The effect of chemical variability and weathering on Raman spectra of enargite and fahlore

Enargite (Cu3AsS4) and tennantite (Cu12As4S13) are typical As-bearing sulfides in intermediate- and high-sulfidation epithermal deposits. Trace and major element variations in enargite and tennantite and their substitution mechanisms are widely described. However, Raman spectra of the minerals with correlative quantitative chemical information are rarely documented, especially for enargite. Therefore, comparative electron and μ-Raman microprobe analyses were performed on enargite and fahlore grains. These spectra can be used in the industrial detection and subsequent removal of As-bearing sulfides prior to ore beneficiation in order to diminish the environmental impact of the metallurgical technologies. A simple Sb5+–As5+ substitution in enargite was confirmed by Raman analyses. Similarly, a complete solid solution series from tetrahedrite to tennantite (i.e., Sb3+–As3+ substitution) can be correlated with a gradual evolution in their Raman spectra. In turn, Te4+ occupies the As3+ and Sb3+ sites in fahlore by the coupled substitution Te4+ + Cu+ → (As, Sb)3+ + (Cu, Fe, Zn)2+. Accordingly, Raman bands of goldfieldite (Te-rich member) are strongly broadened compared with those of tetrahedrite and tennantite. A secondary phase with high porosity and a fibrous or wormlike texture was found in enargite in a weathered sample. The chemical composition, Raman spectrum, and X-ray diffraction signature of the secondary phase resemble tennantite. A gradual transformation of the primary enargite into this secondary phase was visualized by comparative electron and Raman microprobe mapping.</p

Full crystal structure, hydrogen bonding and spectroscopic, mechanical and thermodynamic properties of mineral uranopilite

14 pags., 10 figs., 4 tabs.The determination of the full crystal structure of the uranyl sulfate mineral uranopilite, (UO)(SO)O(OH)·14HO, including the positions of the hydrogen atoms within the corresponding unit cell, has not been feasible to date due to the poor quality of its X-ray diffraction pattern. In this paper, the complete crystal structure of uranopilite is established for the first time by means of first principles solid-state calculations based in density functional theory employing a large plane wave basis set and pseudopotential functions. The computed unit-cell parameters and structural data for the non-hydrogen atoms are in excellent agreement with the available experimental data. The computed X-ray diffraction pattern is also in satisfactory agreement with the experimental pattern. The infrared spectrum of uranopilite is collected from a natural crystal specimen originating in Jáchymov (Czech Republic) and computed employing density functional perturbation theory. The theoretical and experimental vibrational spectra are highly consistent. Therefore, a full assignment of the bands in the experimental infrared spectrum is performed using a normal mode analysis of the first principles vibrational results. One overtone and six combination bands are recognized in the infrared spectrum. The elasticity tensor and phonon spectra of uranopilite are computed from the optimized crystal structure and used to analyze its mechanical stability, to obtain a rich set of elastic properties and to derive its fundamental thermodynamic properties as a function of temperature. Uranopilite is shown to have a large mechanical anisotropy and to exhibit the negative Poisson's ratio and negative linear compressibility phenomena. The calculated specific heat and entropy at 298.15 K are 179.6 and 209.0 J Kmol, respectively. The computed fundamental thermodynamic functions of uranopilite are employed to obtain its thermodynamic functions of formation in terms of the elements and the thermodynamic properties of a set of chemical reactions relating uranopilite with a representative group of secondary phases of spent nuclear fuel. From the reaction thermodynamic data, the relative stability of uranopilite with respect to these secondary phases is evaluated as a function of temperature and under different hydrogen peroxide concentrations. From the results, it follows that uranopilite has a very large thermodynamic stability in the presence of hydrogen peroxide. The high stability of uranopilite under this condition justify its early crystallization in the paragenetic sequence of secondary phases occurring when uranium dioxide is exposed to sulfur-rich solutions.The computation time provided by the Centro Tecnico de Informatica from CSIC is deeply acknowledged. This work has been performed in the framework of a CSIC–CIEMAT collaboration agreement: “Caracterizacion experimental y teorica de fases secundarias y ´oxidos de uranio formados en condiciones de almacenamiento de combustible nuclear”. We thank Jana Ederova (Institute of Chemical Technology in Prague, Czech Republic) for the carefully undertaken thermal analysis. JP acknowledges the support of the Czech Science Foundation through the project GACR 20-11949S. VT is grateful for the financial support of the Ministry of Science, Innovation and Universities of Spain through Project FIS2016-77726-C3-1-P. JCˇ acknowledge the financial support of the Ministry of Culture of the Czech Republic (long-term project DKRVO 2019-2023/1.II.b; National Museum, 00023272)

