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Structure and Thermodynamics of Uranium-Containing Iron Garnets
Abstract
Use of crystalline garnet as a waste form phase appears to be advantageous for accommodating actinides from nuclear waste. Previous studies show that large amounts of uranium (U) and its analogues such as cerium (Ce) and thorium (Th) can be incorporated into the garnet structure. In this study, we synthesized U loaded garnet phases, Ca3UxZr2-xFe3O12 (x = 0.5 - 0.7), along with the endmember phase, Ca3(Zr2)SiFe3+2O12, for comparison. The oxidation states of U were determined by X-ray photoelectron and absorption spectroscopies, revealing the presence of mixed pentavalent and hexavalent uranium in the phases with x = 0.6 and 0.7. The oxidation states and coordination environments of Fe were measured using transmission 57Fe-Mössbauer spectroscopy, which shows that all iron is tetrahedrally coordinated Fe3+. U substitution had a significant effect on local environments, the extent of U substitution within this range had a minimal effect on the structure, and unlike in the x = 0 sample, Fe exists in two different environments in the substituted garnets. The enthalpies of formation of garnet phases from constituent oxides and elements were first time determined by high temperature oxide melt solution calorimetry. The results indicate that these substituted garnets are thermodynamically stable under reducing conditions. Our structural and thermodynamic analysis further provides explanation for the formation of natural uranium garnet, elbrusite-(Zr), and supports the potential use of Ca3UxZr2-xFe3O12 as viable waste form phases for U and other actinides.
... While almandine, pyrope and spessartine-rich garnets generally contain less than 100 ppb U or Th (Haack and Gramse, 1972;Guo et al., 2016), the garnets of the grossular-andradite series (Ca 3 Al 2 Si 3 O 12 -Ca 3 Fe 2 Si 3 O 12 ) may have a few hundreds of ppb to several ppm U and Th (Lal et al., 1976;DeWolf et al., 1996;Yudintsev et al., 2002). DeWolf et al. (1996) and Meinert et al. (2001) first demonstrated the potential of U-Pb geochronology of grossular-andradite garnet using ID-TIMS methods. ...
... The REE patterns of andradite-rich garnets at Yinan indicate the LREE are partitioned more favourably into garnets relative to the HREE. The exact mechanism of U incorporation into grandite remains not well understood due to difficulties in determining the multiple possible oxidation states (tetravalent, pentavalent, and hexavalent) and the associated coordination and occupancies (dodecahedral and octahedral sites) of U in the garnet structure (Yudintsev et al., 2002;Rák et al., 2011;Guo et al., 2016). Although it is considered that the dodecahedral and octahedral sites respectively incorporate U 4+ and U ions with higher oxidation states (i.e., U 5+ , U 6+ ) based on crystal chemical arguments (Guo et al., 2016), the latter tends to reside in the dodecahedral site via Ca substitution, provided that there are sufficient Fe atoms in its vicinity to facilitate the charge transfer. ...
... The exact mechanism of U incorporation into grandite remains not well understood due to difficulties in determining the multiple possible oxidation states (tetravalent, pentavalent, and hexavalent) and the associated coordination and occupancies (dodecahedral and octahedral sites) of U in the garnet structure (Yudintsev et al., 2002;Rák et al., 2011;Guo et al., 2016). Although it is considered that the dodecahedral and octahedral sites respectively incorporate U 4+ and U ions with higher oxidation states (i.e., U 5+ , U 6+ ) based on crystal chemical arguments (Guo et al., 2016), the latter tends to reside in the dodecahedral site via Ca substitution, provided that there are sufficient Fe atoms in its vicinity to facilitate the charge transfer. Thus, the presence of Fe is significant for lowering the total energy of structure with the incorporation of U in ferric garnet (Rák et al., 2011). ...
... A major concern in garnet U-Pb dating is that uranium may be incorporated as mineral inclusions (e.g., zircons or monazite) rather than in the garnet crystal lattice. Guo et al. (2016) synthesized U-loaded garnet phases and used X-ray photoelectron and absorption spectroscopy, to show that the U oxidation state of the garnet is a mixture of U 5+ and U 6+ . Elbrusite-(Zr) (Ca 3 U x Zr (2-x) Fe 3 O 12 ) is identified as a natural U-bearing garnet. ...
... Based on the flat time-resolved signals obtained from depth profile analyses for U, combined with the correlations between Uranium and REE of Pingbao garnets, indicates that the incorporation of U into the garnet is largely controlled by substitution mechanisms (Deng et al., 2017). Gaspar et al. (2008) and Deng et al. (2017) Guo et al. (2016) suggested that U enters into the crystal lattice under the relatively hightemperature oxidized solution calorimetry, where Fe is mostly trivalent, indicated that these garnet are thermodynamically stable under reducing conditions. Li et al. (2018) and Zhang et al. (2018) IV . ...
... Besides, the Fe composition shows a positive correlation with U, suggesting that the coupling of these two elements when they enter the garnet lattice. This is consistent with the suggestion by Guo et al. (2016) Li et al. (2018) and Zhang et al. (2018). Lower U content is detected in Rim IV, but the In, Sn and Al contents are high indicating that the U incorporation regime is varied within rims, may probably due to the oreforming fluid composition (Fig. 8). ...
In this study, we discuss the application of U-Pb dating on U-rich, common-Pb bearing grossular-rich garnet and the issues of common Pb correction. Grossular-rich garnet samples were collected from the Baoshan and Huangshaping areas of the Pingbao skarn district (Hunan, South China). The Baoshan garnet comprises Gr64.3 And34.0 to And99.0 Gr0.2 (Gr = Grossular; And = Andradite), with minor spessartine (0.43–
1.45 %), almandine (0–0.61 %) and pyrope (0.01–0.91 %). The Huangshaping 304 garnet comprises Gr41.7 And56.0 to Gr70.2 And25.9 with minor spessartine (1.42–2.10 %), whereas the Huangshaping 301 garnet comprises Gr62.0 And26.6 Spe11.2 to Gr74.5 And20. 9 Pyr2.5 Spe2.1 (Spe = Spessartine, Pyr = Pyrope).
The Baoshan garnet grains are subhedral to euhedral with well-developed core-rim textures and contain approximately 1–10 ppm U. The Huangshaping 304 garnet is also euhedral with grains size ranging from several to tens of millimeters. The core-rim textured garnet grains are compacted together with interstitial quartz and contain about 5 to 10 ppm U. The Huangshaping 301 garnet is anhedral to tabular with interstitial quartz and magnetite, and contains up to 579 ppm U. In-situ U-Pb dating of the Baoshan, Huangshaping 304 and 301 garnet samples yielded consistent anchored y-intercept ages at 162.6 ± 2.9 Ma (n = 60; MSWD =1.5); 160.2 ± 1.5 Ma (n = 59; MSWD = 1.9) and 160.9 ± 0.3 Ma (n = 60; MSWD = 4.1), respectively. These ages are consistent with the new and published zircon and titanite U-Pb ages (162 to 160 Ma), indicating that the garnet U-Pb ages are reliable and the technique is robust. We suggest that the non-U-bearing mineral
correction has involved in the third-party common Pb ratio, which is believed to be a better correction for the system common Pb.
... Furthermore, 43.3% of U(V) is present, which could be attributed to U 3 O 8 . In the narrow scan, 27.5% of U(IV) is also observed as UO 2 [43,[52][53][54][55][56]. Aerial oxidation of the UO 2 powder, along with exposure to water, likely explains the low UO 2 presence in the original powder ( Figure S8c). ...
... Furthermore, 43.3% of U(V) is present, which could be attributed to U3O8. In the narrow scan, 27.5% of U(IV) is also observed as UO2 [43,[52][53][54][55][56]. Aerial oxidation of the UO2 powder, along with exposure to water, likely explains the low UO2 presence in the original powder ( Figure S8c). ...
Understanding the corrosion of spent nuclear fuel is important for the development of long-term storage solutions. However, the risk of radiation contamination presents challenges for experimental analysis. Adapted from the system for analysis at the liquid–vacuum interface (SALVI), we developed a miniaturized uranium oxide (UO2)-attached working electrode (WE) to reduce contamination risk. To protect UO2 particles in a miniatured electrochemical cell, a thin layer of Nafion was formed on the surface. Atomic force microscopy (AFM) shows a dense layer of UO2 particles and indicates their participation in electrochemical reactions. Particles remain intact on the electrode surface with slight redistribution. X-ray photoelectron spectroscopy (XPS) reveals a difference in the distribution of U(IV), U(V), and U(VI) between pristine and corroded UO2 electrodes. The presence of U(V)/U(VI) on the corroded electrode surface demonstrates that electrochemically driven UO2 oxidation can be studied using these cells. Our observations of U(V) in the micro-electrode due to the selective semi-permeability of Nafion suggest that interfacial water plays a key role, potentially simulating a water-lean scenario in fuel storage conditions. This novel approach offers analytical reproducibility, design flexibility, a small footprint, and a low irradiation dose, while separating the α-effect. This approach provides a valuable microscale electrochemical platform for spent fuel corrosion studies with minimal radiological materials and the potential for diverse configurations.
... Among all these elements, Zr shows most significant correlation with U, although the data points are still quite scattered. This may be accounted for by U incorporation into garnet lattices as elbrusite-(Zr), Ca 3 U x Zr 2-x Fe 3 O 12 (Grew et al., 2013;Guo et al., 2016). The consistent positive correlation between U and Zr (Fig. 2) implies that U in most natural garnets may occur hexavalently (Grew et al., 2013) and possibly with some mixing of pentavalent U (Guo et al., 2016), as revealed by studies on elbrusite-(Zr). ...
... This may be accounted for by U incorporation into garnet lattices as elbrusite-(Zr), Ca 3 U x Zr 2-x Fe 3 O 12 (Grew et al., 2013;Guo et al., 2016). The consistent positive correlation between U and Zr (Fig. 2) implies that U in most natural garnets may occur hexavalently (Grew et al., 2013) and possibly with some mixing of pentavalent U (Guo et al., 2016), as revealed by studies on elbrusite-(Zr). ...
Garnet occurs in a wide range of rock types, from mantle peridotites to granites, from eclogites to skarns. In recent years, garnet LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) U‐Pb dating provides a powerful solution for retrieving the ages of rock formation, but successful dating is often prohibited by the low concentration of U. However, the concentration of U, a trace element of garnet, is unknown prior to the LA‐ICP‐MS analysis. In this study, we propose that the U concentration in garnet can be predicted by the contents of major and minor elements, which can be quantitatively obtained by EPMA (electron probe microanalysis). Using supervised machine learning method (neural network), a model is trained to discriminate U‐rich (>2 ppm) and U‐poor (<2 ppm) garnets based on EPMA results. Results of cross validation shows that the model has an average accuracy of ∼92%, and is powerful in finding datable U‐rich garnet. To facilitate the use of the discriminator, it is programed as a stand‐alone Microsoft Excel spreadsheet (HighUGarnet), and users directly paste the molar proportions of garnet end members into it and get the result of discrimination.
... Generally, crystal chemical arguments dictate U 4+ merely fits dodecahedral sites, while U 6+ occupies octahedral sites. In case of U 5+ presents in octahedral sites, based on the first principle calculation, it needs to transfer its extra electron to neighboring tetrahedral Fe 3+ (Rak et al., 2013;Guo et al., 2016). Therefore, the octahedral site may be more flexible to incorporate U with higher oxidation states (Guo et al., 2016). ...
... In case of U 5+ presents in octahedral sites, based on the first principle calculation, it needs to transfer its extra electron to neighboring tetrahedral Fe 3+ (Rak et al., 2013;Guo et al., 2016). Therefore, the octahedral site may be more flexible to incorporate U with higher oxidation states (Guo et al., 2016). Compared with the reduced magmatic-hydrothermal system, the higher oxidized magmatic rocks and skarns may enrich U into the grandite garnet, such as the Ertsberg diorite and the related Big Gossan Cu\ \Au skarn deposit (Indonesia) with U concentration up to 200 ppm in the grandite garnet (Wafforn et al., 2018). ...
