Abstract High Resolution Image. In this Article, the fundamental thermodynamic properties TP of schoepite and metaschoepite minerals, including their temperature dependence, have been obtained by using theoretical solid-state methods based on density functional theory using plane waves and pseudopotentials. The reactions A to D describe the formation of schoepite in terms of the uranium trioxide and the transformations of dehydrated schoepite, rutherfordine, and soddyite minerals into schoepite, respectively. Reactions E to H are analogous to the previous reactions but for metaschoepite.
Because, with the exception of reaction E , the experimental values of the TPs of these important reactions are not known, our theoretical calculations have allowed to predict the corresponding enthalpies and Gibbs free energies of reaction EOR and GFEOR and associated reaction constants RC for an extended range of temperature. Similarly, the relative TSs of metaschoepite with respect to schoepite in the presence of H 2 O 2 and water and under high H 2 O 2 concentrations, respectively, were determined by studying the following transformation reactions: K L. These results extend a previous work 30 in which the TPs of a large set of reactions including other UCMs were obtained.
This Article is organized in the following manner. The theoretical methodology employed is described in Section II. Subsection III. Subsections III. The results allowed to determine the relative TS of schoepite and metaschoepite with respect to a series of the most important secondary phases of the SNF under different conditions. The solubility products of schoepite and metaschoepite are studied in Subsection III.
Finally, the main conclusions of this Article are presented in the Section IV. The crystal structures of schoepite and metaschoepite mineral phases were modeled using the CASTEP code, 31 included in the Materials Studio package of programs. The inclusion of dispersion corrections improved significantly the description of the dense hydrogen bond network present in the unit cell structures of these materials.
The thermodynamic properties of some uranyl containing materials such as dehydrated schoepite, studtite, and metastudtite have also been determined using theoretical solid-state methods by Weck and Kim 44,45 and Sassani et al. The details of the computational treatment of schoepite were described in a previous article, 43 and those corresponding to metaschoepite are given in an Appendix of the Supporting Information. The calculations were carried out with two different values of the kinetic cutoff, and eV, to study the variation of the computed properties with respect to the increase of this calculation parameter.
Since the variation was small, the calculations performed with the cutoff value of eV were considered to be well converged, and the corresponding results were reported here. The structural results and the X-ray diffraction pattern obtained for schoepite in the previous work 43 and provided in the Appendix of the Supporting Information for metaschoepite are in very good agreement with their experimental counterparts.
The computed Raman spectrum of schoepite was in very good agreement with the one recorded experimentally. In order to determine the TPs of these materials, phonon calculations were carried out at the optimized structures of schoepite and metaschoepite. The phonon spectrum at the different points of Brillouin zone were obtained employing the density functional perturbation theory DFPT technique as second order derivatives of the total energy.
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Results and Discussion. Phonon calculations were performed at the optimized structure of schoepite. The TPs were evaluated from the computed phonon spectra. Figure 1 A—C display the computed isobaric heat capacity C p , entropy, enthalpy, and Gibbs free energy functions, respectively. High Resolution Image. Since the TPs of schoepite have not been measured experimentally, their values were predicted. The calculated values of the isobaric specific heat and entropy at The calculated values of the TPs of schoepite at selected temperatures are given in Tables S.
From the corresponding phonon calculations for metaschoepite, performed at its optimized equilibrium structure, the TPs were evaluated. Figures 2 A—C display the computed isobaric heat capacities, entropies, and Gibbs free energies, respectively. Although the range of thermal stability of metaschoepite appears to be from 0 to K, 56 it will be shown below that the thermal stability of metaschoepite in the presence of H 2 O 2 is much larger.
For this reason, the values of the TPs of this material are provided in Tables S. The results for the TPs of metaschoepite at While the agreement with the experimental data of Tasker 56 is only reasonable, the agreement with those of Barin 55 is very good. The calculated isobaric specific heat of metaschoepite is, as expected, lower than the corresponding calculated specific heat of schoepite by about 6. The values measured by Tasker et al. Table 1. It must be emphasized that Tasker et al. Besides, the authors state that the X-ray diffraction pattern of their sample was identical to that of Debets and Loopstra, 57 which corresponds again to metaschoepite, not schoepite.
A comparison of the calculated TPs of schoepite and metaschoepite with the corresponding experimental data for metaschoepite measured by Tasker et al. Similarly, the experimental values of the heat capacity, entropy, enthalpy and Gibbs free energy for metaschoepite reported by Barin 55 in the range of temperatures from The last experimental value of the isobaric specific heat reported by Tasker et al. As can be seen, the difference between the experimental results of Tasker et al.
