In a broad project designed to examine uranium transport by surface water from Sierra Peña Blanca to Laguna del Cuervo in the Chihuahuan Desert, sediments from intermittent streams and the lagoon have been extracted and studied. Two samples were sediments from the high area of the Sierra, close to the uranium deposit “El Nopal.” Moreover, 23 core segments extracted for dating sediments were analyzed to consider changes in the fine component concentrations. The techniques of scanning electron microscopy–energy dispersive X-ray spectroscopy, XRD in a conventional diffractometer, and high-resolution synchrotron XRD analysis were applied. The crystallographic objective of the present work was to evaluate the functionality of various methodologies when applied to cases of a detailed analysis of many polyphase samples with cryptocrystals. The methods for processing the experimental data were the Rietveld method in the current multi-pattern variant of the Fullprof program and the degree of crystallinity method for the rapid estimation of the proportion of cryptocrystals in a mixture. This last technique was developed with an ad hoc software package deposited in the GitLab public repository.

Some of the largest Mexican uranium (U) deposits are located in Chihuahua. The most important is in Sierra Peña Blanca, northwest of the capital, which was explored and partially exploited in the 1980s. After the closure of activities, the mining projects were left exposed to weathering. To characterize the spread of U minerals towards the neighboring Laguna del Cuervo, sediment samples were collected in the main streams of the drainage pattern of the largest deposits. The U mineral fragments from the fine sand portion were extracted using fluorescence light at 365 nm. The morphology and elemental composition of these particles were analyzed by focused ion beam microscopy (FIB) and scanning transmission electron microscopy (STEM). The particle density in samples close to the U sources was quantified using gamma spectrometry. The highest density was 2500 part./g, and the lowest was 124 part./g. X-ray absorption spectroscopy (XAS) allowed us to establish via XANES the speciation of U in the U particles, confirming the U(VI) oxidation state, while the exploitation of the EXAFS spectrum put in evidence of the presence of uranophane. Finally, the Fe, Sr, and U distributions in the particle and its matrix were obtained via X-ray fluorescence microtomography (XRF-µCT). It was concluded that the particle is composed of uranophane, imbricated with quartz and other oxides.

The Sierra Peña Blanca (SPB) region in Chihuahua, Mexico contains a significant uranium deposit representing about 40% of the country’s reserves. Common uranium minerals in this area include uranophane, schoepite, and weeksite/boltwoodite, with several superficial occurrences. Mining activities in the 1980s left unprocessed uranium ore exposed to weathering, with potential transport towards Laguna del Cuervo. This study presents an experimental simulation of uranium transport in SPB sediments using three approaches: (i) a batch experiment to evaluate the ideal adsorption of (UO2)2+ by fine sediment; (ii) a column system fed with 569 mgU L−1 UO2(NO3)2 to simulate adsorption by different sediment particle sizes; (iii) a column system with an upper horizon of uranophane from the area, fed with deionized water, to simulate uranium weathering and transport in particulate material, determined by liquid scintillation counting, revealed that the clay fraction had the highest adsorption capacity for U. X-ray Absorption Fine Structure (XAFS) analysis at the U L3 edge confirmed the U(IV) oxidation state and the fittings of the extended XAFS spectra confirmed the presence of the uranophane group of minerals. X-ray tomography further corroborated the distribution of particulate minerals along the column. The results suggest that the primary transport mechanism in SPB involves the fragmentation of uranium minerals, accompanied by eventual dissolution and subsequent adsorption of U onto sediments.

La margaritasita [Cs2(UO2)2(VO4)2·nH2O] es un uranovanadato descrito hasta el día de hoy únicamente en la Sierra Peña Blanca, Chihuahua. En este trabajo se comparten los resultados obtenidos de la caracterización de margaritasita, empleando las técnicas de difracción de rayos X de alta resolución (DRX-AR) y espectroscopía de estructura fina por la absorción de rayos X (EXAFS), las cuales utilizan luz sincrotrón. Para este trabajo se realizó la caracterización en el sincrotrón de Diamond Light Source estación B18 (en Reino Unido) y en el difractómetro de alta precisión de Elettra en Triestre (Italia). La margaritasita, con hábito tabular micrométrico entre 1 y 3 μm, presenta las fases margaritasita pura y carnotita en solución sólida desordenada con una relación de potasio (K) y cesio (Cs) de 1:1, acompañadas de cuarzo, montmorillonita, feldespato potásico y calcita. Mediante estructura de los rayos X cerca del borde de absorción (por sus siglas en inglés XANES) y EXAFS se comprobó la asociación de vanadio y cesio con uranio en la fase margaritasita. En el estudio se caracterizó la estructura del prisma pentagonal uranilo determinando las distancias de unión entre el uranio y el oxígeno U-O, característica de los uranovanadatos. DOI: https://doi.org/10.54167/tch.v19i1.1675