Although garnet U–Pb dating method has been reported recently, yet the accurate concordia ²⁰⁶ Pb/ ²³⁸ U ages and growth histories of multi generation of garnets based on ages were still lacked. LA-ICP-MS U–Pb dating on multi-generational grandite (grossular-andradite) garnet from the large Tonglvshan Cu-Fe-Au skarn deposit was applied in this study. Based on petrographic observation, in chronological order, three generation garnets have been distinguished, namely homogeneous Grt1-exo (in the exoskarn zone) and Grt1-endo (in the endoskarn zone), oscillatory zoning Grt2 and vein-type Grt3 cutting magnetite ores. LA-ICP-MS U–Pb dating on four grandite samples from the Grt1-exo, Grt1-edno, Grt2 and Grt3 yields Tera-Wasserburg lower intercept ²⁰⁶ Pb/ ²³⁸ U ages of 139.1 ± 1.0 Ma (2σ, MSWD = 0.79), 134 ± 11 Ma (2σ, MSWD = 2.5), 143.4 ± 8.3 Ma (2σ, MSWD = 2.3) and 140.3 ± 1.4 Ma (2σ, MSWD = 0.95), respectively. More importantly, two concordia ²⁰⁶ Pb/ ²³⁸ U ages of 139.2 ± 0.6 Ma (2σ, MSWD = 1.4) and 139.8 ± 1.5 Ma (2σ, MSWD = 0.13) were firstly obtained from the sample of Grt1-exo with highest U concentrations ([U] avg > 80 ppm) contents. The precision U–Pb ages of 139–140 Ma from Grt1-exo and Grt3 can be considered as the timing of Cu-Fe-Au skarn mineralization, and consistent with the majority of published zircon U–Pb ages of the quartz dioritic stock and ⁴⁰ Ar– ³⁹ Ar plateau ages of phlogopite at Tonglvshan (142–140 Ma). The precision grandite U–Pb ages also indicate that the entire metasomatic hydrothermal mineralization activity in the Tonglvshan Cu-Fe-Au skarn deposit occurred within a relatively short time span of <1 (or 2.5 considering errors) Myr. In addition, we found that the grandite garnet is more easily to be enriched in U and can obtain the high-precision concordia U–Pb ages with higher andradite Mol%, euhedral and larger crystals, and relevant oxidized magmatic rocks or skarns.
... 36,[39][40][41] The use of oxide melt drop solution calorimetry with the sodium molybdate solvent has proven to be a valuable technique in promoting lanthanide and actinide research, especially on light actinides (uranium, thorium), in terms of fundamental understanding of actinide bonding, [30][31][32]42 probing the thermochemistry of uranyl minerals, 17,33,[43][44][45][46] interpreting nuclear fuel alteration, 11,20,24,31,36 and predicting the long-term stability of actinideimmobilized solid waste matrices. 19,22,23,25,28 These studies rely on accurate measurements of drop solution enthalpies of binary light-actinide-containing oxides, such as ThO 2 , UO 2 , U 3 O 8 , and UO 3 10,13,24,27,47 Recently, ∆H ds values of NpO 2 and Np 2 O5 in molten 3Na 2 O⋅4MoO 3 have been reported. 35 However, no ∆H ds value is available for PuO 2 in the literature, partly due to the safety (radiotoxicity) concerns and limited accessibility associated with Pu. ...
... To refine this value, we plan to directly characterize the oxidation state of Pu in the solidified solvent after the dissolution of PuO 2 using synchrotron X-ray absorption spectroscopy. 22,28,30,52 In addition, structural characterization (e.g., by synchrotron X-ray scattering and Raman spectroscopy at high T/P 53-55 ) will be conducted on AnO 2 series to establish its structure-energetics relationship. ...
Thermodynamic properties of refractory materials, such as standard enthalpy of formation, heat content, and enthalpy of reaction, can be measured by high temperature calorimetry. In such experiments, a small sample pellet is dropped from room temperature into a calorimeter operating at high temperature (often 700 °C) with or without a molten salt solvent present in an inert crucible in the calorimeter chamber. However, for hazardous (radioactive, toxic, etc.) and/or air-sensitive (hygroscopic, sensitive to oxygen, pyrophoric, etc.) samples, it is necessary to utilize a sealed device to encapsulate and isolate the samples, crucibles, and solvent under a controlled atmosphere in order to prevent the materials from reactions and/or protect the personnel from hazardous exposure during the calorimetric experiments. We have developed a sample seal-and-drop device (calorimetric dropper) that can be readily installed onto the dropping tube of a calorimeter such as the Setaram AlexSYS Calvet-type high temperature calorimeter to fulfill two functions: (i) load hazardous or air-sensitive samples in an air-tight, sealed container and (ii) drop the samples into the calorimeter chamber using an “off-then-on” mechanism. As a case study, we used the calorimetric dropper for measurements of the enthalpy of drop solution of PuO2 in molten sodium molybdate (3Na2O·4MoO3) solvent at 700 °C. The obtained enthalpy of −52.21 ± 3.68 kJ/mol is consistent with the energetic systematics of other actinide oxides (UO2, ThO2, and NpO2). This capability has thus laid the foundation for thermodynamic studies of other Pu-bearing phases in the future.
... The garnet mineral system has presented a challenge for U-Pb geochronology, in part because almandine, pyrope, and spessartine-rich garnets typically contain less than~100 ppb U or Th (Haack and Gramse, 1972;Guo et al., 2016). Pioneering garnet U-Pb geochronology on almandine and pyrope-rich garnets from regional metamorphic environments using ID-TIMS proved consistent with other geo/thermochronometers (Mezger et al., 1989 andMezger et al., 1991), but measurable U and Th within Fe-Mg garnets is likely sourced from inclusions (DeWolf et al., 1996). ...
... Preferentially selecting grandites for high [U] will also increase the precision of U-Pb dates. The exact mechanism of U incorporation into grandite remains unclear despite numerous studies (Yudintsev et al., 2002;Rák et al., 2011;Guo et al., 2016). Of the garnets surveyed in this study, higher andradite content correlates with [U], with the exception of Mali Red Grandite. ...
This study presents a new laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) based U-Pb geochronometric method for dating grossular-andradite (grandite) garnet. Grandite is a primary skarn mineral, therefore dating its growth directly dates hydrothermal activity. As zircon U-Pb geochronometry provides a high-resolution record of magmatic processes, grandite U-Pb dating has the potential to provide a complementary record for hydrothermal systems. This study characterizes four garnets of variable grossular-andradite content and age as potential reference materials for LA-ICPMS U-Pb geochronology: Willsboro Andradite (~ 1020 Ma, Adirondacks, USA), Red and Yellow Mali Grandite (~ 200 Ma, Southern Mali) and Lake Jaco Grossular (~ 35 Ma, Coahuila, Mexico). Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb analyses of Willsboro andradite yield a weighted mean ²⁰⁶Pb/²³⁸U date of 1022 ± 16 Ma. We use Willsboro Andradite as a primary reference material for LA-ICPMS U-Pb characterization of the two other garnets. The non-radiogenic Pb (Pbc) concentration in grandite is variable between specimens. Willsboro Andradite and Mali Grandites contain almost purely radiogenic Pb (Pb*), however Lake Jaco Grossular has a higher Pbc concentration comparatively and yields discordant U/Pb ratios. For specimens with highly variable Pb*/Pbc, linear regression is employed to derive a lower-intercept age. LA-ICPMS U-Pb dates for Yellow Mali Grandite (202 ± 2 Ma; ²⁰⁶Pb/²³⁸U weighted mean) and Lake Jaco Grossular (35 ± 2 Ma; lower intercept) agree with independent ID-TIMS U-Pb (202.0 ± 1.2 Ma) and (U-Th)/He dates (35 ± 5 Ma) derived for each specimen. The precision of LA-ICPMS U-Pb dates varies between 1–10% (2σ) for the analyzed garnets. Considering the short estimated lifespans of pluton-related hydrothermal systems (tens of thousands to a few million years), Neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels. As a performance test for the method, we present new U-Pb andradite data from a Late Miocene skarn system on Serifos Island, Greece. This garnet yields a lower-intercept age of 9.15 ± 0.36 Ma, in agreement with biotite Rb-Sr and zircon U-Pb age data from the causative pluton.
... In spite of the many studies that have applied LA-ICP-MS garnet U-Pb dating, mechanisms of U incorporation remain illconstrained (e.g., Guo et al., 2016;Rák et al., 2011;Smith et al., 2004). Li et al. (2022) measured anomalously high U content (27-76 ppm) in garnet from the Prairie Lake alkaline complex, Canada. ...
... In addition, our methodological development has broadened the applications of hightemperature drop calorimetry in thermochemical studies of various materials. 33,[45][46][47][48][49][53][54][55][56][57][61][62][63][64][80][81][82][83][84] ...
Molten salt reactors (MSRs) are a promising alternative to conventional nuclear reactors as they may offer more efficient fuel utilization, lower waste generation, and improved safety. The state of knowledge of the properties of liquid salts is far from complete. In order to develop the MSR concept, it is essential to develop a fundamental understanding of the thermodynamic properties, including the heat capacities (Cp) and enthalpies of mixing (ΔHmix), of molten salts at MSR operating conditions. Historically, the Cp values of molten salts were determined by drop calorimetry or differential scanning calorimetry, whereas their ΔHmix values were typically measured using specialized high temperature calorimeters. In this work, a new methodology for measuring both the Cp and the ΔHmix of molten chloride salts was developed. This novel method involves sealing a chloride salt sample in a nickel capsule and performing conventional transposed temperature drop calorimetry using a commercially available Setaram AlexSYS-800 Tian–Calvet twin microcalorimeter. This methodology may be applied to calorimetric measurements of more complex salt mixtures, especially mixtures containing actinides and fission products.
... et al. synthesized U loaded garnet phases, Ca 3 U x Zr 2x Fe 3 O 12 (x = 0.5-0.7), along with the endmember phase, Ca 3 (Zr 2 )SiFe 2 O 12 , for comparison [30]. Inspired by this, using iron-rich garnet to contain actinides is a promising strategy [31][32][33]. ...
... Changes in uid oxidation state might be concomitant with variable incorporation o multi-valence elements (e. g., Sn, W, and U) into garnet structure. Tin and W preer to substitute into the octahedral site o garnet in their oxidized states (Dhivya et al., 2013;Xu et al., 2016;Park et al., 2019), whereas U is more likely incorporated into the dodecahedral site in its reduced state, although some substitution into the octahedral site with higher oxidation states (U 5+ , U 6+ ) may also occur (Smith et al., 2004;Gaspar et al., 2008;Guo et al., 2016). In our study, the behaviors o Sn and W in zoned garnets, especially those rom Yaguila, are distinct (Figs. 8 and 9), while moderate correlations between U and W, Sn are observed, especially in Lawu garnets (Fig. 7c, 8 and 9). ...
... Já o sistema U-Pb em granada é utilizado para solução sólida dominada pelos membros finais grossulária-andradita (temperatura de fechamento da andradita é em torno de 750°C, DeWolf et al., 1996) --visto que piropo, espessartita e almandina apresentam, de forma geral, teores mais baixos desses elementos, na ordem de ~100 ppb de U ou Th (Haack e Gramse, 1972;Guo et al., 2016). Granadas da série grossulária-andradita são mais comuns em escarnitos, frutos do metamorfismo de contato ou em algumas rochas calcissilicáticas, em metamorfismo regional, e a datação U-Pb por LA-ICP-MS, conforme descrita por Seman et al. (2017), se faz possível para granada com concentrações na ordem de ppm para esses isótopos. ...
Rochas metapelíticas são importantes marcadores petrocronólogicos, não apenas pelas variadas e sensíveis paragêneses minerais e presença de fases datáveis, mas também por sua ampla e contínua distribuição ao longo de terrenos, permitindo, assim, estudos integrados e detalhados do metamorfismo e eventos em diferentes regiões. O presente trabalho visou sintetizar aspectos relevantes à caracterização petrológica e geocronológica de metapelitos para regimes de pressão média. Considerando o sistema químico KFMASH (K2O, FeO, MgO, Al2O3, SiO2, H2O), minerais como clorita, muscovita, cloritoide, biotita, estaurolita, granada, cordierita, andaluzita, cianita, sillimanita e feldspato potássico são típicos da paragênese de metapelitos para baixas ou médias pressões, desde que haja disponibilidade química e condições P-T para sua formação. De forma geral, tem-se como distintivas, com o aumento das condições P-T e o aparecimento dos respectivos minerais-índice, as zonas metamórficas da clorita, biotita, granada, estaurolita, cianita, sillimanita e sillimanita + ortoclásio. Para a determinação petrogenética e das condições do metamorfismo desses litotipos, estudos macro- e microestruturais, associados com análises de química mineral e de rocha total, propiciam a aplicação de métodos termobarométricos diversos, desde os convencionais, passando pelos otimizados e diagramas isoquímicos de fases e chegando nos termômetros monoelementares, cada qual com suas especificidades e aplicações. De modo a promover um estudo petrocronológico dos litotipos, fases minerais como zircão, granada, monazita e rutilo, em seus respectivos sistemas isotópicos, possibilitam atribuir idades a esses eventos metamórficos e, com a integração e interpretação dos dados obtidos, construir a trajetória P-T-t-d de formação dessas rochas e dos eventos/estágios dos processos envolvidos. Uma abordagem sistemática, de acordo com as particularidades da rocha, deve ser empregada, garantindo, assim, a acurácia dos resultados obtidos.
... Although element substitution is known to be common in garnet (Guo et al., 2016;Dą browa et al., 2021;Kitaura et al., 2021), some trace elements (e.g., U) were proposed to occur as U-rich mineral inclusions in garnet (Vance et al., 1998;Lima et al., 2012). This becomes a challenge for garnet U-Pb dating and geochemical applications (Deng et al., 2017). ...