The same is not true for the experimental TPs reported by Barin, 55 which agree well with the calculated properties even at temperatures of the order of K. The percent differences of the calculated specific heat, entropy, and Gibbs free energy with the experimental results of Barin 55 are 8. Whereas the TPs reported by Barin 55 seems to be accurate, the TPs computed in this work are recommended because they expand the temperature range in which they are known from From this value and our calculated value of the entropy of schoepite at This error estimation seems to be too small if we consider the experimental standard state EOF of metaschoepite reported by Kubatko et al.
Table 2. Table 3. Our calculated value at Based on this discussion, the experimental value of Kubatko et al. This value is very close to the experimental values reported by Cordfunke, 67 Hemingway, 59 and Tasker et al. In Table S. The calculated and experimental values at K differ by only 2.
However, the calculated results cover a much larger range of temperatures from 0 to K. These reactions represent the formation of schoepite in terms of the oxides and the transformations of dehydrated schoepite, rutherfordine, and soddyite minerals into schoepite, respectively. The results are given in Table S.
The error estimates for the temperatures at which these two changes of the stability of schoepite are found were of the same order as those found in our previous work. Finally, as shown in Figure 4 C,D, the GFEORs of reactions D and E are positive within the entire range of temperatures considered from to K , and therefore, both rutherfordine and soddyite do not transform into schoepite under normal H 2 O 2 free conditions. These reactions represent the formation of metaschoepite in terms of the oxides and the transformations of dehydrated schoepite, rutherfordine, and soddyite minerals into metaschoepite, respectively.
Schoepite is shown to be only slightly more stable than metaschoepite. This was expected since the distinction between schoepite and metaschoepite both at laboratory and natural settings is complicated, and the samples usually involve a mixture of both phases.
The TGA studies of Dawson et al. Most of these observations have been confirmed by several studies as those of Hoekstra and Siegel. Since dehydrated schoepite is more stable than UO 3 30 and dehydrates to UO 3 at about K, metaschoepite must dehydrate first at K becoming dehydrated schoepite, and then, this compound dehydrates to UO 3 at about K. This final temperature of dehydration to UO 3 is in very good agreement with the experimental value of K.
According to our previous work, 30 further heating leads to U 3 O 8 , the transformation beginning at a temperature of K. The conversion rate reaches a maximum at about K. The reaction of dehydration of schoepite into metaschoepite may be written as 7. As was already mentioned in the previous section, schoepite has a TS that is very similar to that of metaschoepite at H 2 O 2 free conditions.
Schoepite is found to be only slightly more stable than metaschoepite. The calculated TPFs of schoepite and metaschoepite were combined with those of metastudtite and studtite reported in our previous work 29 to study the reactions I to L of the Introduction Section. Reactions I and J represent the transformation of schoepite into metastudtite and studtite in the presence of water and H 2 O 2 and in the presence of H 2 O 2 and absence of water, respectively. Reactions K and L represent the transformations of metaschoepite into schoepite under the same conditions. The last situation is very important since it is the one expected under high radiation fields, which cause the radiolysis of the water reaching the surface of the SNF.
Since the GFEORs of reaction 1 are positive everywhere see Figure 7 A , schoepite will not transform spontaneously into metastudtite in the presence of water and H 2 O 2. Therefore, schoepite is highly stabilized under the presence water and H 2 O 2 , becoming more stable than metastudtite.
The opposite behavior is observed for reaction J. In this case the GFEORs are negative within the full range of temperature considered see Figure 7 B , and consequently, schoepite will be converted into studtite under high H 2 O 2 concentrations. This means that the stabilization of schoepite is not as large as that of studtite phase, which, as shown in the next section, is the most stable secondary phase of the spent nuclear fuel at these conditions among those considered in this work.
In fact, the difference in the stabilities of these phases increase as the amount of H 2 O 2 increases. At K, the free energies of the transformation of schoepite into metaschoepite increases from 8. Dehydrated schoepite and soddyite readily transform into studtite under high H 2 O 2 concentrations as shown by Forbes et al.
The transformation of uranium trioxide, rutherfordine, and metastudtite into studtite was also predicted in our previous work. The relative TS of these phases at these conditions is displayed in Figure 8. As shown in Figure 8 A, in the absence of H 2 O 2 , soddyite is the most stable phase and rutherfordine is also more stable than schoepite and metaschoepite.