Crystal Structure, Infrared Spectrum and Elastic Anomalies in Tuperssuatsiaite

14 pags., 8 figs.The full crystal structure of the phyllosilicate mineral tuperssuatsiaite, including the positions of the hydrogen atoms in its unit cell, is determined for the first time by using first-principles solid-state methods. From the optimized structure, its infrared spectrum and elastic properties are determined. The computed infrared spectrum is in excellent agreement with the experimental spectrum recorded from a natural sample from Ilímaussaq alkaline complex (Greenland, Denmark). The elastic behavior of tuperssuatsiaite is found to be extremely anomalous and significant negative compressibilities are found. Tuperssuatsiaite exhibits the important negative linear compressibility phenomenon under small anisotropic pressures applied in a wide range of orientations of the applied strain and the very infrequent negative area compressibility phenomenon under external isotropic pressures in the range from 1.9 to 2.4 GPa. The anisotropic negative linear compressibility effect in tuperssuatsiaite is related to the increase of the unit cell along the direction perpendicular to the layers charactering its crystal structure. The isotropic negative area compressibility effect, however, is related to the increase of the unit cell dimensions along the directions parallel to the layers.Te supercomputer time provided by the CTI-CSIC center is greatly acknowledged. JP acknowledges the support through the project no. LO1603 of the Ministry of Education, Youth and Sports National Sustainability Program I of the Czech Republic. JS was supported by the Ministry of Culture of the Czech Republic (long-term project DKRVO 2019–2023/1.II.b)

The crystal structures and mechanical properties of the uranyl carbonate minerals roubaultite, fontanite, sharpite, widenmannite, grimselite and čejkaite

25 pags., 17 figs., 5 tabs.The research involving the crystal structures and properties of uranyl carbonate minerals is essential in actinide environmental chemistry due to the fundamental role played by these minerals in the migration of actinides from uranium deposits and nuclear waste repositories and in the investigation of accidental site contaminations. In this work, the crystal structure, hydrogen bonding network, X-ray diffraction pattern and mechanical properties of six important uranyl carbonate minerals, roubaultite (Cu2[(UO2)3(CO3)2(OH)2]·4H2O), fontanite (Ca[(UO2)3(CO3)2(OH)2]·6H2O), sharpite (Ca[(UO2)3(CO3)4(OH)2]·3H2O), widenmannite (Pb2[(UO2)(CO3)2(OH)2]), grimselite (K3Na[(UO2)(CO3)3]·H2O) and čejkaite (Na4[(UO2)(CO3)3]), are investigated using first principles solid-state methods based in density functional theory. The determination of the positions of the hydrogen atoms in the unit cells of fontanite, sharpite and grimselite minerals, defining the hydrogen bonding network in their crystal structures, has not been feasible so far due to the low quality of their experimental X-ray diffraction patterns. The full crystal structures of these minerals are obtained here and their hydrogen bonding networks are studied in detail. Furthermore, the experimental structures of roubaultite, widenmannite and čejkaite, obtained by refinement from X-ray diffraction data, are confirmed. In the six cases, the computed unit-cell parameters and the associated geometrical variables are in excellent agreement with the available experimental information. Furthermore, the X-ray diffraction patterns computed from the optimized structures are in satisfactory agreement with their experimental counterparts. The knowledge of the full crystal structures, being extraordinarily relevant for many scientific fields, is also extremely interesting because it opens the possibility of determining their physico-chemical properties using the first principles methodology. The measurement of these properties under safe conditions is very expensive and complicated due to the radiotoxicity of these minerals. In this paper, a large set of relevant mechanical properties of these minerals are determined including their bulk, shear and Young moduli, the Poisson's ratio, ductility, hardness and anisotropy indices and bulk modulus pressure derivatives. These properties have not been measured so far and, therefore, are predicted here. Four of these minerals, roubaultite, fontanite, sharpite and widemmannite, are highly anisotropic and exhibit negative mechanical phenomena under the effect of small external pressures. This journal isThe supercomputer time provided by the CTI-CSIC center is greatly acknowledged. This work has been carried out in the context of a CSIC–CIEMAT collaboration agreement: “Caracterización experimental y teórica de fases secundarias y óxidos de uranio formados en condiciones de almacenamiento de combustible nuclear”. JP acknowledges the support of the Czech Science Foundation through the project GACR 20- 11949S. JS was supported by the Ministry of Culture of the Czech Republic (long-term project DKRVO 2019–2023/1.II.b; National Museum, 00023272)