Origin of garnet in skarn (magmatic vs. hydrothermal) and the prograde skarn fluid evolution are still controversial. Two generations of garnet (Grt1, Grt2) were identified at the Tongshankou deposit: Grt1 is anisotropic with oscillatory zoning and resorbed boundary, whilst Grt2 grew around Grt1 and formed oscillatory rims. In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept ²⁰⁶Pb/²³⁸U age of 142.4 ± 2.8 Ma (n = 57; MSWD = 1.16) and 142.3 ± 9.6 Ma (n = 60; MSWD = 1.06), respectively, coeval with the ore formation and ore-related granodiorite emplacement. Positive Eu anomaly, non-CHARAC Y/Ho value and low TiO2 content, together with the mineral assemblages indicate that both Grt1 and Grt2 have a hydrothermal origin. The existence of melt and melt-fluid inclusions in Grt1, together with similar LREE-enriched patterns to the granodiorite, further indicate that Grt1 may have formed in the magmatic-hydrothermal transition. Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO2. The inner gray Grt2 rims (Grt2A) is HREE-enriched with low U contents, indicating that fluid II has nearly neutral pH and high fO2. The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims (Grt2B) show that the evolved magmatic fluid II had mixed with an external fluid, characterized by being mildly acidic pH and with high fO2. Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou, and the mixing of meteoric water may have been prominent in the prograde skarn stage.
... For example, the U/Th substitution of Ca could be expressed as 2Ca 2+ ↔ U 4+ /Th 4+ + □ or 3Ca 2+ ↔ U 4+ /Th 4+ + 2Na + (□ for vacancies or H 2 O molecules) for vesuvianite (Moiseev et al., 2020), and Ca 2+ + 2Si 4+ ↔ U 4+ + 2(Fe 3+ , Al 3+ ) for grandite (Gaspar et al., 2008). But the geochemical behavior of uranium or thorium in these minerals remains enigmatic, e.g., there is no consensus on why andradite (Ca 3 Fe 2 [SiO 4 ] 3 ) usually can accommodate more uranium and is more suitable for U-Pb dating than grossularite (Ca 3 Al 2 [SiO4] 3 ) (Rák et al., 2011;Guo et al., 2016;Deng et al., 2017), although both minerals bear abundant Ca 2+ in their lattice and are usually intergrown in nature. It is true for vesuvianite (Ca 10 (Mg, Fe) 2 Al 4 [SiO 4 ] 5 [Si 2 O 7 ] 2 (OH, F) 4 ) and grossularite (Figs. 5 and 6). ...
Vesuvianite is one kind of common U-bearing minerals in various skarns and skarn mineralization. As an early phase mineral, vesuvianite can be well preserved since it usually occurs at the front of skarn close to the marble and far away from the pluton, therefore, accurately determining its growth history can directly date hydrothermal activity in the skarn system, just like the zircon U-Pb geochronometer recording magmatic processes. In order to demonstrate reliability of the vesuvianite U-Pb geochronometer, we choose six vesuvianite samples from two skarn deposits in South China for the LA-ICPMS dating. Accordingly, the U-Pb dating of four vesuvianite samples from the Shizhuyuan W-Sn ore district yielded precise ages of 160.3 ± 1.2 Ma, 155.6 ± 2.6 Ma, 154.5 ± 3.1 Ma and 150.4 ± 3.1 Ma, respectively; these dates coincide well with the previously published ages of the relevant granite complex emplacement and hydrothermal mineralization in the region with the ages of ca. 160 Ma, ca. 155 Ma and ca. 150 Ma. The other two vesuvianite samples from the Furong Sn ore district yielded precise ages of 159.3 ± 1.8 Ma and149.1 ± 1.6 Ma，which are consistent with the multi-stage magmatic activities in the studied area with emplacement time of ca. 160 Ma and ca. 149 Ma. Theoretically, the site of the Ca²⁺ position in vesuvianite is slightly larger than that in intergrown grossularite, which gives the former a better ability to incorporate uranium. Therefore, vesuvianite with high uranium concentrations and relatively low common Pb is beneficial for obtaining precise results during the LA-ICPMS U-Pb dating, it is an ideal mineral for accurately determining the time of skarn formation and/or various skarn mineralization.
... Interestingly, for those euhedral grandite crystals with chemical zoning, more andradite-rich domains (including Grt-I cores and Grt-II Ti-rich rims) contain detectable levels of U and thus tend to be more favorable for LA-ICP-MS U-Pb dating, whereas measurement attempts in the relatively Al-rich, Fe-poor mantle regions have not been successful. Although it may be attributed to a multi-element coupled substitution mechanism promoting the incorporation of U into andradite lattices (Rák et al., 2011;Guo et al., 2016;Huang et al., 2022), the precise mechanism of U incorporation into grandite garnet still remains insufficiently constrained. ...
This contribution presents new U-Pb geochronological data and attempts to elucidate the complex evolution history of various garnet types identified from two kimberlite pipes in the Wafangdian diamond mining district, southern Liaoning Province. These calcic garnets are dominated by andradite with relatively low proportions of schorlomite, grossular and pyrope. Abundant euhedral to subhedral, highly brecciated andradite phenocrysts hosted by LN30 “carbonatite-like” kimberlite samples yield a lower-intercept age of 459.3 ± 3.4 Ma, which is in excellent agreement with the previously reported phlogopite Ar-Ar (463.9 ± 0.9 Ma) and Rb-Sr ages (461.7 ± 4.8 Ma). Based on their trace element and C-O isotopic compositions of associated groundmass carbonate, we infer that these primary magmatic andradites probably originated from kimberlitic magmas. By comparison, three compositionally and texturally distinct groups of Ti-bearing andradites from LN42 hypabyssal kimberlites separately define three well-fitted regression lines with lower intercept ages at 581 ± 12 Ma, 414.9 ± 9.3 Ma and 292.0 ± 5.7 Ma, respectively. Relict andradite xenocrysts implies that ancient lower crust of the North China Craton (NCC) might have been affected by a significant but less-known tectonothermal event to varying degrees at ∼0.6 Ga. By contrast, fresh grains of magmatic Ti-andradites with chemical zoning produce a relatively young age of ∼415 Ma, which can still provide minimum age estimates for the most recent pulses of Paleozoic kimberlite magmatism in this study area. Noteworthily, a yet unrecognized local-scale hydrothermal alteration event at ∼292 Ma has been recorded in the texturally distinct population of secondary hydroandradites, whose age reported here for the first time is geologically meaningful. To sum up, this study further highlights andradite U-Pb dating as a potential robust geochronometer for constraining the late-stage evolution of kimberlite magmas as well as post emplacement hydrothermal alteration.
... Despite the increasing number of studies that apply U-Pb LA-ICP-MS garnet dating, the precise mechanism of U incorporation into andradite-grossular garnet (grandite) still remains insufficiently constrained (cf. Guo et al. 2016;Rák et al. 2011;Smith et al. 2004). However, andradite-rich zones in grandite tend to be favorable for U-Pb LA-ICP-MS geochronology (cf. ...
Here, we present in situ U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) ages of andradite-grossular garnet from four magmatic-hydrothermal polymetallic skarn prospects in the Schwarzenberg District, Erzgebirge (Germany), located in the internal zone of the Variscan Orogenic Belt. Within the geochronological framework of igneous rocks and hydrothermal mineralization in the Erzgebirge, the obtained garnet ages define three distinct episodes of Variscan skarn formation: (I) early late-collisional mineralization (338–331 Ma) recording the onset of magmatic-hydrothermal fluid flow shortly after the peak metamorphic event, (II) late-collisional mineralization (~ 327–310 Ma) related to the emplacement of large peraluminous granites following large-scale extension caused by orogenic collapse and (III) post-collisional mineralization (~ 310–295 Ma) contemporaneous with widespread volcanism associated with Permian crustal reorganization. Our results demonstrate that the formation of skarns in the Schwarzenberg District occurred episodically in all sub-stages of the Variscan orogenic cycle over a time range of at least 40 Ma. This observation is consistent with the age range of available geochronological data related to magmatic-hydrothermal ore deposits from other internal zones of the Variscan Orogenic Belt in central and western Europe. In analogy to the time–space relationship of major porphyry-Cu belts in South America, the congruent magmatic-hydrothermal evolution in the internal zones and the distinctly later (by ~ 30 Ma) occurrence of magmatic-hydrothermal ore deposits in the external zones of the Variscan Orogenic Belt may be interpreted as a function of their tectonic position relative to the Variscan collisional front.
... Such single-mineral isochrons (e.g., Chew et al., 2014) can be challenging to create regardless of the mineral, predominately because the analyst must access some geochemical process that naturally differentiates U/Pb ratios within the sample. As an informative counterexample from an arguably simpler procedure, during whole-rock U-Pb analysis (e.g., Dickin, 2018), three or more mineral separates from a single rock may form a cogenetic U/Pb mixing line because each mineral contains a unique quantity of inherited U (Guo et al., 2016), thereby creating the spread along the whole-rock isochron. However, in a single-mineral isochron, the variability in inherited U concentrations can be much lower, since a single mineral typically has a narrow U-substitution capacity, such as U into calcite (Kelly et al., 2003). ...
Over two decades of technical and application-based advances to the speleothem U-Pb chronometer have cemented this terrestrial archive at the forefront of landscape reconstruction, palaeoclimatology, and palaeoanthropology. The ability to access speleothem palaeoclimate records beyond the 650 ka limit of the U-Th system has opened many avenues to such ‘deep-time’ considerations. Yet still this chronometer remains a challenging analytical exercise, more-so as the technique becomes routinely applied to carbonates with less-than-ideal U/Pb ratios. In this contribution, we review the vadose speleothem U-Pb system, as revealed by 68 previously published isochrons produced from 474 solution-mode analyses across three separate geographical regions. We develop a new statistical parameter of ‘average distance’ to quantify the dispersion or ‘spread’ along the previously published isochrons and compare this with available U and Pb elemental concentrations. Our findings highlight the importance of regional geology and karst morphology in controlling the speleothem's overall U/Pb ratio. Furthermore, we show that variability in the amount of inherited Pb across the sampling layer (average variability of 63% relative to sample average) – not uranium (23% variability) or by extension radiogenic Pb – is a main factor controlling the resulting isochron's quality. This is demonstrated using Z-score distributions of the U and Pb concentrations and isochron average distance values. By making this distinction we hope to initiate further research into Pb-specific transportation vectors through the karst, in addition to physical/chemical processes that fractionate inherited Pb at the speleothem precipitation site. We go on to extend the use of Tera-Wasserburg common Pb anchors to the ‘classical’ U-Pb isochrons as an example of the utility gained by considering the role of inherited Pb within speleothems. Finally we argue that an improved theoretical understanding of inherited Pb distributions within speleothems would greatly benefit solution mode sampling strategies, especially for poor-to-moderate U/Pb speleothems.
... Such single-mineral isochrons (e.g., Chew et al., 2014) can be challenging to create regardless of the mineral, predominately because the analyst must access some geochemical process that naturally differentiates U/Pb ratios within the sample. As an informative counterexample from an arguably simpler procedure, during whole-rock U-Pb analysis (e.g., Dickin, 2018), three or more mineral separates from a single rock may form a cogenetic U/Pb mixing line because each mineral contains a unique quantity of inherited U (Guo et al., 2016), thereby creating the spread along the whole-rock isochron. However, in a single-mineral isochron, the variability in inherited U concentrations can be much lower, since a single mineral typically has a narrow U-substitution capacity, such as U into calcite (Kelly et al., 2003). ...
John Engel describes a chronometer method that enables rapid generation of U–Pb ages for cave speleothems.
... Andradite-rich garnets are able to accommodate U via suspected coupled substitutions involving Fe 3+ , leading to a common positive correlation between increasing mol% andradite and U concentration (Smith et al., 2004;Guo et al., 2016;Gevedon et al., 2018). Thus, andradite-rich garnets are more likely to contain high U concentrations and are more suitable for U-Pb analysis in many skarn deposits (Li et al., 2021). ...
The Haobugao skarn Zn-Pb ore deposit (reserves of 0.29 Mt @ 4.24% Zn, 0.15 Mt @ 2.25% Pb) is located in the southern Great Xing’an Range (SXGR) Cu-Mo-Ag-Au-Pb-Zn-Fe metallogenic province in northeastern China. The ore bodies mainly occur near the contact zones between the early Cretaceous granite and the lower Permian carbonates and sometimes in distal locations. Two types of garnet are identified, i.e., green and fine-grained garnet (Grt A), and brown and coarse-grained garnet (Grt B). The Grt A contains a wider compositional range (Adr71.46-95.07Grs0.16-24.10), whereas the Grt B contains a relatively narrow compositional range (Adr87.65-97.52Grs0.07-10.11). The Grt A displays chondrite-normalized HREE-enriched and LREE-depleted patterns with negative Eu anomalies, suggesting that the Grt A was likely crystallized from a nearly neutral fluid. In contrast, the Grt B is LREE-enriched, HREE-depleted with pronounced positive Eu anomalies, indicating that the Grt B was probably formed from a mildly acidic fluid. The Grt A contains lower U contents than the Grt B, implying that the Grt A was likely formed under a relatively oxidized condition, while the Grt B crystallized from a more reduced condition.
