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1.
晶质铀矿是花岗岩中广泛产出的副矿物,其铀离子价态的变化及比例关系,对于岩浆氧逸度具有一定的指示意义。笔者利用硅酸盐熔浆中U~(6+)-U~(4+)的平衡关系,推导出花岗岩副矿物——晶质铀矿的氧逸度计热力学计算方程,以及氧逸度(f O2)、温度(T)和不同价态铀离子比值(U~(6+)/U~(4+))的对数关系。以赣南五里亭花岗岩体为例,借助X射线光电子能谱分析,获得U~(6+)与U~(4+)的比例关系(U~(6+)/U~(4+)=0.30～0.46);利用晶质铀矿氧逸度计热力学方程,计算出该岩体形成时的平均氧逸度(为FMQ+0.58)。在温度(T)与氧逸度(lg f O2)协变图中,该结果靠近矿物反应缓冲线(FMQ),显示岩浆为弱还原性。因此,晶质铀矿U~(6+)/U~(4+)氧逸度计算结果能够代表岩浆源区的氧逸度特征,可作为常规氧逸度计算方法的一种有效补充。 相似文献
2.
鄂尔多斯盆地纳岭沟铀矿床铀矿物特征与形成机理 总被引：1，自引：0，他引：1
《矿物学报》2017,(4)
纳岭沟铀矿床位于鄂尔多斯盆地北东部,是我国近年发现的一个特大型砂岩铀矿床,铀矿体赋存于中侏罗统直罗组下段河流相砂体中,受古层间氧化带控制,呈板状。铀矿物主要为铀石、沥青铀矿和钛铀矿,其中铀石是最主要的铀矿物。沥青铀矿中w(CaO)较高;铀石中w(UO_2)偏低、w(SiO_2)偏高,w(UO_2)/w(SiO_2)达到1:1.29,远低于正常值;钛铀矿则呈现U低Ti高的特点。根据各铀矿物的形成、富集机理,认为纳岭沟铀矿床在古层间氧化发育阶段赋矿砂体属于酸性环境,SiO_2活度较低,形成铀矿物主要为沥青铀矿。始新世晚期及以后,氧化作用不发育,赋矿砂体被二次还原,岩石地球化学环境由酸性转变为弱碱性,SiO_2活度增大,使早期阶段形成的沥青铀矿转变为铀石。钛铀矿主要是在氧化—还原作用下由含铀溶液中的铀(UO_2~(2+))交代重矿物—钛铁矿中的Fe2+而形成。 相似文献
3.
诸广山岩体南部中段花岗岩晶质铀矿标型特征的探讨 总被引：1，自引：0，他引：1
本文以花岗岩的副矿物组合类型为基础,从剖析晶质铀矿的晶形标型入手,探讨了诸广山岩体南部中段花岗岩晶质铀矿的标型特征。花岗岩的副矿物组合类型不同,晶质铀矿的产出特点以及某些矿物学特征也存在差异。晶质铀矿的含量、产状、粒度、晶形、晶胞参数及含氧系数等可反映出岩浆中的铀浓度、U~(4 )和UO_2~(2 )所占份额、氧逸度特点及其变化趋势的定性信息。 相似文献
4.