The state of Chihuahua, Mexico, has several uranium deposits. The most important is the Peña Blanca deposit at north of the Chihuahua City. After an intensive exploration in the 80’s, the mineral extracted and unprocessed was confined in rock stacks exposed to weathering. To characterize the transport of uranium by surface water in the form of particles or dissolved, a sampling was carried out in the bed of the stream “El Tigre”. Nine sediment samples were collected near the Nopal 1 mine, and separated into coarse sand, fine sand, silt, and fine silt + clay fractions. The activity concentration of the uranium series isotopes was determined by gamma spectrometry, applied to the different fractions of each sample. The fine silt + clay and fine sand fractions were analyzed by X-ray diffraction in the 2 most active samples, Nopal 1 and Nopal 2. The phases are quartz, calcite, montmorillonite, sanidine, orthoclase, albite, kaolinite and magnetite. Particles of d < 100 μm (P1-Bulk) were extracted from the fine sand fraction of Nopal 1 with the help of ultraviolet light fluorescence; uranium and silicates were confirmed in them by SEM-EDS. P1-Bulk sample was subjected to X-ray absorption spectroscopy measurements on the U L3-edge, as well as to the samples fine silt + clay fraction of the Nopal 1 (P1-Clay) and Nopal 2 (P1-Silt). The X-ray absorption near edge (XANES) region analyzed indicates the presence of hexavalent uranium in the three samples. The fit of the Extended X-ray absorption fine structure (EXAFS) spectrum of the P1-Bulk sample indicates that it is α-uranophane. This result is consistent with the reports of the area.

Peña Blanca, located 50 km to the north of Chihuahua's city, hosts about 70% of the natural U deposits in Mexico, which contains the uranophane mineral. Uranium could be present in the environmental matrices of rock and soil; it could be redistributed naturally or anthropogenically in the environment. The adverse health effects of uranium are mainly ascribed to its chemistry, causing damage to the kidneys. The main goal of this work is to model the transport of U minerals by leaching at Peña Blanca-Laguna del Cuervo area using sediment columns. XDR, SEM, Alpha Spectrometry, and X-Ray Absorption Fine Structure techniques were applied to identify favored processes. It was possible to correctly reproduce the granulometric sequence of the topography from the Peña Blanca's deposits to the alluvial fans in Laguna del Cuervo. The mineralogical composition of the sediments corresponds to the erosion of felsic volcanic rocks. Activity concentrations of the effluent solutions and the fine fractions of the sediments could be explained from the properties of the grain and its distribution. Through X-Ray Absorption Near Edge Structure (XANES) study of silt and (fine silt + clay) sediment fractions, the U(VI) oxidation state was verified, referable to the adsorption of uranyl ions.

To study uranium transport by surface water from Sierra Peña Blanca to Laguna del Cuervo in the Chihuahuan Desert, sediments from intermittent streams and the lagoon have been extracted and studied. Two samples are sediments from the high area of ​​the Sierra, close to the uranium deposit “El Nopal.” Moreover, 23 segments of a sedimentary core were analyzed to study changes in the fine component concentrations. The techniques of scanning electron microscopy-energy dispersive X-ray spectroscopy, XRD in a conventional diffractometer, and high-resolution synchrotron XRD analysis were applied. The crystallographic objective has been to evaluate the functionality of various methodologies when applied to cases of detailed analysis of many polyphase samples with cryptocrystals. The methods for processing the experimental data have been the Rietveld method, in the current multi-pattern variant of the Fullprof program, and the Degree of Crystallinity method for rapid estimation of the proportion of cryptocrystals in a mixture. This last technique has been developed with an Ad-Hoc software package, which has been deposited in the public repository GitLab.