Skarn mineralization at Haobugao (139.10 ± 5.40 and 140.70 ± 1.89 Ma, garnet LA-ICP-MS U-Pb ages) and Zn-Pb mineralization (138.27 ± 0.14/0.69/0.81 and 138.82 ± 0.07/0.68/0.80 Ma, molybdenite ID-N-TIMS Re-Os ages) were associated with the emplacement of the granitoids dated from 143.49 ± 0.76 to 140.85 ± 0.75 Ma (zircon LA-ICP-MS U-Pb ages). The integrated geochronological data suggest that the ore-related granitoids, skarn, and Zn-Pb mineralization in the Haobugao deposit all formed in the early Cretaceous, which occurred in an extensional tectonic setting associated with regional Paleo-Pacific slab roll-back. This study highlights the reliability and viability of garnet U-Pb dating for constraining the timing of skarn formation, and utilizing its age with textural and trace element concentrations could well reconstruct the ore-forming process of skarn deposits.
... Thermodynamic modeling of uranothorite and implications for material degradation As a conclusion of RMC analyses and DFT calculations, the above proposed geometrical distortion coupled with the alteration in electronic structures is responsible for stabilizing intermediate U/ Th composition in uranothorites with negative enthalpies of mixing and volume of mixing 54,70 . Another important implication for thermodynamics of uranothorite is that the configurational entropy may be determined by using the Boltzmann entropy formula (ΔS Boltzmann ¼ À4R 71,72 , where R is the ideal gas constant and x is the U concentration in the dodecahedral site. The configurational entropy due to mixing for uranium concentration of 0.11, 0.35, and 0.84 were calculated to be 2.88, 5.38, and 3.65 J·mol −1 ·K −1 (Supplementary Table 8). ...
Non-ideal thermodynamics of solid solutions can greatly impact materials degradation behavior. We have investigated an actinide silicate solid solution system (USiO 4 –ThSiO 4 ), demonstrating that thermodynamic non-ideality follows a distinctive, atomic-scale disordering process, which is usually considered as a random distribution. Neutron total scattering implemented by pair distribution function analysis confirmed a random distribution model for U and Th in first three coordination shells; however, a machine-learning algorithm suggested heterogeneous U and Th clusters at nanoscale (~2 nm). The local disorder and nanosized heterogeneous is an example of the non-ideality of mixing that has an electronic origin. Partial covalency from the U/Th 5 f –O 2 p hybridization promotes electron transfer during mixing and leads to local polyhedral distortions. The electronic origin accounts for the strong non-ideality in thermodynamic parameters that extends the stability field of the actinide silicates in nature and under typical nuclear waste repository conditions.
... Garnet is a common and often voluminous minerals within skarns and is among the earliest mineral to crystallize during skarn formation, meaning that garnet crystallization can accurately capture the timing of the onset of hydrothermal activity (Gevedon et al., 2018) but the low concentrations of U (typically <1 ppm) present in garnet can limit the possible use of garnet U-Pb dating (DeWolf et al., 1996). However, andradite-rich garnet and andradite-rich zones within grossular garnet are both common within the majority of skarns (Meinert et al., 2005) and have the ability to accommodate U as a result of suspected coupled substitutions involving Fe 3+ (DeWolf et al., 1996;Smith et al., 2004;Guo et al., 2016). This yields a generally positive correlation between increasing mol% andradite values and U concentration, indicating that many skarns are likely to contain garnet with high U concentrations that are suitable for U-Pb analysis. ...
The Magushan skarn Cu–Mo deposit is a representative example of the skarn mineralization present within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt. The precise age of an ore deposit is important for understanding the timing of mineralization relative to other geological events in an area and to fully place the formation of a mineral deposit within the geological context of other processes that occur within the study area. Here, we present new molybdenite Re–Os, titanite and andradite garnet U–Pb ages for the Magushan deposit and use these data to outline possible approaches for identifying genetic relationships in geologically complex areas. The spatial and paragenetic relationships between the intrusions, alteration, and mineralization within the study area indicates that the formation of the Magushan deposit is genetically associated with the porphyritic granodiorite. However, this is not always the case, as some areas contain complexly zoned plutons with multiple phases of intrusion or mineralization may be distal from or may not have any clear spatial relationship to a pluton. This means that it may not be possible to determine whether the mineralization formed as a result of single or multiple magmatic/hydrothermal events. As such, the approaches presented in this study provide an approach that allows the identification of any geochronological relationships between mineralization and intrusive events in areas more complex than the study area. Previously published zircon U–Pb data for mineralization-related porphyritic granodiorite yielded an age of 134.2 ± 1.2 Ma (MSWD = 1.4) whereas the Re–Os dating of molybdenite from the study area yielded an isochron age of 137.7 ± 2.5 Ma (MSWD = 0.43). The timing of the mineralizing event in the study area was further examined by the dating of magmatic accessory titanite and skarn-related andradite garnet and yielded U–Pb age of 136.3 ± 2.5 Ma (MSWD = 3.2) and 135.9 ± 2.7 Ma (MSWD = 2.5), respectively. The dating of magmatic and hydrothermal activity within the Magushan area yields ages around 136 Ma, strongly suggesting that the mineralization in this area formed as a result of the emplacement of the intrusion. The dates presented in this study also provide the first indication of the timing of mineralization within the Xuancheng district. These data provide evidence of a close relationship between the formation of the mineralization within the Xuancheng district and the Early Cretaceous magmatism that occurred in this area. This in turn suggests that other Early Cretaceous intrusive rocks within this region are likely to be associated with mineralization and should be considered highly prospective for future mineral exploration. This study also indicates that the dating of garnet and titanite can also provide reliable geochronological data and evidence of the timing of mineralization and magmatism, respectively, in areas lacking other dateable minerals (e.g., molybdenite) or where the relationship between mineralization and magmatism is unclear, for example in areas with multiple stages of magmatism, with complexly zoned plutons, and with distal skarn mineralization.
The study examined garnets in lamprophyric and carbonatitic dykes and massif melilitolites from the Devonian Kola Alkaline Carbonatite Province (North-Eastern European craton). We analyzed major and trace element compositions of garnet in 14 well-characterized samples of aillikite, carbonatite, alnöites, monchiquites, nephelinite and turjaites and correlated the garnet zoning with the sequence of magmatic crystallization and late deuteric changes. The garnets occur in all textural positions, from early phenocrysts to groundmass phases and deuteric pseudomorphs. The garnets are calcic and classified into five compositional types, 1) high-Zr, 2) high-Ti, 3) medium-Ti, 4) low-Ti, and 5) high-Al. These garnet types combine in distinct ways comprising four zoning patterns (turjaite, nephelinite, monchiquite and carbonate-related) repeated in different rock types. Fractional crystallization controls the observed garnet evolution from high-Ti and high-Zr to garnets poorer in Ti, but richer in Al. Garnets progressing from magmatic to deuteric crystallization evolve from heavy rare earth elements (HREE)-enriched to light rare earth elements (LREE)-enriched and become depleted in trace elements. We interpret high-Zr garnets as antecrysts crystallized from deep alkaline carbonate-rich melts and subsequently destabilized in evolved shallow lamprophyric melts. High-Al garnets have late magmatic or deuteric origin as they replace melilite or carbonate, have low Y/Ho ratio and occur only in a location where carbonatites formed by melt and fluid immiscibility. Significant compositional changes accompanying magmatic crystallization and variations in accessory mineralogy in a single rock type result in a very wide range of major and trace element compositions of garnets and complicate their use as petrogenetic indicators.
Pressure-induced phase transitions from the zircon structure-type (I4 1 /amd) to the scheelite structure type (I4 1 /a) are known for many ternary oxides systems (ABO 4). In this work, we present the first high-pressure study on synthetic stetindite (CeSiO 4) by a combination of in situ high-pressure synchrotron powder X-ray diffraction up to 36 GPa, implemented with and without dual sided laser heating, and in situ high-pressure Raman spectroscopy up to 43 GPa. Two phase transitions were identified: zircon to a high-pressure low-symmetry (HPLS) phase at 15 GPa and then to a scheelite at 18 GPa. The latter from HPLS scheelite phase was found irreversible; i.e., scheelite is fully quenchable at ambient conditions, as in other zircon-type phases. The bulk moduli (K 0) of stetindite, HPLS, and high-pressure scheelite phases were determined, respectively, as 171(5), 105(4), and 221(40) GPa by fitting to a second-order Birch−Murnaghan equation of state. The pressure derivatives of vibrational modes and Gruneisen parameters of the zircon-structured polymorph are similar to those of other orthosilicate minerals. Due to the larger ionic radii of Ce 4+ , with respect to Zr 4+ , stetindite was found to possess a softer bulk modulus and undergo the phase transitions at a lower pressure than zircon (ZrSiO 4), such observations are consistent with what were found in coffinite (USiO 4).
The garnet structure Y3Fe5O12 (YIG) possesses a high immobilization capacity for actinides. However, the related research on the sensitivity of garnet structures to irradiation damage is still in its infancy. Herein, we synthesize Y3-xNdxFe5O12 (x = 0.1, 1.8) ceramics and irradiate them with 2 MeV α-particle irradiation to fluences ranging from 1 × 10¹⁴ to 1 × 10¹⁷ ions/cm² at room temperature. We evaluated the effects of irradiation damage on the phase transition and chemical stability. An amorphous layer with a thickness of 84 nm is formed by irradiation, though the grain shape and element distribution remain uniform after irradiation. In addition, Y3-xNdxFe5O12 (x = 0.1, 1.8) ceramics exhibit excellent chemical stability (∼ 10⁻⁵ g · m⁻² · d⁻¹) in the leaching process. Restricted by the topological structure of garnet, the increase in solid solubility does not lead to a change in the radiation tolerance of the garnet structure, which means that garnet of different compositions should have similar and predictable radiation tolerance.
The SIMS U–Pb dating method for andradite-rich garnet with the andradite composition ranging from 52 to 95 mol% was developed.
Advanced materials and processes are needed to immobilize fission products in the form of chemically durable waste. Nd-doped Ca3Zr2Fe2SiO12 (GZG) ceramics with different sintering temperatures (1100–1300 °C), sintering times (0.5–6 h) and solid solutions (0 ≤ x ≤ 2.5) were prepared by microwave sintering. The results show that by adjusting a series of key parameters, natural garnet structures with excellent chemical stability are obtained. XRD shows that a single phase of solid solution can be obtained by increasing the sintering temperature and controlling the actinide solid solution below the solid solution limit. The sintering temperature only affects the compactness of the solid solution. Thermodynamic calculations show that high temperature is favorable for the synthesis of GZG, and it is positive and irreversible. The normalized leaching rate of Nd³⁺ in the GZG structure is approximately ∼ 10⁻⁶ g m⁻¹ d⁻¹. The release form of Nd in aqueous solution is inner diffusion. The experimental results show that the naturally occurring garnet structure is an excellent waste form.
The Qin–Hang suture belt in South China formed during the Neoproterozoic amalgamation between the Cathaysia and Yangtze blocks. There are several Jurassic porphyry–skarn Cu deposits in this region, and the factors controlling the mineralization potential remain unclear. This study reports geochemical data for the Tongshan skarn deposit and zircon trace element data for five porphyry- or skarn-type Cu deposits in this region, in order to identify geochemical indexes for the mineralization potential of the ore-related intrusions. A garnet U–Pb age (170 Ma) for the Tongshan skarn indicates that Cu mineralization occurred in the Middle Jurassic, which is consistent with the zircon U–Pb ages (173 Ma–171 Ma) of the host granodiorite porphyries. Granodiorite porphyry samples from the Tongshan deposit have moderately high SiO2 contents (64.4–65.0 wt.%), low MgO contents (1.39–1.47 wt.%), and high (La/Yb)N ratios (14.7–16.8), typical of low-Mg adakitic rocks. The whole-rock geochemistry, bulk Earth-like εNd (t) values (–1.5 to –1.6) and positive zircon εHf(t) values (+2.7 to +6.0), corresponding to two-stage Hf model ages of 1014–832 Ma, suggest the porphyries were derived from thickened juvenile lower crust initially generated during Neoproterozoic amalgamation. In addition, the porphyry samples have arc-like geochemical characteristics (Nb–Ta–Ti depletion), low Th/Yb ratios (average = 4.7), high Ba/Th ratios (average = 346), zircon Eu/Eu* values (average = 0.71) and 10000×(Eu/Eu*)/Y values (average = 6.80), and moderate zircon Ce/Ce* values (average = 705) and Ce/Nd ratios (average = 17.1). These features demonstrate that the parental magmas of the porphyries were formed by melting of a mantle wedge
that had been metasomatized by moderately oxidized slab-derived fluid near a Neoproterozoic subduction zone. Previous studies suggest porphyries from large to giant deposit (e.g., Dexing and Yinshan) have similar two-stage Hf model ages and were derived from Neoproterozoic juvenile crust. But these porphyries have higher whole–rock Th/Yb ratios and zircon Ce/Ce* and Ce/Nd values than those of intrusions from medium-sized deposits (e.g., Tongshan, Chuankeng and Jiande). This indicates that the magma source of the former may have been distant from the trench in the Neoproterozoic, and formed by melting of a mantle wedge that had been metasomatized by highly oxidized slab-derived melt. Our results and previously published data indicate that the Neoproterozoic two-stage Hf model ages of the intrusions are positive indicators of Jurassic Cu mineralization, and granitoids with relatively high Th/Yb ratios and zircon Ce/Ce* and Ce/Nd values likely have more potential to form large to giant Cu deposits.
