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Cation Substitution in Uranyl Phosphates of the Autunite Group: Equilibrium Relations and Crystallization Between Metatorbernite and Metauranocircite

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Cation Substitution in Uranyl Phosphates of the Autunite Group: Equilibrium Relations and Crystallization Between Metatorbernite and Metauranocircite Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/208 Comunicações Geológicas (2015) 102, Especial I, 27-30 ISSN: 0873-948X; e-ISSN: 1647-581X Cation substitution in uranyl phosphates of the autunite group: equilibrium relations and crystallization between metatorbernite and metauranocircite Substituição catiónica em fosfatos de uranilo do grupo da autunite: relações de equilíbrio e cristalização entre metatorbernite e metauranocircite M. Andrade1, J. Duarte1, I. Martins 1, J. Reis 1, J. Mirão3, M. A. Gonçalves1,2* Artigo original Original article © 2015 LNEG – Laboratório Nacional de Geologia e Energia IP Abstract: Uranyl phosphate minerals play an important role in the 1. Introduction uranium immobilization within weathering and supergene enrichment profiles. This work consists on the morphological, structural and Uranyl phosphate minerals are major constituents in weathered U chemical characterization of natural and synthetic minerals of Cu and Ba deposits and can display a multi-stage evolving history in the – metatorbernite and metauranocircite, respectively. SEM imaging has environment they crystalize. Their importance is two-fold: as revealed an extended range of morphologies, from tabular to rosette-like main U-bearing phases in weathering profiles with potential crystals, with the presence of epitaxial growths. These studies have also economic value (as in Nisa and Tarabau, where natural uranyl revealed natural heterogeneities affected by cationic substitution along phosphates of Cu and Ba were identified; Pinto et al., 2012; preferred crystallographic directions. The experimental results suggest Prazeres, 2011) and as fixing phases of U limiting its long-term, that the precipitation of metatorbernite is easier than metauranocircite. Simulations of the chemical system show that precipitation depends on million-year scale, dispersion in the oxidized surface supersaturation evolution, which in turn in a function of aqueous complex environment. It is thus important to understand the formation between phosphate and free uranyl ions. An electron probe thermodynamic stability of these phases and how they respond to microanalysis suggests that the failure to precipitate metauranocircite chemical changes of the surrounding environment. Building on may be due to later ionic depletion of the solution media (phosphate and previous knowledge on substitution mechanisms and uranyl), because of the early metatorbernite precipitation. crystallization relations between metatorbernite and Keywords: uranyl phosphates, uranium, metatorbernite, metauranocircite (Pinto et al., 2012; Sanchez-Pastor et al., 2013) metauranocircite, supergene environment. this work further investigates this relation in order to understand what drives the often late substitution of the Cu-phase by the Ba- Resumo: Os fosfatos de uranilo têm um papel determinante na phase, and how they compete for crystallization in the same imobilização do urânio em perfis de meteorização e enriquecimento chemical media. Hence, it’s of major importance to consider the supergénico. O presente trabalho incide na caracterização morfológica, general chemical formula of the autunite group estrutural e química destes minerais, naturais e sintéticos de Cu e/ou Ba – metatorbernite e metauranocircite. Análises em SEM revelaram uma (A(UO2)2(XO4)2·10-12H2O with A = Cu, Ca, Ba, or Mg and X = gama diversificada de morfologias, desde formas tabulares a rosáceas, P or As) in order to understand the cationic substitutions marcadas pela presença de crescimentos epitáxicos. Verificou-se também mentioned above. que os cristais naturais são heterogéneos, com substituições catiónicas ao longo de direcções cristalográficas preferenciais. Os resultados 2. Methods experimentais mostraram haver maior facilidade na precipitação de metatorbernite face à metauranocircite, mesmo nos sistemas com Cu e Ba The crystals studied were crystalized in laboratory in a silica-gel em solução. A modelação do sistema químico mostra que a precipitação é medium as described by Sánchez-Pastor et al. (2013). Also, função da evolução da sobressaturação, dependente da complexação do natural metatorbernite crystals from Musonoi Mine, Kolwezi, fosfato com o uranilo que baixa as respectivas actividades. Atendendo às análises de microssonda electrónica sugere-se que a precipitação de Katanga (Shaba) Province, Democratic Republic of Congo were metauranocircite seja inibida pelo empobrecimento em fosfato e uranilo immersed in silica gel and left to react with a BaCl2 solution. no sistema resultante da cristalização precoce de