Margaritasite is a mineral compound discovered in the early 1980s in Chihuahua, Mexico. It is a natural cesium uranyl vanadate found only, so far, in the Margaritas mine of the Peña Blanca highlands. In this work, a thorough characterization of the aforementioned mineral is presented. The portfolio of the techniques employed includes high-resolution X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, transmission electron microscopy in selected area electron diffraction (SAED) mode, and X-ray absorption spectroscopy (XAS). After extensive data analysis and modeling, new information on the mineral has been retrieved. Its phase composition is margaritasite–carnotite: a solid solution of cesium and potassium uranyl vanadate [(Cs,K)2(UO2)2(VO4)2·nH2O], and margaritasite, which is practically pure cesium uranyl vanadate [Cs2(UO2)2(VO4)2·nH2O]. The crystal structure of both components presents the space group P 1 21/c 1. Yet, each phase has similar, but appreciably different, lattice parameters. The mineral has a lamellar tabular and prismatic morphology. SAED patterns confirm the crystal structure of margaritasite. XAS spectra of Cs, V, and U confirm the elemental composition, oxidation states, and interatomic distances of this structure. These findings are consistent with the presence of cesium in this unique mineral from the paragenesis point of view.

The multiple minima hypersurface (MMH) approach is a reliable theoretical model for characterizing uranylwater complexes in aqueous media. It provides insights into probable conformations, solubility, and reactivity through accurate exploration of the local energy landscape via quantum calculations. The approach predicts equilibria and populations of local minima, identifying pre-reactive sites and hydrolyzed ligands. By applying MMH to a wider range of molecular species, a robust theoretical framework is established for understanding the complex chemistry of environmental and nuclear materials. The approach here aims to predict the behavior of weathered uranium mining waste and enhance the study of actinide compounds.

Naica’s ”Cueva de los Cristales” was discovered in 2000. It has been considered particularly interesting for its beauty and the challenges it poses to crystallography. This article focuses on the study of the wall-selenite interface by various techniques, particularly X-ray diffraction (XRD), scanning electron microscopy (SEM), with emphasis on micro-X-ray fluorescence (micro-XRF) and micro-X-ray absorption near edge structure (micro-XANES). The main phases calcite, quartz, goethite and montmorillonite were identified by XRD, as well as the association of crystalline and amorphous minor and trace phases of Zn, Mn, Cu, As and Pb. The latter were identified in micro-XRF maps and micro-XANES spectra. The results for the morphology and the chemical description of the crystal-wall interface may contribute to propose a nucleation and growth mechanism for Naica megacrystals.

The water management initiatives in freshwater systems focus on water availability to preserve this resource for human uses and the health of aquatic ecosystems. This work presents an assessment of the potential pollution risk caused by the metal availability in suspended sediments. The objective of this study was to determine the partitioning, association, and geochemical fractionation of metals in suspended sediments from a surface water body. Additionally, the environmental assessment for this reservoir was estimated using geoaccumulation, enrichment, and pollution indices of metals and the related potential risk by their elemental availability (RAC). Chemical, mineralogical, and morphological characterizations were obtained by inductively coupled plasma spectrometry, alpha spectroscopy, X-ray crystallography, and scanning electron microscopy. Clay, quartz, montmorillonite, and calcite were the main minerals of suspended sediments. Chemical fractionation was the parameter affecting the concentrations of metals in suspended sediments. The sediment composition is of natural origin; however, these finer particles can promote the scavenging of toxic metals. It contributes to obtaining moderate to high levels for enrichment/contamination indices. Although Ca, Mg, and U were the most accessible metals for aquatic biota, metals such as Sr, Mn, Li, Cu, and Ni in the exchangeable phase of suspended sediments are the potentially toxic elements in this aquatic ecosystem.

In Chihuahua, an important source of environmental radioactivity is found in the Sierra Peña Blanca, in the center of the state. The site comprises about 70% of uranium reserves in Mexico. The uranium of Peña Blanca was explored and partially exploited in the '80s. Due to the closure of operations, the extracted and unprocessed ore (hundreds of tons) was confined to rocky stacks, exposed to weathering. Subject to leaching, this uranium is transported from the mountains to Laguna del Cuervo. The mineral exposed in the repository and the uranium transport by surface water and recent sediments must be studied, to assess the effects on the environment, with radiometric and materials science techniques in conventional laboratories and synchrotron light. This work presents the study of sediment and pore water samples at various points along the lagoon, and the values of the activity ratio of the 234U/238U isotopes and the sediment-water distribution coefficient of these isotopes, obtained by applying uranium liquid scintillation alpha spectrometry, gamma-ray spectrometry, scanning electron microscopy and X-ray diffraction methods.