Zircon (ZrSiO4, I41/amd) can accommodate actinides, such as thorium, uranium, and plutonium. The zircon structure has been determined for several of the end-member compositions of other actinides, such as plutonium and neptunium. However, the thermodynamic properties of these actinide zircon structure types are largely unknown due to the difficulties in synthesizing these materials and handling transuranium actinides. Thus, we have completed a thermodynamic study of cerium orthosilicate, stetindite (CeSiO4), a surrogate of PuSiO4. For the first time, the standard enthalpy of formation of CeSiO4 was obtained by high temperature oxide melt solution calorimetry to be -1971.9 ± 3.6 kJ/mol. Stetindite is energetically metastable with respect to CeO2 and SiO2 by 27.5 ± 3.1 kJ/mol. The metastability explains the rarity of the natural occurrence of stetindite and the difficulty of its synthesis. Applying the obtained enthalpy of formation of CeSiO4 from this work, along with those previously reported for USiO4 and ThSiO4, we developed an empirical energetic relation for actinide orthosilicates. The predicted enthalpies of formation of AnSiO4 are then determined with a discussion of future strategies for efficiently immobilizing Pu or minor actinides in the zircon structure.
As efficient and stable nuclear waste forms, single-phase uranium (U6+)-incorporated La2Zr2O7 nanoparticles were designed and synthesized in an air atmosphere. To obtain a high U loading, divalent magnesium (Mg2+) was introduced to balance the extra charge from the substitution of tetravalent zirconium (Zr4+) by U6+ with a minimized impact to the lattice. There is a composition-driven phase transition from order pyrochlore to defect fluorite as the U concentration increases from 10 to 30 mol %, demonstrating both good solubility and stability of the La2Zr2O7 host for U and potentially for other actinides. La2(U
x
Mg
x
Zr1-2x)2O7 (x = 0-0.3) nanoparticles showed good dispersity and crystallinity with an average particle size of ∼48 nm. Furthermore, X-ray photoelectron spectroscopy, Raman spectroscopy, and emission spectroscopy revealed that U was stabilized in the hexavalent state in the form of a UO22+ ion. Spectroscopic methods also demonstrated that our samples caused a scintillating response with an orange emission (597 nm) by 230 nm excitation. In addition, density functional theory simulations were employed to investigate the atomic structures and electronic properties of the U-incorporated pyrochlores. The calculated bond lengths, atomic charges, and charge density confirm the existence of UO22+ ions. Supported by both experimental and computational results, a novel geometrical structure was proposed to explain the Mg2+-U6+ substitution. This work demonstrated the successful development of U-incorporated La2Zr2O7 nanoparticles and provided an efficient way to immobilize U in these ceramic waste matrixes.
Bulk moduli are important parameters to assess the mechanical performance of materials including nuclear fuels. However, little experimental data exist for U3Si2, a potential accident-tolerant nuclear fuel, whose bulk modulus has only been measured by resonant ultrasonic spectroscopy (RUS). In addition, the knowledge for high-pressure structural behavior and phase equilibrium of U3Si2 is largely lacking. Here we studied pressure dependence of the crystal structure of U3Si2 using high-energy synchrotron X-ray diffraction coupled with Rietveld analysis. The pressurization was achieved using a diamond anvil cell (DAC) which provides quasi-hydrostatic pressures up to 37.6 GPa. Crystal structural variation and equations of state of U3Si2 were obtained, and its bulk modulus, a- and c-axial moduli were derived to be 107.11 ± 5.65 GPa, 82.87 ± 4.78 GPa and 194.52 ± 12.03 GPa, respectively. The determined elastic parameters are compared with those obtained by RUS and nanoindentation.
UeSi intermetallic compounds have drawn great attention due to their potential application as nuclear fuels. However, the thermodynamic properties and phase equilibria of this binary system from ambient to high temperature conditions are not fully understood. Via high temperature oxidative drop calorimetry and detailed characterization of the initial and final phases, we have experimentally determined the standard enthalpies of formation of USi and U3Si5.07 at 298 K to be -43.2 ± 6.2 and -43.8 ± 9.0 kJ/mol atom, respectively. The energetics of the tetragonal USi (t-USi, space group I4/mmm) phase has also been calculated with Density Functional Theory (DFT) for the first time. Combining the obtained formation enthalpies with the heat capacities measured previously, we assessed the thermodynamic stability of t-USi relative to a phase assemblage of two other UeSi phases, U3Si5.07 and U3Si2, from ambient temperature to 1200 K. The tetragonal USi is thermodynamically more stable than U3Si5.07 + U3Si2, which supports previously published phase diagram (H. Okamoto and T. Massalski, 1990 [1]): specifically, at least one stable USi phase exists when the U content is 50 at.%. Further thermodynamic and phase equilibrium studies are needed for a more comprehensive understanding of the U-Si system across broader compositional and temperature ranges.
This contribution presents U-Pb geochronological data for Ca-Fe-Ti garnets from compositionally diverse alkaline and carbonatitic intrusive rocks ranging from Neoarchean to Permian-Triassic (Cinder Lake and Eden Lake in Manitoba, Canada; Belaya Zima and Odikhincha in Siberia, Russia; Afrikanda in the Kola Peninsula, Russia) obtained using isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) and their trace-element compositions measured by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS). The studied garnets yield U-Pb concordant or subconcordant ages obtained with a precision of <0.5% owing to their relatively high content of U and negligible common Pb. The new ID-TIMS data are in excellent agreement with the previously reported zircon, baddeleyite and perovskite ages. The results of the present work demonstrate that postmagmatic alteration does not disturb or reset the U-Pb isotopic budget of these minerals, and that garnets representing the andradite-schorlomite-morimotoite system can serve as a robust reference material for micro-analytical geochronological studies of a wide spectrum of igneous and contact-metasomatic rocks. The new LA-ICPMS data demonstrate that the abundances of rare-earth, high-field-strength and other trace elements in calcic garnets from alkaline and carbonatitic rocks vary by at least two orders of magnitude and can therefore be used as reliable magma-evolution tracers. The rare-earth budget of these minerals is best described in terms of the chondrite-normalized ratios (Sm/La) cn and (Sm/Yb) cn , which are sensitive to lanthanide fractionation, and Y/Ho, which is interpreted to respond to garnet re-equilibration with a fluid.
Gd 2 Zr 2 O 7 nano-grain ceramics with high densities (>90%) and small average grain sizes (66–96 nm) were successfully fabricated for the first time by the fast microwave pressureless sintering method (processing time: 10–90 min). The sintering behavior and microstructural evolution of fluorite structure Gd 2 Zr 2 O 7 nano-grain ceramics were investigated from 1100 °C to 1300 °C. Comparative analyses were conducted to unveil the influence of temperature and holding time on densification. The results suggest the optimized conditions for sintering dense nano-grain ceramics (66–96 nm) can be achieved at relatively low temperatures (<1250 °C) and long sintering time (60–90 min), or at high temperatures (>1250 °C) and short sintering time (10–60 min). As a demonstration, dense Gd 2 Zr 2 O 7 ceramics with average grain size of 66 nm were prepared for the first time by rapid microwave pressureless sintering.
Yttrium aluminium garnet (YAG, Y3Al5O12) transparent ceramics were fabricated by vacuum sintering the co-precipitated raw powders with calcium oxide (CaO) as the sintering aid. The influence of CaO content on the phase composition, particle size and the morphologies of resultant YAG powders, as well as the microstructural and optical properties of YAG ceramics, was investigated in detail. Our results show that with increasing of CaO additives, the particle size and agglomeration degree of the powders were slightly increased and the grain growth of the ceramics was inhibited. Specifically, the grain size initially decreased dramatically till reaching a minimum value of 1.85 μm with 0.3 at% CaO, then slightly increased. In addition, for YAG ceramics with a high CaO doping level, the existence of liquid phase was observed for the first time and its sintering mechanism was discussed. The excess of CaO causes the formation of grain boundary phases and residual pores, which are detrimental for maintaining higher optical quality of YAG ceramics. We obtained a fully dense and pore-free YAG ceramic with an in-line transmittance of 80.8% at 1100 nm by adopting 0.1 at% level of CaO during sintering of the green bodies at 1780 °C for 20 h in vacuum.
Defect-fluorite structured Gd2Zr2O7 nanoparticles were successfully synthesized via a homogeneous precipitation-solvothermal method using urea as a precipitant. The obtained nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis and transmission electron microscopy (TEM). Compared to the traditional solvothermal method, this homogeneous precipitation-solvothermal method has the advantage of producing nanoparticles with small grain sizes, a narrow size-distribution, high surface areas and little agglomeration. Particularly, the mean crystallite size of Gd2Zr2O7 obtained by this method is 20–30 nm, providing a great opportunity of using these nanoparticles as starting nano-sized building blocks for low temperature preparation of homogeneous and dense ceramics.
High-temperature oxide melt solution calorimetric measurements were completed to determine the enthalpies of formation of the uranothorite, (USiO4)x - (ThSiO4)1-x, solid solution. Phase - pure samples with x = 0, 0.11, 0.21, 0.35, 0.71, and 0.84 were prepared, purified, and characterized by powder X-ray diffraction, electron probe microanalysis, thermogravimetric analysis and differential scanning calorimetry coupled with in situ mass spectrometry, and high temperature oxide melt solution calorimetry. This work confirms the energetic metastability of coffinite, USiO4, and of U-rich intermediate silicate phases with respect to a mixture of binary oxides. However, variations in unit cell parameters and negative excess volumes of mixing, coupled with strongly exothermic enthalpies of mixing in the solid solution, suggest short-range cation ordering that can stabilize intermediate compositions, especially near x = 0.5.
Heating a mixture of uranyl(vi) nitrate and tantalum(v) oxide in the molar ratio of 2 : 3 to 1400 °C resulted in the formation of a new compound, UTa3O10. The honey colored to yellow brown crystals of UTa3O10 crystallize in an orthorhombic structure with the space group Fddd (no. 70), lattice parameters a = 7.3947(1), b = 12.7599(2), c = 15.8156(2) Å, and Z = 8. Vertex sharing [TaO6](7-) octahedra of two crystallographically distinct Ta cations form a three dimensional tantalate framework. Within this framework, six membered rings of [TaO6](7-) octahedra are formed within the (001) plane. The center of these rings is occupied by the uranyl cations [UO2](+), with an oxidation state of +5 for uranium. The pentavalence of U and Ta was confirmed by X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy. The enthalpy of formation of UTa3O10 from Ta2O5, β-U3O7, and U3O8 has been determined to be 13.1 ± 18.1 kJ mol(-1) using high temperature oxide melt solution calorimetry with sodium molybdate as the solvent at 700 °C. The close to zero enthalpy of formation of UTa3O10 can be explained by closely balanced structural stabilizing and destabilizing factors, which may also apply to other UM3O10 compounds.
The thermal decomposition of studtite (UO2)O2(H2O)2·2H2O results in a series of intermediate X-ray amorphous materials with general composition UO3+x (x = 0, 0.5, 1). As an extension of a structural study on U2O7, this work provides detailed calorimetric data on these amorphous oxygen-rich materials since their energetics and thermal stability are unknown. These were characterized in situ by thermogravimetry, and mass spectrometry. Ex situ X-ray diffraction and infrared spectroscopy characterized their chemical bonding and local structures. This detailed characterization formed the basis for obtaining formation enthalpies by high temperature oxide melt solution calorimetry. The thermodynamic data demonstrate the metastability of the amorphous UO3+x materials, and explain their irreversible and spontaneous reactions to generate oxygen and form metaschoepite. Thus, formation of studtite in the nuclear fuel cycle, followed by heat treatment, can produce metastable amorphous UO3+x materials that pose the risk of significant O2 gas. Quantitative knowledge of the energy landscape of amorphous UO3+x was provided for stability analysis and assessment of conditions for decomposition.
Mössbauer study of synthesized ferrite-garnet samples containing Zr, Th, Ce and Gd of the following composition: 1C — Ca2, 5 Ce0, 5 Zr2 Fe3 O12, 2C — Ca1, 5 GdCe0, 5 ZrFeFe3 O12, 1T — Ca2, 5 Th0,5Zr2 Fe3 O12 and 2T — Ca1, 5 GdTh0, 5 ZrFeFe3 O12 are carried out. As a result of 57Fe Mössbauer study it is found that iron atoms in all investigated samples of garnets are in a trivalent state. The analysis of experimental Mössbauer spectra definitely specifies a various structural state of iron atoms in two investigated groups of samples: 1T, 1C and 2T, 2C. X-ray study have shown that 1T and 1C garnet samples crystallize in tetragonal space group I41/acd, but 2T and 2C samples crystallize in cubic space group Ia3d.