电子探针化学测年在攀枝花大田晶质铀矿中的应用及其意义 总被引：1，自引：1，他引：0
晶质铀矿和沥青铀矿是热液铀矿床的主要工业铀矿物,在研究热液铀矿床成因及成矿规律方面具有重要的意义。攀枝花大田地区是我国混合岩型热液铀矿分布区,已发现粗粒特富铀矿滚石(铀含量10%)及较富基岩矿石(铀含量为0.1%~2%),主要铀矿物为晶质铀矿,对两种晶质铀矿成分及形成时代的研究对该区混合岩型热液铀矿成矿规律研究具有重要的价值。本文通过对大田地区滚石中的晶质铀矿和基岩矿石中的晶质铀矿进行矿物学及电子探针分析,研究了晶质铀矿的成分及形成时代。结果表明:(1)大田地区滚石和基岩矿石中的晶质铀矿除铅之外化学成分较为相似,两类矿石晶质铀矿中UO_2含量为77.36%~84.04%,ThO_2含量为0.98%~5.59%,PbO含量为1.79%~8.8%,其中滚石晶质铀矿中的铅含量低于基岩晶质铀矿,钍含量高于基岩晶质铀矿;(2)电子探针化学定年结果表明,基岩矿石晶质铀矿的形成时代为774.9~785.5 Ma,滚石晶质铀矿的形成时代为783.7 Ma,与传统同位素测年结果(775~777.6 Ma)非常一致,一方面说明滚石晶质铀矿和基岩晶质铀矿为同一时代的产物,另一方面说明电子探针原位测年方法是可靠的;(3)在后期的热液蚀变中,晶质铀矿先后发生了硅化、碳酸盐化及赤铁矿化,蚀变发生的时间分别为730.6Ma、699.8 Ma和664.0 Ma。此结论对研究攀枝花大田地区热液铀矿成矿时代及成矿作用过程提供了依据。 相似文献
5.
《矿产与地质》2017,(6)
松辽盆地北部是近年来发现的砂岩型铀矿找矿潜力较大的地区。采用电子探针及化学分析等方法,对该地区砂岩型铀矿中铀矿物的赋存状态及铀的价态进行研究。发现松辽盆地北部砂岩型铀矿铀的赋存状态以独立铀矿为主,主要为沥青铀矿和铀石;铀矿物颗粒细小,多以粒状、斑点状、团块状、条带状、网状、环带状及分散显微颗粒、显微颗粒集合体的形式存在,颗粒大小几微米至200μm;铀矿石主要与黄铁矿、石英、长石共生,分布在碎屑颗粒间或裂隙中,或充填于砂岩胶结物中,或围绕黄铁矿生长形成镶边状。从铀矿物的赋存状态及类型看,松辽盆地北部铀矿化可能存在两个成矿期,即同生沉积期和后生改造叠加期。矿石中U~(6+)/U~(4+)比值为0.30~2.07,平均为0.93,铀含量较高的矿石U~(6+)/U~(4+)比值高于铀含量较低的矿石,普遍大于1,有利于铀矿地浸开采。上述研究为松辽盆地北部砂岩型铀矿找矿、成矿机理研究提供了基础资料,同时对砂岩型铀矿选冶开采工艺的确定具有借鉴意义。 相似文献
6.
《华北地质》2021,(2)
采用电子探针、能谱及背散射分析、扫描电镜、微区X射线荧光等实验方法,详细研究了钱家店铀矿床铀的赋存形式、含铀矿物类型,并对铀矿物相关的矿物组合类型及成因进行探讨。结果表明:(1)钱家店铀矿区铀的赋存形式包括铀矿物、吸附态铀以及含铀矿物三大类。其中,铀矿物类型主要为沥青铀矿,铀石次之,仅在局部可见含钛铀矿物等;吸附铀主要为高岭石吸附和有机质吸附。铀矿物常与黄铁矿、铁白云石、高岭石等密切共生,多呈胶状发育在黄铁矿、铁白云石边缘及裂隙内,也有呈粒状、球粒状独立分布,少数分布在碎屑颗粒内部。(2)研究区至少存在两期铀成矿相关流体:一期中性-弱碱性含铀流体形成的铀矿物围绕黄铁矿边缘发育,形成环边、似环边状,同时形成铁白云石为主的碳酸盐围绕黄铁矿外围发育;另一期含铀流体形成的铀矿物呈脉状充填在黄铁矿内部,无碳酸盐形成。(3)铀矿物(特别是铀石)中含有丰富的磷元素,扫描电镜也发现部分样品中存在富磷元素的微球粒状的铀石集合体,初步认为这是低温流体下微生物参与铀成矿作用的有力证据。 相似文献
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8.