metatorbernite. Crystals were separated and cleaned for imaging and analysis Palavras-chave: fosfatos de uranilo, urânio, metatorbernite, metaurano- with an Environmental-SEM (HITACHI 3700N) working at 20 circite, ambiente supergénico. kV, a current ranging from 68 to 89 A, and low vacuum conditions (40 Pa) and equipped with a BRUKER Xflash 1Departamento de Geologia, Faculdade de Ciências da Universidade de 5010SDD EDS for qualitative chemical analyses. X-Ray micro- Lisboa, Edifício C6, Piso 4, Campo Grande, 1749-016 Lisboa, Portugal. diffraction of the crystals used a BRUKER Discovery X-ray 2 Instituto Dom Luiz, Faculdade de Ciências da Universidade de Lisboa, diffractometer with a Linxeye linear detector, a 0.3 mm Edifício C1, Piso 1, Campo Grande, 1749-016 Lisboa, Portugal. 3Departamento de Geociências e Laboratório HERCULES, Universidade de collimator, and a Göbel mirror. Radiation was generated from a Évora, Palácio do Vimioso, Largo Marquês de Marialva, 8, 7000-809 Évora, Cu-K lamp at 40 kV tension and 40 mA current. Scan ranged Portugal. from 3-75º 2, with 0.05º steps, and 2 s readings per step. Crystal *Autor correspondente/Corresponding author:[email protected] 28 Andrade et al. / Comunicações Geológicas (2015) 102, Especial I, 27-30 mounts were prepared for electron probe microanalysis (EPMA) limited Ba incorporation, as observed in SEM and EPMA (see and the crystals were analyzed for U, P, Cu, Ba, and Si (because below). of the gel medium). Astimex standards were used for Si The crystals from Cu-dominated experiments show (Diopside), Ba (Benitoite), U (UO2), P (Apatite), and Cu aggregates with tabular and micaceous habit, with sizes from 5 to (Cuprite). Analytical conditions used were a beam diameter of 20 10 m, also exhibiting macroscopic green colored dendritic m (reduced to 10m for the smaller crystals), 9 nA current (10 morphologies. Crystals from Ba-dominated experiments show nA for the smaller crystals), and 15 kV voltage. rosette-like morphologies, with sizes from 50 m to more than The porous silica-gel crystallizing environment was modelled 500 m, essentially yellow to brownish-white colored, in with PHREEQC (Parkhurst and Appelo, 1999) for transport and macroscopic acicular, capilar or dendritic aggregates. In the Cu multicomponent counter-diffusion of cationic and anionic species and Ba experiments, the crystals show similar morphologies to in the gel, without precipitation. The database used was the the previously described for both metatorbernite and phreeqc.dat with data on the solubility of metatorbernite and metauranocircite ranging from tabular to spheroidal, with sizes metauranocircite from Cretaz et al. (2013) and Vochten et al. from 50 to 100 m. They form macroscopic aggregates showing (1992), respectively. The U speciation was taken from the dendritic to tabular habits, along crystallization zones, where the llnl.dat. U diffusion coefficient was taken from Awakura et al. abundance and density of small crystals is significant. (1987). 3.2 Chemical features of the crystals 3. Results and Discussion The images showing EDS qualitative chemical maps revealed some particular aspects worth emphasizing: 3.1 Crystal morphologies 1. Epitaxial crystal growth is a mechanism of precipitation of Only 3 experiments were described by Sánchez-Pastor et al. a Ba-rich phase over metatorbernite (Fig. 2), which is shown by a (2013) which had a successful outcome, while the rest had higher signal of Ba over the topographic highs as opposed to a crystallization times > 2 years or did not crystallize anything. It lower signal elsewhere. was these ones that we used in this study, and their major 2. Natural metatorbernite crystals showed chemical difference to the set of experiments of Sánchez-Pastor et al. heterogeneities where a square domain core of metautunite (Ca) (2013) was that the gel in the horizontal column was reactive is surrounded by metatorbernite (Fig. 3), indicating that the (containing a fixed constant concentration of 50 ppm of U) cationic substitution in uranyl-phosphates of the metautunite instead of being diffused along with the remaining cations (Cu group affects a wider range of cations and mineral species than and/or Ba). Both experiment groups with either Cu or Ba in previously expected, as it has also been recently confirmed in solution revealed the presence of metatorbernite or natural environments (Prazeres et al., submitted). metauranocircite, respectively, showing different morphologies 3. Cation substitutions of both Ca2+ and Cu2+ for Ba2+ in and crystal aggregates. However, experiments with Cu and Ba in natural samples (Fig. 4). This substitution occurs pervasively solution showed only metatorbernite crystals with no direct along structural discontinuities, essentially through cleavage indication of the presence of a Ba-phase (Fig. 1).This was only plans and other crystallographic directions with a clear detected in some crystals as mixed
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