Abstract :
[en] Since 1960, the number of described uranyl mineral species has been multiplied by three.
To date more than 200 mineral species containing uranium as a main structural element are reported in
Nature, and the crystal structures have been determined and refined for only one third of them.
Furthermore, many new uranyl minerals are still reported to data, showing sometimes novel chemical
compositions and structural arrangements. The uranium mineralogy is a fascinating subject, and an
essential field of research as it contributes to the improvement of the management techniques of
nuclear waste.
The first part of this work was dedicated to the mineralogical characterisation of uranyl
minerals, and especially of the autunite-type minerals, occurring in the Rabejac deposit. Indeed, the
identification analyses have shown that many samples contain uranyl arsenates of the autunite and
meta-autunite groups. Electron-microprobe chemical analyses performed on these minerals species
indicate that the P content in (meta-)zeunerite and (meta-)uranospinite is significant in several
samples, confirming the solid-solution series between uranyl arsenates and phosphates. Also, cationic
substitutions occurring on the A site were observed, especially in uranospinite samples, in which
significant amount of Ca is substituted for Mg. Single-crystal X-ray diffraction experiments have been performed on many members of the
(meta-)zeunerite-(meta-)torbernite series, and the crystallographic parameters were put into correlation
with the variation of the chemical composition. The crystal structure of arsenuranospathite, the Asanalogue
of the Al-bearing uranyl phosphate uranospathite, was solved for the first time, indicating
that these two minerals are isostructural (Dal Bo et al., 2015). Also, a structure model is proposed for
metauranocircite-I, one of the Ba-bearing uranyl phosphate of the meta-autunite group.
Investigations of uranyl mineral samples from other localities have allowed to discover single-crystals
of bassetite, one of the Fe end-members of the autunite group. The crystal structure of bassetite was
also solved for the first time, and these new structural data were presented in parallel to those obtained
on saléeite, the Mg-analogue of bassetite (Dal Bo et al., 2016). Thanks to these new data, a discussion
about the mineral hierarchy of the (meta-)autunite group is proposed. In addition, the crystal structure
of the rare Al-bearing uranyl phosphate furongite from the Kobokobo pegmatite was determined,
showing that this mineral species has unique and novel structural features. This specific study, as well
as the redefinition of the structural formula of phuralumite, was the occasion to provide a review about
the formation of Al polyhedra clusters in uranyl minerals, and to try to elucidate their physicochemical
conditions of crystallisation.
Many new crystallographic data were also collected on uranyl minerals for which the crystal
structure was already reported in the literature. Therefore, it was possible to confirm the previous
studies, or to highlight important discrepancies, especially in the case of the Al-bearing uranyl
phosphate phuralumite. The H-bonds networks, which are essential for the stability of the structure,
have been established in the structure of the uranyl carbonate liebigite, and in the structure of the
uranyl silicates cuprosklodowskite, kasolite and soddyite. The second part of this work was devoted to the hydrothermal synthesis of uranyl oxysalts.
The experimental conditions, as well as the chemical systems, were chosen in agreement with the
conditions occurring in natural uranium deposits, or in geological repositories. A new compound,
Be[(UO2)(PO4)]2(H2O)2.5, has been synthesized at low temperature. This is the third Be-bearing uranyl
phosphate reported to date, and its crystal structure is based upon sheets showing the uranophane
anion topology, typical of uranyl silicates occurring in the oxidation zone of the uranium deposits.
The well-known Ba[(UO2)3(MoO4)4(H2O)](H2O)3 and (NH4)2[(UO2)(MoO4)2](H2O) compounds have
been synthesized, and new crystallographic data were collected on them. In addition, it was possible to
determine the hydrogen-bonds networks for the first compound, and to redefine the Mo6+ coordination
polyhedra in the second one.