鄂尔多斯盆地是我国重要的砂岩型铀矿成矿区之一。铀矿物赋存状态研究对砂岩型铀矿的成因认识、找矿勘查及选冶开采具有重要意义,但其矿物组成复杂,铀矿物粒度细小、种类繁多且赋存状态多样,致使研究初始的鉴定阶段就存在难点。目前普遍使用放射性照相法和电子探针(EMPA)两种方法开展铀矿物鉴定分析工作。放射性照相可一次性得到光片中所有铀矿物赋存位置、赋存状态和放射性形态,但无法鉴定矿物种类,耗时较长且需在暗室中进行;电子探针可得到铀矿物背散射图像和各元素含量,但在高倍数下薄片中寻找含量少、粒度小的铀矿物费时费力,并且在黑白背散射图像中无法快速判断伴生矿物种类。本文以鄂尔多斯盆地北缘-南缘-西缘砂岩型铀矿为研究对象,将自动矿物分析系统(AMICS)运用于砂岩型铀基础研究中,结合扫描电镜(SEM)、能谱仪(EDS)鉴定出研究区铀矿物有铀石、晶质铀矿、沥青铀矿和硅钙铀矿,黄铁矿和钛氧化物与铀矿物关系密切,识别出其他伴生矿物还有石英、金红石、长石、云母、高岭石等。本文建立的AMICS-SEM-EDS分析方法,实现了铀矿物及其共生矿物组合的快速识别鉴定和赋存状态研究。 相似文献
9.
大田铀矿床位于扬子陆块西缘康滇地轴中段,矿区内出露地层为古元古界康定群咱里组,矿体主要产于咱里组一、二段碎裂的混合岩化斜长角闪岩、含黑云母混合岩中。混合岩化斜长角闪岩型矿石w(SiO_2)54.50%～57.13%,w(Na_2O)2.07%～5.68%;含黑云母混合岩型矿石w(SiO_2)67.31%～85.24%,w(Na_2O)1.45%～4.31%,显示成矿作用伴有硅质增加和钠质交代作用。矿石稀土元素配分型式为右倾型,δEu为0.17～0.46,指示铀成矿与深熔作用晚期结晶分异形成的熔融体有关。铀矿体的形成和分布受韧—脆性构造系统、钠质交代蚀变、不同岩性结构面、含碳(石墨)层位、变质深熔作用等因素控制。电子探针数据显示,区内铀矿物主要为晶质铀矿(UO_2:81.93%～86.96%)、钛铀矿(UO_2:52.49%～62.41%,TiO_2:16.91%～23.52%),显示高温成矿作用特点。矿床成因类型为与构造有关的变质热液矿床。 相似文献
10.
紫云山岩体是赣中地区与钨铀成矿关系极为密切的过铝质花岗岩体,但目前该岩体的成岩时代尚不明确.通过偏光显微镜、扫描电镜、电子探针等手段,首次开展了紫云山花岗岩中赋存晶质铀矿的精细矿物学研究.结果表明:晶质铀矿主要赋存于黑云母之中,少数被黄铁矿包裹,部分晶质铀矿被不同程度溶蚀和交代,表明晶质铀矿是本区花岗岩型铀矿的主要铀源矿物之一.利用电子探针U-Th-Pb化学定年法测得蕉坑单元 (J3J)5颗晶质铀矿年龄为154.5~168.9 Ma,加权平均年龄为161.8±2.4 Ma (MSWD=0.26,n=26),庙前单元 (J3M) 三颗晶质铀矿年龄为152.8~164.7 Ma,加权平均年龄为159.7±3.2 Ma (MSWD=0.2,n=15).获得的年龄与南岭地区主要含钨花岗岩的侵入时间高度一致,对应华南中生代大规模岩浆活动的第二阶段.晶质铀矿年龄与华南含钨花岗岩锆石U-Pb年龄非常一致,验证了过铝质富铀花岗岩中晶质铀矿电子探针定年方法的可行性. 相似文献
11.
