The corrosion of uraninite under oxidizing conditions

Nous avons synthetise la torbernite, Cu[(UO 2 )(PO 4 )] 2 (H 2 O) 1 2 , la zeunerite, Cu[(UO 2 )(AsO 4 )] 2 (H 2 O) 1 2 , la metatorbernite, Cu[(UO 2 )(PO 4 )] 2 (H 2 O) 8 , et la metazeunerite, Cu[(UO 2 )(AsO 4 )] 2 (H 2 O) 8 , par diffusion dans des gels ou bien par voie hydrothermale Les intensites des raies en diffraction X sur monocristal ont ete mesurees a temperature ambiante en utilisant un rayonnement MoKa et un detecteur de type CCD Nous avons resolu la structure des quatre composes par methodes directes, avec affinement sur matrice entiere par techniques de moindres carres en utilisant les facteurs F2 pour toutes les reflexions uniques jusqu'a un residu wR 2 (torbernite, zeunerite, metatorbernite et metazeunerite) de 56, 35, 48 et 51% pour toutes les donnees, et R1 de 24, 16, 20 et 23%, calcule pour 764, 468, 794 et 1447 reflexions uniques observees (‖F o ‖ ≥ 4σ F ), respectivement La torbernite est tetragonale, groupe spatial P4/nnc, a 70267(4), c 20807(2) A, V 10273(1) A 3 , Z = 2, D c a l c 3264(1) g/cm 3  La zeunerite est isostructurale avec la torbernite, a 71797(3), c 20857(1) A, V 10751(1) A 3 , Z= 2, D c a l c 3391(1) g/cm 3  La structure de la metatorbernite a ete determinee a partir d'un cristal macle par meroedrie Elle est tetragonale, groupe spatial P4/n, a 69756(5), c 17349(2) A, V 8442(1) A 3 , Z = 2, D c a l c 3689(1) g/cm 3  La metazeunerite est isostructurale avec la metatorbernite, a 71094(1), c 17416(1) A, V 8803(1) A 3 , Z = 2, D c a l c 3869(1) g/cm 3 ; le cristal qui a servi a l'ebauche aussi est macle Ces mineraux contiennent un feuillet de type autunite, de composition [(UO 2 )(PO 4 )] - , dans lequel il y a partage des coins equatoriaux des bipyramides carrees a uranyle avec des tetraedres de phosphate Dans chacune des structures, les cations Cu 2 + sont situes entre les feuillets dans des octaedres rendus difformes (4 + 2) a cause de l'effet de Jahn-Teller, avec des liaisons courtes a quatre groupes H 2 O dans un agencement carre planaire, et deux liaisons plus longues aux atomes d'oxygene des ions uranyle Un groupe H 2 O symetriquement independant est present dans chaque structure, lie seulement par liaisons hydrogene, et dans la torbernite (et la zeunerite) forme des agencements planaires carres de groupes interstitiels de H 2 O par-dessus et par-dessous les plans contenant les cations Cu 2 +  L'affinement des quatre structures avec certaines contraintes a propos des longueurs des liaisons impliquant l'hydrogene a mene a des descriptions cristallochimiquement raisonnables des liaisons hydrogene

Crystal structures and synthesis of the copper-dominant members of the autunite and meta-autunite groups: torbernite, zeunerite, metatorbernite and metazeunerite

The formation of autunite (Ca(UO2)2(PO4)2nH2O) within the leached layer of dissolving apatite: incorporation mechanism of uranium by apatite

Kinetic dissolution studies were conducted on four prominent U-Ca-PO4 minerals (metaschoepite, becquerelite, chernikovite and metaautunite). Synthetic samples were contacted with four extractants (acetic acid, deionized water, EDTA and sodium bicarbonate) at room temperature at two concentrations, 100 mM and 1 mM. Dissolution progress was monitored by periodic sampling for dissolved U, and dissolution rates were obtained from fits to a three term exponential model. Significant variations were observed in the rate and extent of dissolution among the mineralsexamined. The uranyl phosphates chernikovite and metaautunite proved resistant to dissolution in non-carbonate systems, with dissolution half-times of days to weeks in 100 mM systems and weeks to years in 1 mM systems. In contrast, the uranyl oxide hydrates schoepite and becquerelite were solubilized over much shorter time scales. While 100 mM bicarbonate was successful in dissolving U in all forms, dissolution rates varied among the four minerals. Overall, EDTA was the least sensitive to a 100 to 1 mM drop in its concentration in its solubilization of all four mineral phases, underscoring the importance of organic complexation for the environmental mobility of uranium.

The impact of mineralogy in the U(VI)—Ca—PO4 system on the environmental availability of uranium

/pdf/sodium-meta-autunite-colloids-synthesis-characterization-and-5apt8ej15n.pdf

Sodium meta-autunite colloids: Synthesis, characterization, and stability

Genesis of metaautinute [Ca(UO2/PO4)2 · 7H2O] starting from curite hints at the existence of an intermediate hydrogen autunite stage [HUO2PO4 · 4H2O]. The substitution of protons in hydrogen autunite by Ca2+ ions is proved by electrokinetic measurements. As a consequence of the similarity between X-ray powder patterns of hydrogen autunite and meta-autunite a glycolation method has been applied in order to distinguish the two species. The cell dimensions have been determined from Guinier X-ray diffraction patterns. Both minerals are tetragonal with a=6.981±0.005 A and c=8.448±0.005 A for metaautunite and a=7.084±0.005 A and c=8.777±0.005 A for hydrogen autunite. For both minerals, the zeta-potential is mostly negative and is strongly influenced by temperature, pH and concentration of cations in the suspension. The surface conductivity has been calculated from the value of the zetapotential. The electrokinetic properties of metaautunite are very similar to those of metatorbernite.

Transformation of curite into metaautunite paragenesis and electrokinetic properties

A discussion is given concerning the transformation of curite into meta-torbernite in an acid environment. Crystallographic data together with differential thermal analysis show that the end-product obtained by this method is identical with meta-torbernite. Measurements of the zeta-potential on these meta-torbernite specimens result in negative values, which are strongly influenced by pH and NaCl concentrations. The surface conductivity is calculated from the measured zeta-potential.

Formation of meta-torbernite starting from curite: Crystallographic data and electrokinetic properties

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Transformation of curite into metaautunite paragenesis and electrokinetic properties

Formation of meta-torbernite starting from curite: Crystallographic data and electrokinetic properties