Although pentavalent uranium can exist in aqueous solution, its presence in the solid state is uncommon. Metal monouranates, MgUO4, CrUO4 and FeUO4 were synthesized for detailed structural and energetic investigations. Structural characteristics of these uranates used powder X-ray diffraction, synchrotron X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and 57Fe-Mössbauer spectroscopy. Enthalpies of formation were measured by high temperature oxide melt solution calorimetry. Density functional theory (DFT) calculations provided both structural and energetic information. The measured structural and thermodynamic properties show good consistency with those predicted from DFT. The presence of U5+ has been solidly confirmed in CrUO4 and FeUO4, which are thermodynamically stable compounds, and the origin and stability of U5+ in the system was elaborated by DFT. The structural and thermodynamic behaviour of U5+ elucidated in this work is relevant to fundamental actinide redox chemistry and to applications in the nuclear industry and radioactive waste disposal.
Comprehensive analysis of 'murataites' compositions and structures derived using optical and electron microscopy, including high resolution, X-ray diffraction, Mossbauer techniques and their comparison with that of pyrochlore allow concluding the similarity of model and experimentally observed phase compositions. It is also concluded that 'murataites' represent one more group of subtraction derivatives in fluorite-like structure family and can be regarded as modular members of polysomatic series, in which ultimate members are pyrochlore and murataite Mu-3.
The use of confinement matrices is a key part of safe management of high-level radioactive wastes (HLWs) derived in the nuclear fuel cycle. The matrices should immobilize radioisotopes after HLW deposition in the geological environment with possible groundwater filtration. The glasses currently used for this purpose on an industrial scale are not capable of incorporating sufficient amounts of plutonium and have low stability to chemical corrosion. This paper summarizes the results of structural analysis of crystalline phases that could be used for immobilization of actinide wastes of various compositions. It was suggested that pyrochlore-type phases can be used for incorporation of the actinide-zirconium-rare-earth element fraction of HLWs, while ferrites with a garnet structure could be used for immobilization of wastes of complex composition with high contents of corrosion products (Fe, Al, Ga). Ceramics of such composition were synthesized and analyzed for concentrations of actinides (Th, U), rare-earth elements (Gd, Ce), and Zr. It is necessary to study the stability of these phases to radiation and chemical corrosion to select suitable matrix materials.
Structural damage to actinide-bearing matrices upon their irradiation with Kr and Xe ions with energies of 1.0 and 1.5 MeV was studied. Actinides are incorporated into oxides with fluorite-type structures (zirconolite, pyrochlore, and murataite), uranium titanate (brannerite), silicates and ferrites with a garnet lattice, and a (Ca, REE) silicate with an apatite structure (britholite). The radiation doses for complete structural amorphization at 25°C were as follows (in units of 1014 ions/cm2): 2.9 for zirconolite, 1.8-2.4 for pyrochlore, 1.5-2 for garnet, 1.7-1.9 for murataite, 1.4 for brannerite, and 0.4 for britholite. The radiation resistance of phases expressed as the number of displacements per atom (dpa) ranges from 0.1 to 0.4 dpa. According to these data, a phase containing 10 Wt % 239Pu will be completely amorphized over 500-2000 yr. This will increase actinide leaching from the matrix by tens of times. Amorphization doses are several times higher for natural analogues because of healing of radiation damage to the mineral structure with time. Disposal of highly radioactive waste matrices in deep-well repositories with an elevated temperature of ambient rocks favors an increase in the resistance of the crystal structure to radiation and maintains the immobilizing properties of radionuclide-bearing matrices over longer periods.
Significance
Coffinite, USiO 4 , is an important alteration mineral of uraninite. Its somewhat unexpected formation and persistence in a large variety of natural and contaminated low-temperature aqueous settings must be governed by its thermodynamic properties, which, at present, are poorly constrained. We report direct calorimetric measurements of the enthalpy of formation of coffinite. The calorimetric data confirm the thermodynamic metastability of coffinite with respect to uraninite plus quartz but show that it can form from silica-rich aqueous solutions in contact with dissolved uranium species in a reducing environment. These constraints on thermodynamic properties support that coffinitization in uranium deposits and spent nuclear fuel occurs through dissolution of UO 2 (often forming hexavalent uranium intermediates) followed by reaction with silica-rich fluids.
A series of Fe3+-bearing Li7La3Zr2O12 (LLZO) garnets was synthesized using solid-state synthesis methods. The synthetic products were characterized compositionally using electron microprobe analysis and inductively coupled plasma optical emission spectroscopy (ICP-OES) and structurally using X-ray powder diffraction and 57Fe Mössbauer spectroscopy. A maximum of about 0.25 Fe3+ pfu could be incorporated in Li7-3x Fe
x
La3Zr2O12 garnet solid solutions. At Fe3+ concentrations lower than about 0.16 pfu, both tetragonal and cubic garnets were obtained in the synthesis experiments. X-ray powder diffraction analysis showed only a garnet phase for syntheses with starting materials having intended Fe3+ contents lower than 0.52 Fe3+ pfu. Back-scattered electron images made with an electron microprobe also showed no phase other than garnet for these compositions. The lattice parameter, a0, for all solid-solution garnets is similar with a value of a0≈12.98 Å regardless of the amount of Fe3+. 57Fe Mössbauer spectroscopic measurements indicate the presence of poorly- or nano-crystalline FeLaO3 in syntheses with Fe3+ contents greater than 0.16 Fe3+ pfu. The composition of different phase pure Li7-3x Fe
x
La3Zr2O12 garnets, as determined by electron microprobe (Fe, La, Zr) and ICP-OES (Li) measurements, give Li6.89Fe0.03La3.05Zr2.01O12, Li6.66Fe0.06La3.06Zr2.01O12, Li6.54Fe0.12La3.01Zr1.98O12, and Li6.19Fe0.19La3.02Zr2.04O12. The 57Fe Mössbauer spectrum of cubic Li6.54Fe0.12La3.01Zr1.98O12 garnet indicates that most Fe3+ occurs at the special crystallographic 24d position, which is the standard tetrahedrally coordinated site in garnet. Fe3+ in smaller amounts occurs at a general 96h site, which is only present for certain Li-oxide garnets, and in Li6.54Fe0.12La3.01Zr1.98O12 this Fe3+ has a distorted 4-fold coordination.
Significance
Uranium peroxides, metastudtite and studtite, can be formed on exposure of UO 2 based nuclear fuels to water during geological disposal or as a result of reactor accidents. We report detailed structural and thermochemical analysis of the metastudtite decomposition process. The thermodynamic data confirm the irreversible transformation from studtite to metastudtite and show that metastudtite can be a major oxidized corrosion product at the surface of UO 2 and contribute a significant pathway to dissolution. The prevalence of metastudtite may require additional tailoring of waste forms to minimize this dissolution pathway for uranium.
The garnet structure is a promising nuclear waste form because it can accommodate various actinide elements. Yttrium iron garnet, Y3Fe5O12 (YIG), is a model composition for such substitutions. Since cerium (Ce) can be considered an analogue of actinide elements such as thorium (Th), plutonium (Pu), and uranium (U), studying the local structure and thermodynamic stability of Ce-substituted YIG (Ce:YIG) can provide insights into the structural and energetic aspects of large ion substitution in garnets. Single phases of YIG with Ce substitution up to 20 mol % (Y3–xCexFe5O12 with 0 ≤ x ≤ 0.2) were synthesized through a citrate–nitrate combustion method. The oxidation state of Ce was examined by X-ray absorption near edge structure spectroscopy (XANES); the oxidation state and site occupancy of iron (Fe) as a function of Ce loading also was monitored by 57Fe–Mössbauer spectroscopy. These measurements establish that Ce is predominantly in the trivalent state at low substitution levels, while a mixture of trivalent and tetravalent states is observed at higher concentrations. Fe was predominately trivalent and exists in multiple environments. High temperature oxide melt solution calorimetry was used to determine the enthalpy of formation of these Ce-substituted YIGs. The thermodynamic analysis demonstrated that, although there is an entropic driving force for the substitution of Ce for Y, the substitution reaction is enthalpically unfavorable. The experimental results are complemented by electronic structure calculations performed within the framework of density functional theory (DFT) with Hubbard-U corrections, which reproduce the observed increase in the tendency for tetravalent Ce to be present with a higher loading of Ce. The DFT+U results suggest that the energetics underlying the formation of tetravalent Ce involve a competition between an unfavorable energy to oxidize Ce and reduce Fe and a favorable contribution due to strain-energy reduction. The structural and thermodynamic findings suggest a strategy to design thermodynamically favorable substitutions of actinides in the garnet system.
Kerimasite, ideally Ca3Zr2(Fe3+2Si)O12, is a new calcium zirconium silicate-ferrite member of the garnet group from the extinct nephelinitic volcano Kerimasi and surrounding explosion craters in northern Tanzania. The mineral occurs as subhedral crystals up to 100 m in size in calcite carbonatites, and as euhedral to subhedral crystals up to 180 m in size in carbonatite eluvium. Kerimasite is light to dark-brown in colour and transparent with a vitreous lustre. No cleavage or parting was observed and the mineral is brittle. The calculated density is 4.105(1) g/cm3. The micro-indentation, VHN25, ranges from 1168 to 1288 kg/mm2. Kerimasite is isotropic with n = 1.945(5). The average chemical formula of the mineral derived from electron microprobe analyses (sample K 94-25) and calculated for O = 12 and all Fe as Fe2O3 is (Ca3.00Mn0.01Ce0.01Nd0.01)Σ3.03(Zr1.72Nb0.14Ti0.08Mg0.02Y0.02)Σ1.98(Fe3+1.23Si0.86Al0.82Ti0.09)Σ3.00O12. The largest Fe content determined in kerimasite is 21.6 wt.% Fe2O3 and this value corresponds to 1.66 a.p.f.u. in the tetrahedral site. Kerimasite is cubic, space group Ia3d with a = 12.549(1) Å, V = 1976.2(4) Å3 and Z = 8. The five strongest powder-diffraction lines [d in Å, (I/Io), hkl] are: 4.441 (49) (220), 3.140 (91) (400), 2.808 (70) (420), 2.564 (93) (422) and 1.677 (100) (642). Single-crystal structure refinement revealed the typical structure of the garnet-group minerals. The name is given after the locality, Kerimasi volcano, Tanzania.
The electronic and thermodynamic properties of yttrium iron garnet (Y3Fe5O12, YIG), as a possible uranium-bearing phase, have been investigated using first-principles and semi-empirical methods. The electronic structures of pure and U-doped YIG were calculated and compared in order to obtain a fundamental understanding of the incorporation mechanism and stability of U in a YIG matrix. Uranium at the A-site is in 4 + oxidation state, acting as a single donor and introducing a localized defect state in the band gap. The ionic relaxations show U at the A-site is an off-center impurity. At the B-site, uranium is in 5 + oxidation state giving rise to two localized defect states in the middle of the band gap. At thermodynamic equilibrium the incorporation of U is limited by (i) the relatively narrow stability domain of the host YIG and (ii) the precipitation of uranium oxides as secondary phases. Under Y-rich growth conditions, YIG is unstable with respect to competing phases such as the iron oxides, Y2O3 and YFeO3. Under O-rich conditions, the incorporation U is obstructed by the formation of uranium-oxide precipitates. Under Fe-rich growth conditions, the formation energies of UY (U at the A-site) and UFe (U at the B-site) become negative for 0 ≤ EF ≤ 0.62 eV and 0 ≤ EF ≤ 0.77 eV, respectively, indicating that U might be incorporated in p-type YIG.
Mössbauer study of synthesized ferrite-garnet samples containing Zr, Th, Ce and Gd of the following composition: 1C Ca2, 5 Ce0, 5 Zr2 Fe3 O12, 2C Ca1, 5 GdCe0, 5 ZrFeFe3 O12, 1T Ca2, 5 Th0, 5Zr2 Fe3 O12 and 2T Ca1, 5 GdTh0, 5 ZrFeFe3 O12 are carried out. As a result of 57Fe Mössbauer study it is found that iron atoms in all investigated samples of garnets are in a trivalent state. The analysis of experimental Mössbauer spectra definitely specifies a various structural state of iron atoms in two investigated groups of samples: 1T, 1C and 2T, 2C. X-ray study have shown that 1T and 1C garnet samples crystallize in tetragonal space group I41/acd, but 2T and 2C samples crystallize in cubic space group Ia3d.
High-level nuclear wastes (HLW) are the liquid effluent that result from the reprocessing of spent nuclear fuel. These wastes are typically solidified in a glass for final disposal in deep geologic formations. At present, there is no geologic repository receiving these vitrified wastes. A primary issue in nuclear waste management is whether there can be societal, regulatory, and political confidence that the radiotoxic constituents of HLW can be safely disposed of for hundreds of thousands of years. If a glass waste form, placed at a depth of hundreds of meters, is stable and essentially insoluble in groundwater, it would be almost impossible for radioactivity to reach the environment. This paper summarizes the state of knowledge of glass performance in a geologic repository and examines the question of whether the long-term stability of the glass and radionuclide can be assured.