The quartz-pebble conglomerate (QPC)-hosted detrital uranium mineralization is unique in character in terms of their restricted distribution before 2.2 Ga atmosphere during pre-Great Oxidation Event (pre-GOE). Such QPC paleoplacer deposits over the world are good targets for moderate to high tonnage and low grade uranium deposits and more importantly for their gold content. The Mahagiri Quartzite, dated c. 3.02 Ga for their youngest detrital zircon population, is developed unconformably over the Mesoarchean Singhbhum Granite (3.44 Ga to 3.1 Ga). The Mahagiri Quartzite includes a conglomerate-pebbly sandstone dominated subaerial alluvial fan to coastal braided plain sequence in the lower parts and shallow marine mature quartz arenite in the upper parts. The alluvial fan-braided plain deposits in the lower parts host a number of pyritiferous and uraniferous conglomerate and pebbly sandstone beds. The uraninite grains are rounded to subrounded in outline suggesting mechanical transport and detrital origin. Together with detrital pyrite and uraninite constitute the example of > 3.0 Ga paleoplacer closely comparable to the Witwatersrand Au–U deposits. EPMA and SEM-EDS studies suggest that the uraninite grains are rich in Th (> 4 wt.%), S and REE-Y. Chemical formula calculations from EPMA analyses suggest uraninite grains belong to two populations with different oxidation states as revealed from Y/REE and cation U4 +: U6 + [apfu] ratios. The U contents of the detrital uraninite grains from Mahagiri are significantly lower than that of the ideal stoichiometric composition of UO2. This is mainly due to higher amount of heterovalent cationic substitution by Th, REE, Y, Pb, and Ca in Mahagiri QPC uraninite structures, and partial alteration and metamictization of uraninites. Alteration due to metamictization resulted in elevated concentration of Si, Al, P, and Ca in more altered and metamict uraninite grains. The REE pattern is typically flat with comparable LREE–HREE concentration. The high Th content flat REE-pattern suggests that the uraninitere presents high temperature phases (> 350 °C) and are magmatic in origin. The Mahagiri detrital uraninite grains suggest existence of highly felsic and K-rich (richer than TTG) granodiorite–granite–monzogranite suites (GGM) of rocks older than 3.1 Ga in the Singhbhum craton. 相似文献
12.
Uranium minerals from the San Marcos District, Chihuahua, Mexico 总被引：1，自引：0，他引：1
Manuel Reyes-Cortés Luis Fuentes-Cobas Enrique Torres-Moye Hilda Esparza-Ponce María Elena Montero-Cabrera 《Mineralogy and Petrology》2010,99(1-2):121-132
The mineralogy of the two uranium deposits (Victorino and San Marcos I) of Sierra San Marcos, located 30 km northwest of Chihuahua City, Mexico, was studied by optical microscopy, powder X-ray diffraction with Rietveld analysis, scanning electron microscopy with energy dispersive X-ray analysis, inductively coupled plasma spectrometry, and gamma spectrometry. At the San Marcos I deposit, uranophane Ca(UO2)2Si2O7·6(H2O) (the dominant mineral at both deposits) and metatyuyamunite Ca(UO2)(V2O8)·3(H2O) were observed. Uranophane, uraninite (UO2+x), masuyite Pb(UO2)3O3(OH)·3(H2O), and becquerelite Ca(UO2)6O4(OH)6 ·(8H2O) are present at the Victorino deposit. Field observations, coupled with analytical data, suggest the following sequence of mineralization: (1) deposition of uraninite, (2) alteration of uraninite to masuyite, (3) deposition of uranophane, (4) micro-fracturing, (5) calcite deposition in the micro-fractures, and (6) formation of becquerelite. The investigated deposits were formed by high-to low-temperature hydrothermal activity during post-orogenic evolution of Sierra San Marcos. The secondary mineralization occurred through a combination of hydrothermal and supergene alteration events. Becquerelite was formed in situ by reaction of uraninite with geothermal carbonated solutions, which led to almost complete dissolution of the precursor uraninite. The Victorino deposit represents the second known occurrence of becquerelite in Mexico, the other being the uranium deposits at Peña Blanca in Chihuahua State. 相似文献
13.
