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Inefficient flotation of bastnaesite remains a challenge in the production of rare earth elements. This study aimed to investigate the dissolution and adsorption behaviour of species that are commonly released into bastnaesite flotation pulp from Ca/Ba-bearing gangue minerals. The influence and corresponding mechanisms on the bastnaesite mineral surface and collectors, namely sodium oleate (NaOL), were evaluated experimentally based on micro-flotation, zeta potentials, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and X-Ray photoelectron spectroscopy (XPS) analyses. The flotation recovery of bastnaesite significantly decreased from ∼95% to ∼25%, ∼15%, ∼80%, ∼25% when exposed to calcite, fluorite, barite, and mixed dissolved species, respectively. The zeta potential of bastnaesite was pH sensitive, indicating that H+ and OH− determine the surface potential of bastnaesite. Solution chemistry analyses revealed that the presence of the dissolved species differed at various pH values. In situ ATR-FTIR demonstrated the different effects of the dissolved species from calcite, fluorite, and barite on collector adsorption. The former two dissolved species mainly depressed the chemisorption of the NaOL monomers (RCOO‒), whereas calcite also affected the physical adsorption of the oleic acid molecular dimer (RCOOH·RCOO‒). Moreover, the barite dissolved species only affected the physical adsorption of the NaOL species. The results of XPS analysis revealed that dissolved species from these three gangues could pre-adsorbed onto bastnaesite and affected the interaction with the collector. Density functional theory calculations were employed to provide further theoretical insights into the interactions between the dissolved species from calcite, fluorite, and barite and NaOL.

The mechanism of phthalic acid, a dicarboxylic acid collector, in flotation separation of fluorite and rare earth (RE) was studied in this paper. The experimental data of flotation show that phthalic acid, as the collector, can realize highly efficient separation of fluorite and rare earth under weakly acidic conditions. The adsorption mechanism of phthalic acid on the surface of fluorite and bastnaesite was analyzed in this paper by means of the zeta potential measurement, the Fourier transform infrared (FT-IR), the X-ray photoelectron spectroscopy (XPS) and the stability constant measurement of active metal ion and phthalic acid coordination complex. According to the zeta potential testing results, the surfaces of fluorite adsorb the collector phthalate ion with negative charge under weakly acidic conditions which, in turn, increases its electronegativity and results in the motion of its potential. After the reaction between phthalic acid and fluorite ores under weakly acidic conditions, the peak of the fluorite ores is found to have significant changes in the FT-IR results, indicating strong chemical adsorption on the surfaces of phthalic acid and fluorite ores. According to the XPS analysis, the peak of benzene ring of phthalic acid is as high as 2% on the surface of fluorite, while no obvious characteristic peak of benzene ring is found on the surface of bastnaesite. According to the pH potentiometric titration results, the stability constant Ktotal of calcium phthalate complex within the acid range is higher than the stability constant Ktotal' of cerium phthalate complex, indicating that the complex generated between phthalic acid and Ca2+ is more stable than the complex generated between phthalic acid and Ce3+. The possible reason is that Ca2+, with the highest reticular density, plays a prevailing role in the octahedron structure of fluorite amidst the acidic media. As the active point of flotation, Ca2+ works with the carboxyl groups of the collector phthalic acid (–C=O–) to form polycyclic calcium phthalate complex.

Typical high-entropy fluorite oxides with five metal cations have exhibited improved thermomechanical properties. In this work, we have successfully prepared single-phase high-entropy (Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O2-δ fluorite oxides through the combination of sol-gel method and calcination, using metal nitrate/chloride, citric acid and absolute ethanol as the primary raw materials. A series of characterizations were employed for chemical, morphological and structural analysis of the calcined products. The five cationic elements were shown to be uniformly distributed in the nearly pure fluorite oxide without apparent segregation at the micro/nano-meter scales after 1500 °C calcination. Annealing and water quenching within the test temperature range did not witness any appreciable phase transition. It is revealed that the concentration of Ce3+ in Ce ions reaches 33.84% and δ is equal to 0.234, yielding a nominal composition of (Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O1.766. Cation and anion configurational entropy were found to be 1.609 R and 0.722 R, respectively, where R is the ideal gas constant, for a sum of the total configurational entropy of 2.331 R in the high-entropy system. Line defects inside the crystals and 3 types of crystalline boundary structures have been observed and identified among the mostly atomically sharp grain boundaries.