Synthetic ferrites with garnet-type structures were studied as possible matrices for immobilization of highly radioactive wastes (HLWs). Unlike other promising matrices, such as pyrochlore, monazite, britholite, etc., natural garnets contain no radioactive elements. Thus, the properties of garnet-type matrices, e.g., their capacity to accommodate HLW components, should be studied using synthetic compounds. According to similarity in degree of oxidation and ionic radii, we chose Th4+ and Ce 4+ as imitators of tetravalent actinides (U, Np, and Pu) and Gd3+ as an imitator of trivalent actinides (Am and Cm); Gd3+, together with La3+, composes the REE component of fractionated HLWs. The compositions of the studied samples corresponded to the following formulas: [(Ca1.5-x NaxGdTh0.5) (ZrFe) (Fe3-xSix,) O12] and [(Ca1.5-xNax, (Gd, La)Ce0.5) (ZrFe) (Fe3-xSix) O12], where x = 0.25, 0.5, and 0.75. Our study was aimed at revealing effects of Na2O and SiO2 contents and synthesis temperature (in a system with ThO2) on the phase composition of samples and capacity of the target phases to incorporate REE and actinides. It was found that the increase in Na2O and SiO2 contents in the starting material led to some decrease of ThO2 and CeO2 contents in garnets, while Gd2O3 and La2O3 contents slightly increased. The temperature increase resulted an increase of SiO2 contents in garnets and consequent decrease of ThO2 Solubility in these phases by several weight percent. Our study demonstrated that it is possible to synthesize ferrite garnets with significant amounts of actinides, REE, and admixture elements. Thus, these compounds may be promising matrices for immobilization of complex actinide-bearing HLW.
High-temperature oxide melt solution calorimetry with molten 2PbO.B2O3 as a solvent can be used for determining enthalpies of formation of carbonates and hydrous silicates. Under conditions of gas flow at 1-2 cm3/s, all H2O and CO2 is expelled from the solvent, leading to a reproducible final thermodynamic state. Both analytical data and a number of thermodynamics cycles show that, under these conditions, the volatiles neither dissolve in nor interact energetically with the melt. -Authors
An effective strategy for dealing with high-level waste is to partition the short-lived fission product elements from the long-lived actinides, creating separate waste streams. Once there are two waste streams, the properties and durability of the waste form can be designed to a level appropriate to the toxicity and time required for isolation from the environment. With such a strategy the fission product elements may be incorporated into a borosilicate glass and the actinides into more durable crystalline ceramics. Although special glass compositions may be developed for actinide incorporation, their long-term durability is less easily assured, particularly on the time scales required for actinide immobilization and confinement. The final selection of any waste form should depend on its ability to incorporate the radionuclides of interest, its chemical durability, response to a radiation-field, and physical properties as well as the time required for isolation to protect the environment.
The use of garnet/perovskite-based ceramic, with formula type (Y,Gd,..)3(Al,Ga,..)5O12/(Y,Gd,..)(Al,Ga,..)O3, was tested for immobilizing plutonium residue wastes. Pu residue wastes originate from nuclear weapons production and can contain more than 50% of impurities including such elements as Am, Al, Mg, Ga, Fe, K, La, Na, Mo, Nd, Si, Ta, Ce, Ba, B, W, Zn, Zr, C and Cl. While for some of these residues, direct conversion to typical glass or ceramic forms may be difficult, ceramic forms based on durable actinide host-phases are preferred for Pu, Am and other actinides immobilization. Garnet/perovskite crystalline host-phases are chemically and mechanically durable and desirable for the incorporation of Pu and most of the impurity elements in the Pu residue wastes in the lattices of host-phases in the form of solid solutions. Experiments on the synthesis of garnet/perovskite ceramic samples were carried out using melting in air at temperatures from 1300 °C (for samples doped with 10 wt.% Pu residue waste simulant) to 2000 °C (for samples doped with 10 wt.% Ce or U). Samples were studied by XRD, SEM and cathodoluminescence techniques. It was found that the garnet phase can incorporate up to 6 wt.% Ce and up to 4.0-5.5 wt.% U, which is correlated with the increase of Ga content and decrease of Al content in the melt. In one of the features of the melt, the perovskite phase formation substitutes for the formation of garnet. The capacity of the perovskite lattice to accommodate Ce and U is higher than the capacity of garnet, reaching about 8 and 7 wt.%, respectively. It was shown that cathodoluminescence can be effectively used to determine the valence state of Ce and U, an important step to optimize the starting precursor preparation. In case of U4+ in the melt, the charge-compensating elements (Sn2+, Ca2+...) are needed to successfully incorporate U in the garnet lattice.
The garnet structure has been proposed as a potential crystalline nuclear waste form for accommodation of actinide elements, especially uranium (U). In this study, yttrium iron garnet (YIG) as a model garnet host was studied for the incorporation of U analogs, cerium (Ce) and thorium (Th), incorporated by a charge-coupled substitution with calcium (Ca) for yttrium (Y) in YIG, namely, 2Y 3+ = Ca 2+ + M 4+ , where M 4+ = Ce 4+ or Th 4+. Single-phase garnets Y 3−x Ca 0.5x M 0.5x Fe 5 O 12 (x = 0.1−0.7) were synthesized by the citrate−nitrate combustion method. Ce was confirmed to be tetravalent by X-ray absorption spectroscopy and X-ray photo-electron spectroscopy. X-ray diffraction and 57 Fe−Mö ssbauer spec-troscopy indicated that M 4+ and Ca 2+ cations are restricted to the c site, and the local environments of both the tetrahedral and the octahedral Fe 3+ are systematically affected by the extent of substitution. The charge-coupled substitution has advantages in incorporating Ce/Th and in stabilizing the substituted phases compared to a single substitution strategy. Enthalpies of formation of garnets were obtained by high temperature oxide melt solution calorimetry, and the enthalpies of substitution of Ce and Th were determined. The thermodynamic analysis demonstrates that the substituted garnets are entropically rather than energetically stabilized. This suggests that such garnets may form and persist in repositories at high temperature but might decompose near room temperature.
Ceramic materials based on garnet, (Y,Gd,An,…)3(Al,Ga,An,…)5O12 and perovskite (Y,Gd,An,…)(Al,Ga,An,…)O3 structures have been proposed for the immobilization of weapons-grade actinide-containing waste materials with complex chemical compositions. Cathodoluminescence (CL) images and emission spectra of synthetic garnet and perovskite crystals containing Ce, U and Pu were studied. It was determined that Pu3+ incorporated into the garnet matrix has characteristic CL emission bands at 1.9 and 1.6 eV. The loading capacity of the garnet lattice for Pu4+ incorporation is significantly higher than for Pu3+ and similar to U4+. The maximum amount of Pu3+ that may be incorporated into the garnet structure experimentally is 0.3 wt.% in comparison with 5.3 wt.% for Pu4+. The CL emission spectra of Ce3+ in different materials is a characteristic property and can be used for identification of garnet and perovskite phases into the multiphase ceramic matrices simultaneously with microprobe analysis
Garnet solid solutions, Y3Al5O12-Gd3Al5O 12-Gd3Ga5O12 (YAG-GAG-GGG), are being considered as prospective durable host phases for the immobilization of actinide-containing waste with complex chemical compositions. Garnet samples with the suggested simplified formula: (Gd,Ce,...)3(Al,Ga,Pu,...)5O12 containing from 3.4 to 5.3 wt.% 239Pu and 3.6-5.5 wt.% Ce have been synthesized through melting of oxide starting materials in air using a hydrogen torch. Calcium and Sn were added to increase the Pu incorporation into the garnet lattice through ion charge and size compensation for Pu4+. Polycrystalline materials obtained in the experiments consist of garnet, perovskite and other phases and were studied by scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). Our results confirmed that the use of compensating elements such as Ca and Sn allow for significant incorporation of Pu and Ce (not less than a few wt.%) into the garnet structure. The preliminary conclusions thus so far indicate that garnet solid solution compositions may incorporate simultaneously trivalent and tetravalent actinides in significant quantities because they occupy different positions in the garnet structure.
Zirconolite (CaZrTi2O7) and perovskite (CaTiO3) are key minerals in SYNROC, a ceramic material developed for the immobilization of high level nuclear reactor wastes. When these are incorporated in SYNROC, the long-lived radioactive actinide elements are preferentially partitioned into zirconolite and perovskite which are therefore subjected to the effects of alpha-recoil, resulting from the decay of these elements. These effects have been studied via X-ray and electron diffraction investigations of natural samples of zirconolite and perovskite of varying ages and varying uranium and thorium contents. The samples studied have received cumulative alpha doses ranging from 1.0 × 10¹⁸ to 1.1 × 10²⁰α/g. The upper limit corresponds to the alpha irradiation which would be received by the zirconolite in SYNROC containing 10 percent of high level waste over a period of 5 × 10⁸years. These studies show that zirconolites remain crystalline up to and beyond alpha doses of 2 × 10¹⁹α/g. This dose would have accumulated in such a SYNROC zirconolite after a million years of storage. Electron microscopy revealed that the grains were composed of small crystalline domains which possessed the defect fluorite-type structure. After a dose exceeding that which would be received by SYNROC in 100 million years, zirconolites appeared metamict when studied by X-ray diffraction. However, the electron micrographs and diffraction patterns clearly demonstrate that the mineral continues to retain a large degree of short range order and in no way resembles a glass. The density changes produced in these zirconolites by irradiation are small and range from 0 to 3% at saturation. Perovskite samples which have SYNROC ages up to 20,000 years decrease in density by 1.8 ± 0.1%. Their X-ray powder patterns are essentially unaffected. Comparative studies show that the perovskite lattice is even more resistant to the effects of alpha-recoil than the zirconolite lattice. The results demonstrate that zirconolite and perovskite are extremely resistant to the effects of nuclear radiation and will provide stable crystal structures for the containment of the radioactive waste elements during the time required for the radioactivity to decay to safe levels (typically 10⁵–10⁶years).
To assess progress in high-temperature calorimetry over the past eighteen years, this feature article discusses both technical developments and new areas of application, with primary emphasis on high-temperature oxide melt solution calorimetry. The Calvet calorimeter for such measurements is now commercially available, and an extensive table of enthalpies of drop solution (sample at room temperature dropped into molten oxide solvent in calorimeter and dissolved) is provided for the user community. New developments in methodology are described and applications to nanomaterials, to nitrides and other monoxide materials, and to lanthanides and actinides are given.
The thermodynamic stability of Th-doped yttrium iron garnet (Y 3 Fe 5 O 12 , YIG) as a possible actinide-bearing material has been investigated using calorimetric measurements and first-principles electronic-structure calculations. Yttrium iron garnet with thorium substitution ranging from 0.04 to 0.07 atoms per formula unit (Y 3Àx Th x Fe 5 O 12 , x ¼ 0.04–0.07) was synthesized using a citrate–nitrate combustion method. High-temperature oxide melt solution calorimetry was used to determine their enthalpy of formation. The thermodynamic analysis demonstrates that, although the substitution enthalpy is slightly endothermic, an entropic driving force for the substitution of Th for Y leads to a near-zero change in the Gibbs free energy. First-principles calculations within the density functional theory (DFT) indicate that the main limiting factors for Th incorporation into the YIG structure are the narrow stability domain of the host YIG and the formation of ThO 2 as a secondary phase. Nevertheless, the defect formation energy calculations suggest that by carefully tuning the atomic and electronic chemical potentials, Th can be incorporated into YIG. The thermodynamic results, as a whole, support the possible use of garnet phases as nuclear waste forms; however, this will require careful consideration of the repository conditions.
The structural relaxations and electronic structure of U-containing Ca 3 (Ti,Zr,Hf,Sn) 2 (Fe 2 Si)O 12 garnet systems have been investigated using ab initio methods within density functional theory (DFT) in the generalized gradient approximation with a Hubbard correction U (GGA + U). The calculations provide a fundamental understanding of the role of Fe in the incorporation and stability of U in the garnet structure. The atomic relaxations around U are controlled by a delicate balance between the Coulomb interactions among the ions and the size effect of the large U atom. The relaxation pattern indicates that when U occupies the A site, a charge transfer occurs from U to its nearest-neighbor (NN) Fe atom. This is further verified by the detailed analysis of the electronic band structures and charge density distribution. The double exchange coupling of the U f and the NN Fe d shells via the transfer of electrons lowers the energy of the system when the spins of the f and d shells are antiparallel. The incorporation energy of U at the A site (substituting Ca) increases dramatically with the decrease in the number of Fe atoms in the neighboring tetrahedral sites. The presence of Fe is crucial, since it accommodates the extra valence electrons introduced by U and the electron transfer allows the lowering of the total energy of the structure. Comparing the incorporation energies at the A and B site (octahedral site), U clearly prefers the A site, provided that there are sufficient Fe atoms in its vicinity to facilitate the charge transfer.
With the increasing demand for the development of nuclear power comes the responsibility to address the issue of waste, including the technical challenges of immobilizing high-level nuclear wastes in stable solid forms for interim storage or disposition in geologic repositories. The immobilization of high-level nuclear wastes has been an active area of research and development for over 50 years. Borosilicate glasses and complex ceramic composites have been developed to meet many technical challenges and current needs, although regulatory issues, which vary widely from country to country, have yet to be resolved. Cooperative international programs to develop advanced proliferation-resistant nuclear technologies to close the nuclear fuel cycle and increase the efficiency of nuclear energy production might create new separation waste streams that could demand new concepts and materials for nuclear waste immobilization. This article reviews the current state-of-the-art understanding regarding the materials science of glasses and ceramics for the immobilization of highlevel nuclear waste and excess nuclear materials and discusses approaches to address new waste streams.