《International Geology Review》2012,54(12):1907-1910
Deposition of uraninite in microfissures of uranothorianite crystals, in a proluvialdeluvial placer of titaniferous magnetite (with Pb and many other impurities) is understandably the result of self-oxidation of U4+ to U6+ in the mineral (e.g. by radiogenic oxygen produced by decay of thorium in the presence of lead), its outward migration (as by leaching), and then its partial reduction, UO3 to UO2, deposition, and preservation, under certain conditions, as in the Arctic or Japan. 相似文献
14.
CS Suma K Srinivasamoorthy K Saravanan S Gopinath R Prakash A Faizal Khan 《Arabian Journal of Geosciences》2016,9(18):703
An attempt has been made in Chinnar sub basin of Dharmapuri district, South India to isolate the geochemistry of uranium occurrences in groundwater. The geology of the area is mainly of charnockite and granite gneiss. Groundwater samples were collected for two different seasons post and pre monsoon in two different litho units (granite gneiss and charnockite) and analysed for major, minor and uranium concentrations. Higher uranium (18.45 μg L?1) has been recorded during pre monsoon season in granite gneiss with increasing pH. The saturation index calculation for the groundwater isolated minerals like uaraninite, coffinite, haiweeite and soddyite to be precipitating and uranium oxides like UO2.25, UO2.25beta, UO2.33beta as oversaturated. The Eh-pH diagram attempted represents solubility of uraninite within the pH range of 6.0 to 8.0. The study isolate uranium in groundwater of the study area is controlled by the presence of (U4O9) uranium oxide. 相似文献
15.
This study reports major, minor, and trace element data and Sr isotope ratios for 11 uranium ore (uraninite, UO2+x) samples and one processed uranium ore concentrate (UOC) from various U.S. deposits. The uraninite investigated represent ores formed via different modes of mineralization (e.g., high- and low-temperature) and within various geological contexts, which include magmatic pegmatites, metamorphic rocks, sandstone-hosted, and roll front deposits. In situ trace element data obtained by laser ablation-ICP-MS and bulk sample Sr isotopic ratios for uraninite samples investigated here indicate distinct signatures that are highly dependent on the mode of mineralization and host rock geology. Relative to their high-temperature counterparts, low-temperature uranium ores record high U/Th ratios (>1000), low total rare earth element (REE) abundances (<1 wt%), high contents (>300 ppm) of first row transition metals (Sc, Ti, V, Cr, Mn, Co, Ni), and radiogenic 87Sr/86Sr ratios (>0.7200). Comparison of chondrite normalized REE patterns between uraninite and corresponding processed UOC from the same locality indicates identical patterns at different absolute concentrations. This result ultimately confirms the importance of establishing geochemical signatures of raw, uranium ore materials for attribution purposes in the forensic analysis of intercepted nuclear materials. 相似文献
16.
The southeastern part of the Nanling metallogenic province, China is host to numerous granite-hosted vein-type hydrothermal uranium deposits. The geology and geochemistry of these deposits have been extensively studied. However, accurate and precise ages for the uranium mineralization are scarce because the uranium minerals in these deposits are usually fine grained, and may have formed in several stages. Therefore, the ages previously obtained by the bulk dating techniques are possibly a mixed age.The Xianshi uranium deposit, located in the southeastern part of the Guidong granite complex, is a major uranium deposit in South China. The uranium mineralization from this deposit is mainly fine grained uraninite in quartz or calcite veins which are spatially associated with the Cretaceous mantle-derived mafic dykes. Micro-Raman spectroscopy and X-ray diffraction analyses indicate that the dominant uranium mineral occurs as a rare form of uraninite (U3O7). Three distinct generations of uranium minerals have been identified based on petrographic and field relations. Stage 1 uraninite has the lowest UO2 and highest PbO contents whereas Stage 3 uraninite has the highest UO2 and lowest PbO contents.Uraninite from the Xianshi deposit was dated using an in-situ SIMS U–Pb dating technique. The results show three distinct age groups: 135 ± 4 Ma, 113 ± 2 Ma and 104 ± 2 Ma, which are in excellent agreement with the ages of three episodes of mantle-derived mafic dykes. Therefore, the Xianshi uranium deposit has experienced at least three hydrothermal events that are responsible for the deposition of uranium ores, which are genetically related to the emplacement of three sets of mafic dykes. 相似文献
17.
