synthetic uranyl selenites. Crystal structures of two uranyl selenite minerals guilleminite and demesmaekerite were refined. The structural complexity approach was implemented to determine the preference of a particular topological type, taking into account existing geometrical isomers.