The electronic structure of the Pu- and Np-containing Ca3(Ti,Zr,Hf)2Fe23+SiO12 garnet series has been investigated using ab initio methods within density functional theory (DFT, GGA + U). The calculations provide a fundamental understanding of the incorporation mechanism and stability of Pu and Np in the garnet structure. The detailed analysis of the electronic densities of states (DOS), band structures and charge density distributions confirm that electrons are transferred from the actinides to the surrounding Fe atoms. This occurs through the double exchange coupling of the actinide f shell and the NN Fe d shells and stabilizes the actinide at the A-site, when the spins of the f and d shells are anti-parallel. The presence of Fe is crucial, since it accommodates the extra valence electrons introduced by the actinides, and this electron transfer lowers the total energies of the actinide-doped structure. Comparing the incorporation energies at the A- and B-site, both Pu and Np clearly prefer the A-site, provided that there are sufficient Fe atoms to facilitate the charge transfer. The calculated incorporation energies suggest that ferric garnet is promising material for actinide immobilization.
Technetium is an important environmental contaminant introduced by the processing and disposal of irradiated nuclear fuel and atmospheric nuclear tests. Under oxic conditions technetium is soluble and exists as pertechnatate anion (TcO4−), while under anoxic conditions Tc is usually insoluble and exists as precipitated Tc(IV). Here we investigated abiotic Tc(VII) reduction in mineralogically heterogeneous, Fe(II)-containing sediments. The sediments were collected from a 55 m borehole that sampled a semi-confined aquifer at the Hanford Site, USA that contained a dramatic redox transition zone. One oxic facies (18.0–18.3 m) and five anoxic facies (18.3–18.6 m, 30.8–31.1 m, 39.0–39.3 m, 47.2–47.5 m and 51.5–51.8 m) were selected for this study. Chemical extractions, X-ray diffraction, electron microscopy, and Mössbauer spectroscopy were applied to characterize the Fe(II) mineral suite that included Fe(II)-phyllosilicates, pyrite, magnetite and siderite. The Fe(II) mineral phase distribution differed between the sediments. Sediment suspensions were adjusted to the same 0.5 M HCl extractable Fe(II) concentration (0.6 mM) for Tc(VII) reduction experiments. Total aqueous Fe was below the Feaq detection limit (<2 μM). Technetium(VII) reduction occurred in all anoxic sediments at depths greater than 18.3 m and reaction time differed significantly between the sediments (8–219 d). Mössbauer analysis of the Tc-reacted, 30.8–31.1 m sediment revealed changes in the concentrations of solid-phase Fe(II) and Fe(III). A decrease in the spectral areas of siderite and Fe(II)-containing phyllosilicates illustrated that these phases were oxidized following reaction with Tc(VII). XAS analysis demonstrated that Tc associated with sediments was in the Tc(IV) valence state and immobilized as clusters of a TcO2·nH2O-like phase. The speciation of redox product Tc(IV) was not affected by reduction rate or Fe(II) mineralogy.
The standard enthalpy of formation of thorite and huttonite and the enthalpy of the phase transition between these polymorphs were determined using high-temperature oxide melt solution calorimetry and transposed temperature drop calorimetry. Standard enthalpies of formation of thorite and huttonite are reported for the first time and are −2117.6 ± 4.2 kJ/mol and −2110.9 ± 4.7 kJ/mol, respectively. Based on our measurements, thorite and huttonite are metastable relative to SiO2 (quartz) and ThO2 (thorianite) at standard conditions, but are presumably stabilized at high temperature by the entropy contribution. Based on the measured enthalpy of the thorite-huttonite phase transition of 6.7 ± 2.5 kJ/mol, a dP/dT slope for the transformation was calculated as −1.21 ± 0.45 MPa/K.
The preparation of the uranates of calcium and strontium by solid state reactions is described. A number of new compounds have been found of which thermal stability, heats of formation and X-ray powder data are given.
A consistent methodology for obtaining enthalpy of formation of Fe{sup 2+}-containing binary and multicomponent oxides using high temperature oxide melt solution calorimetry has been developed. The enthalpies of wuestite (FeO) and magnetite (FeO) oxidation to hematite (FeO) were measured using oxidative drop solution calorimetry in which the final product is dissolved ferric oxide. Two methods were applied: drop solution calorimetry at 1073 K in lead borate solvent and at 973 K in sodium molybdate, each under both oxygen flowing over and bubbling through the solvent, giving consistent results in agreement with literature values. The enthalpies of formation of all three iron oxides from the elements were obtained using a thermodynamic cycle involving the directly measured oxidative dissolution enthalpy of iron metal in sodium molybdate at 973 K and gave excellent consistency with literature data. The methodology was then applied to the magnetite - maghemite system. The enthalpy of mixing of the FeO-FeO spinel solid solution is exothermic and, 2 represented by a subregular (Margules) formalism, H{sub mix} = x(1-x)(-63.36 {+-} 8.60(1-x) + 17.65 {+-} 6.40x) kJ/mol, where x is the mole fraction of magnetite. The entropies of mixing of the solid solution were calculated for different assumptions about the distribution of cations, charges, and vacancies in these defect spinels. The different models lead to only small differences in the entropy of mixing. Calculated free energies of mixing show no evidence for a solvus in the magnetite - maghemite system.
Selected values for the entropy (S°), molar volume (v°), and for the enthalpy and Gibbs free energy of formation (..delta..H/sub f/°, and ..delta..G/sub f/°) are given for the elements, 133 oxides, and 212 other minerals and related substances at 298.15 K. For those materials for which high-temperature heat-capacity or heat-content data are also available (H/sub T/° - Hâââ°)/T, S/sub T/°, (G/sub T/° - Hâââ°)/T, C/sub p/°, ..delta..H/sub f,T/°, ..delta..G/sub f,T/° and log K/sub f,T/ are tabulated at 100 K intervals for temperatures up to 1800 K. For substances that have solid-state phase changes or whose melting or boiling point is less than 1800 K, we have also tabulated the properties listed above at the temperature of the phase change so that the enthalpy or entropy changes associated with the transformation form an integral part of the high-temperature tables.
A high-performance titanate ceramic waste form composed of minerals that have survived in various natural environments for long periods, SYNROC should permit safe burial of waste in deep drill holes.
Magnetic and crystallographic properites of Y3-x Ce x Fe5O12 (x=0.0, 0.1, and 0.3) have been studied with X-ray diffraction, vibrating sample magnetometer (VSM), and Mössbauer spectroscopy. A small coercivity (H c =5.8 Oe), was obtained for the sample Y2.9Ce0.1Fe5O12, which is comparable to that of an undoped sample Y3Fe5O12 (H c =54.1 Oe). Mössbauer spectra of Y3-x Ce x Fe5O12 were measured at various absorber temperatures from 4.2 K to Néel temperature. It is found that Debye temperatures of octahedral (16a) and tetrahedral (24d) site for Y2.9Ce0.1Fe5O12 are Theta a =353, Theta d =464 K, respectively, and for Y2.7Ce0.3Fe5O12, Theta a =380, Theta d =444 K, respectively. The intersublattice a d superexchange interaction was found to be antiferromagnetic with the strength of J a-d =-21.42 k B, while the intrasublattice interactions a a, d d were found to be ferromagnetic with strengths of J a a =4.50 k B and J d d =0.02k B , respectively, in the sample Y2.9Ce0.1Fe5O12.
Garnet, A3B2X3O12, has a structure that can incorporate actinides. Hence, the susceptibility of the garnet structure to radiation damage has been investigated by comparing the results of self-radiation damage from α-decay of 244Cm and a 1MeV Kr2+ ion irradiation. Gradual amorphization with increasing fluence was observed by X-ray diffraction analysis and in situ transmission electron microscopy. The critical dose, Dc, for an yttrium–aluminum garnet (Y3Al5O12) doped with 3wt.% 244Cm is calculated to be 0.4 displacements per atom (dpa). While the doses obtained by ion irradiation experiments of garnets with different compositions (Y2.43Nd0.57)(Al4.43Si0.44)O12, (Ca1.64Ce0.41Nd0.42La0.18Pr0.18Sm0.14Gd0.04)Zr1.27Fe3.71O12, and (Ca1.09Gd1.23Ce0.43)Sn1.16Fe3.84O12, varied from 0.29 to 0.55 dpa at room temperature. The similarity in the amorphization dose at room temperature and critical temperature of the different garnet compositions suggest that the radiation response for the garnet structure is structurally constrained, rather than sensitive to composition, which is the case for the pyrochlore structure-type.
The concept of nuclear waste forms based on minerals that contain actinides has led to the development of polyphase and special-purpose crystalline ceramics. These ceramics are considered by many to be attractive media for the long-term storage of actinides in geological repositories. The available data show that monazite, pyrochlore, zircon, and zirconolite are all highly durable in both natural and synthetic aqueous systems at low temperatures. In comparison, perovskite is prone to dissolution and conversion to anatase and other secondary alteration products. The titanate and silicate phases of interest become metamict (amorphous) as a result of irradiation. Several compounds, including monazite, cubic zirconia, and the defect fluorite structure types with Zr on the B site, exhibit the attractive property of radiation "resistance." These results, together with other materials properties, are discussed briefly with respect to criteria for waste form performance.
A low temperature method is described for the preparation of the new garnet compounds YCaLnGa5O12 (LnCe, Pr, Tb). In this set of compounds (Ca2+ + Ln4+) replaces 2Y3+ in the parent gallium based garnet Y3Ga5O12 in order to stabilize as effectively as possible the Ln4+ species in the eight-coordinate “A” site of the garnet structure. Characterization of the oxides by X-ray powder diffraction and thermogravimetric analysis is discussed with regard to the structural relationship of the substituted compound to the parent material. The tetravalent ions Pr4+ and Tb4+ exhibit increased thermal stability in reducing conditions as compared to the Ln4+ states in the fluorite (LnO2) and perovskite (BaLnO3) type structures. This result is discussed with reference to the complex crystal chemistry of these systems.
Calorimetry at 600–900° C, using Calvet-type microcalorimeters, has provided data in several areas of interest to geology. The best-developed application is solution calorimetry in molten oxide solvents (lead borate, sodium molybdate) to determine the enthalpies of formation of anhydrous silicates and related minerals, the enthalpies of phase transformations and order-disorder reactions, and the enthalpies of mixing in molten salts, glasses, and solid solutions. An attractive feature of the technique is the ability to use rather small samples; ~ 300 mg total often being sufficient for the study of a phase synthesized at high pressure. Current developments include the improvement of precision by careful control of solvent composition and water content, the development of alkali borate or borosilicate solvents for use under atmospheres of controlled oxygen fugacity, and the study of compounds containing fluorine as well as oxygen.
The garnet structure, originally solved by Menzer, has become increasingly important in the last ten years. During this period a number of garnet-structure refinements have been carried out; these are reviewed and some of the consequences of the results are discussed. A survey has been made of all the cations which enter the garnet structure and their site preferences are given. Numerous examples of garnets and garnet systems that have been investigated arc listed. Some arc reported here for the first time. The ionic site preference in the garnets is discussed; it appears that relative ionic size is of primary importance, but for certain ions like Cr3+ and Mn3∼. the electronic configuration also plays an important role. Considerable discussion is given to the Co2+ ion for which the evidence maintains that the Co2+ ion prefers, by far, the octahedral sites to the tetrahedral. Garnets have been prepared with Co3+ ion in the tetrahedral and in the octahedral sites. The determination of the distribution of ions in the system Y3Fc5-x.GaxO12 by different techniques is reviewed.
The influence of synthesis conditions on the formation of yttrium iron garnet (YIG) powders starting from the same precursor solution was investigated by employing a citrate–nitrate (C–N) gel combustion process and a precursor plasma spraying technique. Two different C–N ratios were used in the synthesis and their influences on phase formation were studied by thermal analysis (DTA/TGA) and X-ray powder diffraction (XRD). Time-resolved powder XRD experiments were performed for the first time on these C–N precursor materials to understand their mode of decomposition. For a C–N=0.75, the ex-situ XRD data confirmed a single-step conversion to YIG from the amorphous precursor powder without any intermediate phase formation. However, the use of the same C–N precursor solution as a liquid feedstock material in the precursor plasma spraying (PPS) technique revealed an entirely different transformation mechanism to YIG through the intermediate phase YFeO3. The measured values of saturation magnetization (MS) as well as coercive field (HC) of the powder samples annealed at 1500°C are close to those reported for bulk YIG.
As the world faces the consequences of global warming caused by the use of fossil fuels, there has been a resurgence of interest in nuclear power. However, there is no "silver bullet," and each energy-producing system produces waste. This issue of Elements shows the importance of mineralogy and geochemistry in the safe management and disposal of the different types of waste generated by the nuclear fuel cycle.
A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step-scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least-squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.