Thus, at H 2 O 2 free conditions, in the presence of silicate or carbonate ions, schoepite and metaschoepite should be replaced by other mineral phases. In the presence of water and H 2 O 2 , as shown in Figure 8 B, schoepite and metaschoepite are very highly stabilized and become the first and second most stable phases, respectively. Finally, as can be seen in Figure 8 C, and it also occurs to studtite phase, 30 the TS of these phases also increases under high H 2 O 2 concentrations.
However, the stabilization of schoepite and metaschoepite is not as large as that of the studtite phase, and they become the second and third most stable phases at these conditions among those considered in this work. Studtite TS decreases largely with the decrease of H 2 O 2 concentration and the increase of temperature.
When the concentration of H 2 O 2 diminishes with time, as expected from the decrease of the intensity of radiation fields over time in a RNWR, 76 the stability of this hydrated uranyl peroxide phase will decrease, and other secondary phases will be formed. However, in order to evaluate the TS of the secondary phases of the SNF in a precise way, an extended study must be carried out including a more significant number of secondary phases.
The full evaluation of the relative amounts of secondary phases of SNF at FGD conditions over time requires the realization of complete thermodynamic calculations using thermochemical data for an appreciable number of materials including aqueous species, the most important secondary phases, and amorphous phases at different temperature and pressure conditions.
The solubility reactions of schoepite and metaschoepite are, respectively 8 and 9. Schoepite is shown to be more insoluble than metaschoepite. Table 4. The TPs of schoepite and metaschoepite minerals were determined by using theoretical solid-state methods based on density functional theory using plane waves and pseudopotentials. Since these properties have not been measured experimentally for schoepite, their values were predicted. The results obtained of metaschoepite were found to be in very good agreement with the experimental values from Barin. The calculated GFEOFs of metaschoepite are in excellent agreement with the experimental data even at high temperatures.
Under H 2 O 2 free conditions and within the full range of temperatures considered, from to K, schoepite is less stable than rutherfordine and soddyite. The relative TS of schoepite with respect to the uranyl peroxide hydrates metastudtite and studtite was studied under different conditions of temperature and concentrations of H 2 O 2 by considering the corresponding reactions.
Besides, the relative stability of metaschoepite with respect to schoepite at these conditions was evaluated from the TPs of the corresponding transformation reactions. Schoepite and metaschoepite are shown to have very similar TSs, the first being slightly more stable than the second one. These results obtained in this work allowed to determine the relative TS of schoepite and metaschoepite with respect to a series of the most important SNF secondary phases under different conditions. These results show that, among the mineral phases considered in this study, schoepite and metaschoepite are the first and second most stable phases under intermediate H 2 O 2 concentrations.
Besides, they are, after studtite, the second and third most stable phases under high H 2 O 2 concentrations. This situation is important since it is the one expected under high radiation fields producing the radiolysis of water on the surface of the spent nuclear fuel. Finally, the solubility reaction constants of schoepite and metaschoepite were determined. The calculated solubility product of metaschoepite was in excellent agreement with the experimental value.
Supporting Information. The authors declare no competing financial interest. This mineral is named in honor of Prof.
Thermodynamic properties of natural melilites | American Mineralogist | GeoScienceWorld
Alfred Schoep Ghent. It was found with other alteration products of uraninite. The color is sulfur yellow, luster adamantine. The crystals are not more than 1. Congo Belge. Mineralogie , 1 , Fasc. III, 5 — 7. Google Scholar There is no corresponding record for this reference. Schoepiet en Becquereliet. Tabular yellow orthorhombic crystals of paraschoepite occur on pitchblende or with secondary U minerals. Cleavage is perfect and easy, hardness The mineral is optically neg. A summary is given of the properties of the hydrous oxides of U. Schoepite and paraschoepite differ in optical properties.
Epiianthinite is a yellow, orthorhombic alteration product of ianthinite. Its compn. It is optically neg. Mineralogical and crystal chem. Optical orientation and ns were detd. Similarly identified crystals were used to obtain indexed powder patterns for each of the minerals. The results obtained on the phys. Crystals of schoepite and of vandendriesschite that are apparently single consist of several distinct phases in parallel intergrowth.
The spontaneous alteration from one phase to another that occurs in these crystals results from loss of hydration H2O. A reasonable crystal structure, based on these patterns, consists of UO2 OH 2 layers parallel to the cleavage, with H2O of hydration and any cations in interlayer positions.
