*3.1. Uranyl Selenite Minerals*

Guilleminite, Ba[(UO2)3(SeO3)2O2](H2O)3 [1,27], and demesmaekerite, Pb2Cu5[(UO2)2(SeO3)6 (OH)6](H2O)2 [2,28], were the first uranyl selenites found in nature (Table 1). These minerals occur in the lower part of the oxidized zone of the copper-cobalt deposit of Musonoi (Katanga, DR Congo). The first mineral was named after the general director of the Union Minière du Haut-Katanga (UMHK), co-founder of the International Mineralogical Association, French chemist and mineralogist, Jean-Claude Guillemin. Guilleminite crystallizes in the orthorhombic *Pmn*21 space group and forms small tabular crystals and canary yellow crusts. It occurs in association with malachite, uranophane-α, wulfenite, etc. The second mineral was named in honor of the director of the geological department of the UMHK, Belgian geologist Gaston Demesmaeker. Demesmaekerite crystallizes in the triclinic *P*-1 space group in the form of lamellar and elongated crystals of bottle-green to dark olive-green color in association with malachite, uranophane–α, chalcomenite, and other uranyl-selenites: Namely marthozite and derriksite as well as guilleminite.

Marthozite, Cu[(UO2)3(SeO3)2O2](H2O)8 [3,29], was also found in the Musonoi mine within a few years after, and named to honor Aimé Marthoz, former director of the UMHK. Marthozite crystallizes in the orthorhombic *Pbn*21 space group, in the form of well-faceted green crystals, in association with the other selenites, including guilleminite and demesmaekerite, as well as kasolite, cuprosklodowskite, malachite, chalcomenite, and sengierite. Mineral is isotypic with guilleminite.

A few years later, derriksite, Cu4[(UO2)(SeO3)2](OH)6 [4,30], was found at the same deposit in Congo, and named after Jean-Marie François Joseph Derriks, a Belgian geologist and administrator of the UMHK. Derriksite crystallizes in the orthorhombic *Pn*21*m* space group, as sub-green up to bottle-green-colored crystals, elongated at [001] or incrustations and fine-crystalline crusts on digenite and the mineral is associated with marthozite, demesmaekerite, kasolite, malachite, etc.


**1.**Crystallographiccharacteristicsofnaturaluranylselenites.

*Crystals* **2019**, *9*, 639

*Crystals* **2019**, *9*, 639

Next, natural uranyl selenite was discovered in 20 years across the Atlantic, in the Repete mine (Utah, USA). Haynesite, [(UO2)3(SeO3)2(OH)2](H2O)5 [5,31,32] is named after the American geologist Patrick Eugene Haynes. Haynesite is orthorhombic, occurs as amber-yellow tablets, transparent to translucent, elongated at [001], and as acicular prismatic rosettes up to 3 mm in diameter, and is associated with andersonite, boltwoodite, gypsum, and calcite as crusts on mudstones and sandstones.

Piretite, Ca(UO2)3(SeO3)2(OH)4·4H2O [6], calcium uranyl selenite from Shinkolobwe mine (Katanga, DR Congo) is named after the Belgian crystallographer Paul Piret. Piretite is orthorhombic, it crystallizes as lemon-yellow elongated tablets, irregular in outline and up to 3 mm, flattened on (001), or as needle-prismatic crystals up to 5 mm. It occurs in association with a masuyite-like uranyl-lead oxide as crusts on uraninite. It should be noted that crystal structures of haynesite and piretite have still not been determined.

The last to date, uranyl selenite mineral, larisaite, Na(H3O)[(UO2)3(SeO3)2O2](H2O)4 [7], was found in the Repete mine (Utah, UT, USA) and named in honor of Larisa Nikolaevna Belova, a Russian mineralogist and crystallographer who made a significant contribution to the knowledge on uranium minerals. Larisaite occurs as canary-yellow lamellar crystals up to 1 mm long, and as radial aggregates up to 2 mm across; most crystals are fissured and ribbed. The mineral is a supergene product associated with calcite, quartz, gypsum, montmorillonite, wölsendorfite, andersonite, haynesite, and uranophane–α in sedimentary rocks.