In the present work, a combination of various techniques is utilized for the study of nano-mineralogy and -geochemistry of high-grade karst-type bauxite (Al-rich and Fe-depleted samples; Al2O3 ca. 80 wt.%) from the Parnassos-Ghiona mines located in Greece. Initial characterization using PXRD and electron microscopy in microscale and mesoscale (SEM-EDS including STEM mode), proved the presence of “Fe-Cr-Ti-containing diaspore”, anatase and minor rutile. The study by means of 57Fe Mössbauer spectroscopy, in correlation with magnetic susceptibility measurements and, complemented, with Synchrotron-based spectroscopies at the microscale (SR micro-XRF and micro-XANES/-EXAFS), indicated that Fe3+, in contrast to [6]Cr3+, is not exclusively a component of the diaspore structure. While Cr3 + substitutes Al3 + in octahedral sites of diaspore ([6]Cr3+ ↔ [6]Al3+), the electron microscopy in nanoscale (TEM-EDS & EELS) revealed that Fe exists in the form of peculiar Fe3+-bearing nanominerals (most likely maghemite-type phases) between 25 and 45 nm in size, in addition to the Fe3+ ions substituting Al3+ in the diaspore structure. Moreover, it was proven that TiO2 polymorph mineral nanoparticles, particularly rounded anatase mesocrystals and nanocrystals and individual needle-shaped rutiles, are dispersed into the diaspore matrix. Thus, diaspore in the studied bauxite concerns -in fact- a distinct Fe3+-Cr3+-AlOOH low-T authigenic phase, demonstrated for the first time in literature. On the other hand, the observed TiO2 mineral nanoparticles (formed, together with diaspore, during diagenesis) and Fe nanominerals (formed during epigenesis) were hitherto unknown not only for the allochthonous karst-type bauxite deposits of Greece, but also for the overall bauxite deposits, worldwide. 相似文献
18.
Artur P. Deditius 《Geochimica et cosmochimica acta》2007,71(20):4954-4973
Alteration of uraninite from a hydrothermal vein-type U-deposit in Marshall Pass, Colorado, has been examined by electron microprobe analysis in order to investigate the release and migration of trace elements W, As, Mo, Zr, Pb, Ba, Ce, Y, Ca, Ti, P, Th, Fe, Si, Al, during alteration, under both reducing and oxidizing conditions. The release of trace elements from uraninite is used to establish constraints on the release of fission product elements from the UO2 in spent nuclear fuels. Uraninite occurs with two different textures: (1) colloform uraninite and (2) fine-grained uraninite. The colloform uraninite contains 1.04-1.75 wt% of WO3, 0.16-1.70 wt% of As2O3, 0.06-0.88 wt% of MoO3; whereas, the fine-grained uraninite retains 2.25-4.93 wt% of WO3, up to 5.76 wt% of MoO3, and 0.26-0.60 wt% of As2O3. The near constant concentration of incompatible W in the colloform uraninite suggests W-incorporation into the uraninite structure or homogeneous distribution of W-rich nano-domains. Incorporation of W and Mo into the uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of Cs during the corrosion of spent nuclear fuel, as there is a strong affinity of Cs with W and Mo. Zoning in the colloform texture is attributed to variation in the amount of impurities in uraninite. For unaltered zones, the calculated amount of oxygen ranges from 2.08 to 2.32 [apfu, (atom per formula unit)] and defines the stoichiometry as UO2+x and U4O9; whereas, for the altered zones of the colloform texture, the oxygen content is 2.37-2.48 [apfu], which is probably due to the inclusion of secondary uranyl phases, mainly schoepite. The supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of uraninite. Four stages of colloform uraninite alteration are proposed: (i) formation of an oxidized layer at the rim, (ii) corrosion of the oxidized layer, (iii) precipitation of U6+-phases with well-defined cleavage, and (iv) fracture of the uraninite surface along the cleavage planes of the U6+-phases. 相似文献
19.