Mineralogical Association of Canada. The structure has been solved by direct methods and refined on f20 to a weighted R index of 5. The refinement indicates that the formula contains 8 more H2O groups per unit cell than previously assumed. These sheets are topol. There are 12 sym. H-atom positions were not resolved, but an H-bonding scheme is suggested on the basis of stereochem. The structure displays strong Pbca pseudosymmetry, esp. The lower symmetry is primarily due to H-bond interactions between interlayer H2O groups and O uranyl atoms of the structural sheet.
Powder Diffr. American Institute of Physics. We have calcd. Schoepite crystallizes in space group P21ca but is strongly pseudo- centrosym. The six strongest reflections for schoepite are [d A , hkl relative intensity ] 7. The calcd. The a axis of schoepite However, potential overlap of the strongest reflections can make identification and unit-cell detn. Natural samples commonly contain intergrowths of schoepite, metaschoepite, and dehydrated schoepite.
Data for "synthetic schoepite" indicate that this product was a mixt.
Powder data labeled "paraschoepite" in the Powder Diffraction File do not correspond to the mineral of that name. American Chemical Society. The U VI solid phases schoepite, metaschoepite, and dehydrated schoepite are important reservoirs of mobile U in the environment. These simple uranyl oxide hydrates result from weathering of U minerals and the corrosion of anthropogenic U solids radioactive wastes. The authors have studied the role of hydrational H2O among these phases and in subsequent transformation to other secondary metal-U VI oxide hydrates.
Synthetic metaschoepite MS, UO3. Unlike natural DS, the dehydrated material was easily rehydrated, although crystallinity of the rehydrated phase was reduced. Alteration rates were significantly faster when the starting material had been dehydrated. These results are explained in the context of structural aspects of U VI solid phases, and the possible impact of hydration on long-term stability of U VI oxide hydrates in environmental systems is discussed.
Acta Crystallogr. B: Struct. Munksgaard International Publishers Ltd. The structure, at K, space group Pbcn, lattice consts. Three of the layer hydroxyl groups are linked through H bonding to single H2O mols. One of the H2O mols. The relation of the structure of meta-schoepite to that of schoepite is discussed. Ceska Geologicka Spolecnost. A review. This results in replacement of uraninite by weathering products contg. U in hexavalent form, i. The final assemblage of the weathering products, uranyl minerals, and their compns. 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, crystallog. 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 Jiri Cejka on occasion of his 85th birthday anniversary. Jiri 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 Cejka and Urbanec which has served, for a long-time, as a guide for beginning uranium mineralogists. Mineral Composition of Gummite. Gummite is not a mineral species, but a generic term for unidentified yellow to red alteration products of uraninite.
X-ray, optical, and chem. Typically a central core of uraninite is surrounded by a yellow to orange-red zone contg. Pb-U oxides, chiefly fourmarierite, vandendriesscheite, and 2 unidentified phases A and C, with less common clarkeite, becquerelite, curite, and schoepite. Available chem.
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Optical and x-ray powder data are given for the minerals listed above and also for masuyite, meta-autunite, and phosphoruranylite. Mineral A is bright orange, optically pos. Pb, K, Na, Ca, and Ba. Mineral B is straw yellow, optically pos. Mineral C is orange-brown to reddish brown, optically pos. The Corrosion of Uraninite under Oxidizing Conditions. These affect the thermodn. Uraninite can contain a significant amt.
Incongruent alteration of the Pb-UOHs in natural waters produces increasingly Pb-enriched uranyl phases, effectively reducing the amt. The most common end product of Pb-UOH alteration is curite. Curite may provide surface nucleation sites for certain uranyl phosphates, thereby enhancing their formation. Uranyl phosphates are generally less sol. In the absence of Pb, schoepite and becquerelite are the common initial corrosion products. The reaction path for the alteration of Pb-free uraninite results in the formation of uranyl silicates, which are generally more sol.
Thus, the long-term oxidn. Because the presence of Pb effectively reduces the mobility of uranium in oxidizing waters, the concn. Radiochimica Acta , Pt. Partial dehydration of schoepite, UO3. The loss of structural water from the schoepite interlayer results in progressive modification to the structures; expansion parallel to schoepite cleavage planes, and extensive fracturing.
Dehydration of schoepite commences at grain boundaries and progresses inward until the entire grain is converted to dehydrated schoepite, UO3. The vol. These gaps can provide pathways for the access of groundwater, and uranyl silicates and uranyl carbonates have pptd.
Schoepite, however, is not obsd. Thus, while the formation of schoepite early during the corrosion of uraninite may be favored, schoepite is not a long-term soly. I , Chapter V, pp — The deposit is located in a basin and Range horst composed of welded silicic tuff; uranium mineralization presently occurs in a chem. These characteristics are similar to those of the proposed U. These characteristics compare well with spent nuclear fuel. The oxidn. In contrast, secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the Yucca Mountain environment e.
