Oxidation-hydration weathering of uraninite, the most common U-bearing mineral in nature, comprises various physical and chemical processes that lead to the destruction of the fluorite-type structure of uraninite where U is present as tetravalent. This results in replacement of uraninite by weathering products containing U in hexavalent form, i.e. as uranyl ion, UO 2 2+. The final assemblage of the weathering products, uranyl minerals, and their compositions depend on the various factors, namely the composition of the primary minerals and percolating oxidizing fluids that cause the alteration. The knowledge of such processes and stabilities of the uranium minerals is of the great interest namely due to demand for U as the energy source. During the past decade there has been substantial progress in understanding the mineralogy, crystallography and thermodynamics of uranyl minerals and thus a substantially improved understanding of the weathering processes themselves. This review aims to summarize the state-of-art of the current knowledge on uranium-related topics as well and identify some of the important questions that remain unanswered.The following text is dedicated to Jiří Čejka on occasion of his 85 th birthday anniversary. Jiří greatly contributed not only to the spectroscopy and mineralogy of uranyl minerals, but also to the questions pertaining their origin and stability. Many important issues were addressed, even if briefly, in the pioneering book "Secondary Uranium Minerals" by Čejka and Urbanec (1990) which has served, for a long-time, as a guide for beginning uranium mineralogists.

Uranyl sulfates, including those occurring in Nature (∼40 known members), possess particularly interesting structures. They exhibit a great dimensional and topological diversity of structures: from those based upon clusters of polyhedra to layered structures. There is also a great variability in the type of linkages between U and S polyhedra. From the point of view of complexity of those structures (measured as the amount of Shannon information per unit cell), most of the natural uranyl sulfates are intermediate (300–500 bits per cell) to complex (500–1000 bits per cell) with some exceptions, which can be considered as very complex structures (>1000 bits per cell). These exceptions are minerals alwilkinsite‐(Y) (1685.95 bits per cell), sejkoraite‐(Y) (1859.72 bits per cell), and natrozippeite (2528.63 bits per cell). The complexity of these structures is due to an extensive hydrogen bonding network which is crucial for the stability of these mineral structures. The hydrogen bonds help to propagate the charge from the highly charged interlayer cations (such as Y3+) or to link a high number of interlayer sites (i.e. five independent Na sites in the monoclinic natrozippeite) occupied by monovalent cations (Na+). The concept of informational ladder diagrams was applied to the structures of uranyl sulfates in order to quantify the particular contributions to the overall informational complexity and identifying the most contributing sources (topology, real symmetry, interlayer bonding).

Raman spectroscopy has been used to study vanadates inthe solid state. The molecular structure of the vanadate minerals vésigniéite [BaCu 3 (VO 4 ) 2 (OH) 2 ] and volborthite [Cu 3 V 2 O 7 (OH) 2 ·2H 2 O] have been studied by Raman spectroscopy and infrared spectroscopy. The spectra are related to the structure of the two minerals. The Raman spectrum of vésigniéite is characterized by two intense bands at 821 and 856 cm −1 assigned to ν 1 (VO 4 ) 3− symmetric stretching modes. A series of infrared bands at 755, 787 and 899 cm −1 are assigned to the ν 3 (VO 4 ) 3− antisymmetric stretching vibrational mode. Raman bands at 307 and 332 cm −1 and at 466 and 511 cm −1 are assigned to the ν 2 and ν 4 (VO 4 ) 3− bending modes. The Raman spectrum of volborthite is characterized by the strong band at 888 cm −1 , assigned to the ν 1 (VO 3 ) symmetric stretching vibrations. Raman bands at 858 and 749 cm −1 are assigned to the ν 3 (VO 3 ) antisymmetric stretching vibrations; those at 814 cm −1 to the ν 3 (VOV) antisymmetric vibrations; that at 508 cm −1 to the ν 1 (VOV) symmetric stretching vibration and those at 442 and 476 cm −1 and 347 and 308 cm −1 to the ν 4 (VO 3 ) and ν 2 (VO 3 ) bending vibrations, respectively. The spectra of vésigniéite and volborthite are similar, especially in the region of skeletal vibrations, even though their crystal structures differ.

Raman spectroscopy complemented with infrared spectroscopy was used to study the molecular structure of the mineral euchroite, a mineral involved in a complex set of equilibria between the copper hydroxy arsenates: euchroite Cu 2 (AsO 4 )(OH)·3H 2 O → olivenite Cu 2 (AsO 4 )(OH) → strashimirite Cu 8 (AsO 4 ) 4 (OH) 4 ·5H 2 O → arhbarite Cu 2 Mg(AsO 4 )(OH) 3 . The Raman bands observed at 848 and 768 cm −1 are assigned to the ν 1 and ν 3 (AsO 4 ) 3− stretching vibrations. Two Raman bands at 358 and 385 cm −1 are attributed to the ν 2 (AsO 4 ) 3− bending mode and two Raman bands at 441 and 474 cm −1 to the ν 4 AsO 4 3− bending modes. Two sharp Raman bands are observed at 3470 and 3537 cm −1 and are attributed to the stretching vibrations of hydroxyl units. A comparison of the Raman spectrum of euchroite is made with the other copper hydroxy arsenate minerals including strashimirite, olivenite, cornubite, and cornwallite.