Kai-Uwe Ulrich Eugene S. Ilton Jonathan O. Sharp Rizlan Bernier-Latmani John R. Bargar 《Geochimica et cosmochimica acta》2009,73(20):6065-7290
The long-term stability of biogenic uraninite with respect to oxidative dissolution is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO2.00 with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both materials exhibited a similar intrinsic solubility of ∼10−8 M under reducing conditions. The two materials had comparable dissolution rates under anoxic as well as oxidizing conditions, consistent with structural bulk homology of biogenic and stoichiometric uraninite. Carbonate reversibly promoted uraninite dissolution under both moderately oxidizing and reducing conditions, and the biogenic material yielded higher surface area-normalized dissolution rates than the chemogenic. This difference is in accordance with the higher proportion of U(V) detected on the biogenic uraninite surface by means of X-ray photoelectron spectroscopy. Reasonable sources of a stable U(V)-bearing intermediate phase are discussed. The observed increase of the dissolution rates can be explained by carbonate complexation of U(V) facilitating the detachment of U(V) from the uraninite surface. The fraction of surface-associated U(VI) increased with dissolved oxygen concentration. Simultaneously, X-ray absorption spectra showed conversion of the bulk from UO2.0 to UO2+x. In equilibrium with air, combined spectroscopic results support the formation of a near-surface layer of approximate composition UO2.25 (U4O9) coated by an outer layer of U(VI). This result is in accordance with flow-through dissolution experiments that indicate control of the dissolution rate of surface-oxidized uraninite by the solubility of metaschoepite under the tested conditions. Although U(V) has been observed in electrochemical studies on the dissolution of spent nuclear fuel, this is the first investigation that demonstrates the formation of a stable U(V) intermediate phase on the surface of submicron-sized uraninite particles suspended in aqueous solutions. 相似文献
20.
N. I. Kovalenko B. N. Ryzhenko N. I. Prisyagina Ya. V. Bychkova 《Geochemistry International》2012,50(1):18-25
Uraninite solubility in HF solutions (0.0001–0.5 m) was experimentally studied at 500°C, 1000 bar, and hydrogen fugacity corresponding to the Ni/NiO buffer. It was shown that
the predominant U(IV) species in aqueous solution are U(OH)40, U(OH)3F0, and U(OH)2 F20. Using the results of uraninite solubility measurement, the Gibbs free energies of the uranium (IV) species were calculated
at 500°C and 1000 bar (kJ/mol): −986.55 for UO2(aq), −1712.42 for U(OH)3F0, −1755.53 for U(OH)2F20, and the equilibrium constants of the uraninite solubility in water and HF solutions were estimated: UO2(κ) = UO2(aq), which is similar to UO2(cr) + 2H2O = U(OH)40, pK0 = 6.64; UO2(cr) + HF0 + H2O = U(OH)3F0, K1 = 0.0513; UO2(cr) + 2HF0 = U(OH)2F20K2 = 7.00 × 10−4. Approximate values K3 = 5.75 × 10−3 and K4 = 6.7 × 10−2 were obtained for equilibria UO2(cr) + 4HF0 =UF40 + 2H2O and UO2(cr) + 4HF = UF40 + 2H2O. Maximum observed in the uranium concentration curve as a function of HF concentration can be explained by the decrease
(to < 1) of activity coefficient ratio of HF0 to U(OH)3F0 with increasing HF concentrations. 相似文献