This work presents the results of a simulation by the Monte Carlo method, performed through the GEANT4 code, of the irradiation and energy deposition by high flux X-rays on the piezoelectric ceramic Bismuth-Sodium Titanate doped with Barium. X-rays energies were around the Ti-K absorption edge emulating a transmission experiment, and the irradiation with 5300 eV X-rays for a fluorescence experiment. The method consists of introducing the data that describes a characteristic R3c structure of the polarized ceramic, reported in the literature. The absorption coefficients for the energies of interest, as well as the energy deposited in the form of radiation doses, are calculated. Intensity changes for specific energy lines in the micro-fluorescence spectra, which suggest the presence of vacancies in the crystal structure, are verified via simulation. The vacancy density produced by a typical photon flux of a fourth-generation synchrotron beam is calculated through the threshold displacement energy for vacancy production. Consequently, the simulation is carried out for a structure with appropriate Bi and O vacancies, and the ability to detect the radiation damage is verified by comparison with micro-XRF and XAFS experimental results. The simulation predicts a maximum dose of 1.21-1.27x105 Gy irradiating 107 photons for the given energy and a maximum vacancy density of 1.10x108 / micron3 for oxygen atoms and 6.90x107 /micron3 for bismuth atoms.

High-resolution X-ray diffraction is applied to investigate macro and micro crystal structure changes across different synthesis routes in BNBT4 piezoceramics. Synthetic powders were obtained by conventional sol-gel auto combustion and mixed oxides methods; then samples were sintered by combining hot uniaxial pressing and subsequent recrystallization. The crystal structure of the ceramic obtained by sol-gel is rhombohedral (R3c) and the one obtained from mixed oxides is monoclinic (Cc). In the sol-gel case, a poling process produces an intense 001 texture parallel to the polarizing electric field. Poling the mixed oxides ceramics quickly transforms the structure from monoclinic to rhombohedral while texture and anisotropic micro-strains develop further as the poling increases. A correlation between detected structural transformations and variations in selected physical properties is suggested. The anisotropy of the critical temperatures for dielectric and electromechanical depolarization is lower for mixed oxides ceramics than for sol-gel ceramics.

High-resolution X-ray diffraction is applied to investigate macro and micro crystal structure changes across different synthesis routes in BNBT4 piezoceramics. Synthetic powders were obtained by conventional sol-gel auto combustion and mixed oxides methods; then samples were sintered by combining hot uniaxial pressing and subsequent recrystallization. The crystal structure of the ceramic obtained by sol-gel is rhombohedral (R3c) and the one obtained from mixed oxides is monoclinic (Cc). In the sol-gel case, a poling process produces an intense 001 texture parallel to the polarizing electric field. Poling the mixed oxides ceramics quickly transforms the structure from monoclinic to rhombohedral while texture and anisotropic micro-strains develop further as the poling increases. A correlation between detected structural transformations and variations in selected physical properties is suggested. The anisotropy of the critical temperatures for dielectric and electromechanical depolarization is lower for mixed oxides ceramics than for sol-gel ceramics.

Calcium sulfate (CaSO4) is one of the most common evaporites found in the earth’s crust. It can be found as four main variations: gypsum (CaSO4∙2H2O), bassanite (CaSO4∙0.5H2O), soluble anhydrite, and insoluble anhydrite (CaSO4), being the key difference the hydration state of the sulfate mineral. Naica giant crystals’ growth starts from a supersaturated solution in a delicate thermodynamic balance close to equilibrium, where gypsum can form nanocrystals able to grow up to 11–12 m long. The growth rates are reported to be as slow as (1.4 ± 0.2) × 10−5 nm/s, taking thousands of years to form crystals with a unique smoothness and diaphaneity, which may or may not include solid or liquid inclusions. Conservation efforts can be traced back to other gypsum structures found prior to Naica’s. Furthermore, in the last two decades, several authors have explored the unique requirements in which these crystals grow, the characterization of their environment and microclimatic conditions, and the prediction of deterioration scenarios. We present a state-of-the-art review on the mentioned topics. Beyond the findings on the origin, in this work we present the current state and the foreseeable future of these astounding crystals.