Pb, P and V. The end products of these reported lab. These similarities in reaction product occurrence developed despite the differences in time and phys. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system conditions. Wronkiewicz, David J. A pulse of rapid U release, combined with the formation of dehydrated schoepite characterizes reactions between one and two years.
Rapid dissoln. After two years, U release rates decline and a more stable assemblage of uranyl silicate phases form by incorporating cations from the leachant. Uranophane, boltwoodite, and sklodowskite are the final soly. This obsd. Dispersion of particulate matter may be an important release mechanism for U and other radionuclides in spent nuclear fuel. Alteration phases may influence both the dissoln. UO2 pellets serve as surrogates for com. The development of a dense mat of alteration phases after 2 yr apparently trapped loose particles, resulting in reduced rates of U release. The paragenetic sequence of alteration phases is similar to that obsd.
Results from this study and comparisons with natural analog deposits suggest that the migration of fission products from altered spent fuel may be retarded by their incorporation in secondary U phases. An evaluation of water layer thickness effective in the oxidation of UO 2 fuel due to radiolysis of water. The steady-state concns. The rates of diffusion of the oxidants into the surface aq.
The comparison shows that, in most cases, the diffusion from the bulk phase into the surface layer does not make a major contribution to the oxidn. The thickness of the H2O layer, which affects the radiolytic oxidn. Fuel corrosion processes under waste disposal conditions. Elsevier Science B. A review with refs. The release of the majority of radionuclides from spent nuclear fuel under permanent disposal conditions will be controlled by the rate of dissoln.
In this manuscript the mechanism of the coupled anodic fuel dissoln. The primary emphasis is on summarizing the overall mechanistic behavior and establishing the primary factors likely to control fuel corrosion. Included are discussions on the influence of various oxidants including radiolytic ones, pH, temp. The relevance of the data recorded on unirradiated UO2 to the interpretation of spent fuel behavior is included. Based on the review, the data used to develop fuel corrosion models under the conditions anticipated in Yucca Mountain NV, USA repository are evaluated.
American Nuclear Society. Alpha, beta, and gamma dose rates in H2O, in contact with the ref. Procedures to calc. These procedures can be adapted to est. The dose rate information is needed to compare the results of leaching and corrosion expts. Dissolution mechanisms for UO 2 and spent fuel. Waste Manage. The probable dissoln. The dissoln. Therefore, the dissoln. Moreover, the dissoln. Alpha-radiolysis effects on UO 2 alteration in water.
Sattonnay, G. The formation of hydrated uranium peroxide metastudtite UO4. The prodn. From both these observations and literature about hydrated uranium peroxide occurrence, the possibility of metastudtite formation on nuclear spent fuel in storage conditions is discussed. Raman microspectrometric identification of corrosion products formed on UO 2 nuclear fuel during leaching experiments. The corrosion of directly disposed spent nuclear fuel by contact with intruding groundwater will alter the phys. Secondary phases which formed during alteration of UO2 surfaces were measured with Raman microspectrometry and the characteristic vibrational spectra of the materials were recorded.
U phases were synthesized in hydrothermal autoclave syntheses. A Raman spectral library of UO2 corrosion phases was set up for the identification of unknown products found on altered nuclear fuel samples. In a case study, U peroxide UO4 was identified by comparison with a natural sample as the main alteration phase by its characteristic O-O Raman vibration at cm The results demonstrate the differentiation between UO2 and its alteration products U VI oxyhydroxide and U VI peroxide UO4 on one sample with a relatively quick, nondestructive, spatially resolving measurement method which delivers oxidn.
Implications for the anal. MRS Online Proc. Assessment of K Basin Sludge Vol. Topics range from mining, desalination, and radiation to broader physics, biology, and chemistry studies. Some reports include maps, foldouts, blueprints, and other oversize materials. What responsibilities do I have when using this report? Dates and time periods associated with this report. Geographical information about where this report originated or about its content. Thermodynamic Properties of Minerals and Related Substances at You Are Here: home unt libraries government documents department this report.
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Properties of Minerals
Authors Robie, Richard A. Hemingway, Bruce S. Originator Geological Survey U. Publisher United States. Government Printing Office. Place of Publication: Washington D. About Browse this Partner. What Descriptive information to help identify this report. Subjects Keywords entropy minerals oxides. Language English.
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