Naica's "Cueva de los Cristales" (Cave of the Crystals) has been compared to the most beautiful places of worship for the incredible display of columns that populate its vault. Said columns are giant gypsum crystals that have already been the subject of extensive studies. This paper contributes to these studies by focusing on the mineral aggregates found at the wall-selenite interface. A detailed chemical and structural characterization of representative samples has been performed using chemical, mineralogical, elemental, and phase analysis methods, with an emphasis on synchrotron micro-spectroscopic techniques. The following main phases were identified: calcite, silica, goethite, and several Pb-, Mn-, Cu-, and Zn-based aggregates. The role of the mineral aggregates, from their potential incorporation at the very beginning of the formation to the final steps of the crystals' growth, is investigated. Particular attention is paid to their shapes and composition. The data obtained on the morphology of the wallcrystal interface and related phase composition, together with classical nucleation formalisms based on the slightly supersaturated solution, suggest that the nanocrystalline monomers formed in solution undergo adsorption on the wall, which ultimately promotes mega crystal growth.

Uranium is a radioactive element with a special presence in the rocks, waters, sediments, soils, and plants at the state of Chihuahua. The activity ratio of 238U/234U is used to explain the uranium transport by surface water and its deposition in arid environments. In this work, the activity concentration of natural U isotopes is determined by PERALS liquid scintillation and high-resolution alpha spectrometry (AS, Canberra camera 7401VR) in the Environmental Radiological Surveillance Laboratory (LVRA) at CIMAV, Chihuahua. Uranium is extracted from soils through the scintillating liquid extractor URAEX for PERALS, with chemical recovery (CR) of 80 - 85 %, and through the ion exchange resin UTEVA + electrode position by the Hallstadius method, with CR of 85 - 90 %, for AS. The procedures of 234U and 238U activity concentration (AC) determination in soils were validated by their application to the certified reference material IAEA-375. The resulting values were in the reference range of the certified or informative values. Both procedures were applied to 6 representative soil samples, with AC of the same order of magnitude or greater, and similar CR and compatible results. Both procedures are satisfactory for the purposes of LVRA research and in general.

In arid zones, the principal water supply is from groundwater, which can present high concentration of salts, heavy metals, and radioactive elements. The aim of the study was to determine isotopic uranium concentration in groundwater samples with high concentration of salts and its association with other chemical species. Samples were taken from wells with high salt content. The 238,234U radioisotope concentrations were determined by liquid scintillation and alpha-particle spectrometry. In addition, the physical-chemical parameters were recorded in situ; whereas the dissolved ions and elemental composition were measured by UV-Vis and X-ray fluorescence spectrophotometry, respectively. To obtain isotopic uranium concentrations, three radiochemistry procedures were carried out. An ANOVA test was performed to compare the results from procedures, as well as an analysis of Pearson correlation was used between parameters to obtain their associations. Statistically, the U isotopic concentrations did not show differences (p-value 0.82) between procedures. 238U and 234U showed mean concentrations of 6.7 mBq mL-1 and 16.6 mBq mL-1, respectively, with an Activity Ratioby up 7.2. The groundwater under study showed high concentration of TDS, calcium, sulphate, chloride, nitrate, and nitrite. Isotopic U concentrations tend to increase with NO3>Zn>Cl>Br>SO4>Cu>T>SDT>P; meanwhile their contents decrease with T>Cl->NO2>Fe. These findings help us to understand the uranium behavior in groundwater with high salt contents as well as the influence of agricultural supplies on chemical species presents in groundwater.

A computer-aided methodology for the approximate prediction of axially textured polycrystals' properties is presented. The input data for the developed application consist of: (a) the two-dimensional diffraction pattern of the material under investigation and (b) the tensors of the elasto-piezo-dielectric properties of the single crystal case. Program ANAELU 2.0 allows the determination of the fiber axis inverse pole figure by means of a Rietveld-type procedure. The Material Properties Open Database (MPOD, http://mpod.cimav.edu.mx) provides free access to the experimentally determined values of the tensor properties for several crystal species. Practical estimates of polycrystals' properties may be obtained by averaging single crystals' properties tensors, with the orientation distribution function (the symmetry-axis inverse pole figure, in fiber textures) as a weight factor. This treatment, with the application of the Voigt, Reuss and Hill approaches, requires special precautions when it comes to the coupling properties (e.g. piezoelectricity, magnetostriction, magnetoelectricity). Some key physical, mathematical and computational aspects related to the considered topic are discussed. Program GISELLE systematizes the calculation of polycrystal properties under the considered treatments. The application of the proposed methodology to real-world events is illustrated by means of a case study. (C) 2019 SECV. Published by Elsevier Espana, S.L.U.