The paper provides the results of experiments on the hydrometallurgical processing of finely dispersed technogenic raw materials for primary aluminum production in Soderberg cells (case study of the Irkutsk Aluminum Smelter) – aged sludge. The components of this sludge are dust from electrostatic precipitators (79.7 %), wet gas cleaning sludge (4.4 %) and coal froth flotation tailings (15.8 %). According to the grain-size analysis carried out, aged sludge sample particles have a size of –50 μm. According to the chemical composition analysis of the sludge sample, main components in it are carbon, cryolite, chiolite with a small amount of other compounds (corundum, ralstonite, spodumene, fluorite). Fluorine leaching experiments were carried out with a 2 % sodium hydroxide solution at a stirrer speed of ~1020 rpm. Using the mathematical planning of a three-factor experiment, it was found that the maximum concentration of fluorine in the solution (15.844 g/dm3) is achieved with the following optimal parameters of fluorine alkaline leaching: temperature of 90 °C, liquid-to-solid ratio of 9 : 1, and time of 90 min. The multidimensional polynomial equation was obtained for fluorine alkaline leaching from aged sludge. Cryolite was obtained from fluorine-containing solutions (by the reaction of sodium fluoride interaction with sodium bicarbonate and an aluminate solution), which was confirmed by X-ray phase analysis data.

Interactions of cadmium fluoride with other metal fluorides are studied. Two compound formation areas can be distinguished in the cation cumulative moment versus electronegativity diagram. Cadmium fluoride is an amphoteric compound in terms of the Lewis theory of acids and bases. Solid solutions (ss) with the fluorite, tysonite (in particular, berthollide phases in CdF2–RF3, R = Tb–Tm, Y, systems), and α-YF3 structures are formed in cadmium fluoride systems with rare-earth fluorides. It might be expected that cadmium fluoride would form compounds with francium fluoride, radium fluoride, fluorides of tervalent (Al, Fe, and Ni), tetravalent (Hf, Si, Ge, Re, and Ir) and pentavalent (Ta, Mo, and V) elements, and with uranium hexafluoride.

Tailoring electrical and mechanical properties in the fluorite oxides family is of great interest for technological applications. Other than doping and substitution, entropy-driven stabilization is an emerging technique for new solid solutions formation and enhancing or exploring new functionalities. However, there is a high number of possible combinations for higher-order diagram investigations, and the current state of the art shows limited possibilities in predicting phase formation and related properties. In this paper, we expand the compositional space of fluorite oxides in ZrO2-HfO2-CeO2-Nb2O5-RE2O3 systems. X-ray diffractometry and scanning electron microscopy measurements showed the formation of cubic fluorite-type structures when processing compositions at 1600 °C.

In this paper, we explored the possibility of forming single-phase high-entropy fluorite oxides (HEFOs) in the (Ce0.2Zr0.2Ti0.2Sn0.2M0.2)O2-δ (M = Mg, Y, Yb, and Ca) system. The result showed that single-phase high-entropy fluorite can only be obtained from (Ce0.2Zr0.2Ti0.2Sn0.2Ca0.2)O2-δ. Compared with other reported anion-deficient HEFOs, this material has the lowest rare earth element content, the smallest average cationic radius and the lowest theoretical density. The effects of cation radius and starting materials on the formation of single-phase HEFOs are discussed.

A2B2O7 compounds have unique properties, such as good thermal stability, oxygen mobility, ionic conductivity, etc. Such compounds with tailored compositions exhibit good catalytic properties in a variety of high-temperature reactions, such as reform and oxidative coupling of methane (OCM). An effective catalyst for OCM must have electrophilic oxygen mobility and alkalinity on its surface. A typical A2B2O7 compound that contains a trivalent rare earth metal at site A, such as La, can provide the necessary surface alkalinity for a good OCM catalyst. Previous studies show that La2Ce2O7 (LCO) catalysts perform well in OCM due to the selective mobile oxygen sites, suitable alkaline sites, and high thermal stability. Doping LCO with Ca increases the alkalinity, which can considerably increase selectivity for ethylene in the OCM reaction. We have studied calcium-doped lanthanum ceria materials as a catalytic layer in solid oxide fuel cells reactors for methane conversion to ethylene. We have explored the combustion synthesis to obtain homogeneous La2-xCaxCe2O7 powders with x = 0, 0.25, and 0.50 (LCO, LCa25, and LCa50, respectively). X-ray diffraction (XRD) showed that the calcium addition in LCO formed a single-phase solid solution. The crystalline structure of the synthesized materials is the disordered fluorite-type, as indicated by both XRD and Raman analysis. Raman spectroscopy data evidenced the presence of surface oxygen vacancies on all materials, which may benefit the OCM reaction. SEM images of as-prepared powders obtained evidenced a similar microstructure composed of porous sponge-like agglomerated particles with irregular shape, expected from the combustion synthesis (Fig. 1). Ca-doping changed the sintering behavior by reducing the onset of shrinkage during the sintering process. Impedance spectroscopy data showed increased ionic conductivity with increasing Ca-doping. Such an increase is more evident at high measuring temperature (800 oC) and indicates enhanced oxygen mobility that are expected to contribute to OCM reaction. Figure 1: Scanning electron microscope micrographs of La2-xCaxCe2O7 (x = 0, 0.25, and 0.50) as-prepared powders obtained by combustion method. Figure 1

Experiments on the melting of samples of fluorite-containing ongonites from the Ary-Bulak massif at 700–800°C at a pressure of 100 MPa and oxygen fugacity corresponding to the Ni–NiO and Mt–Hem buffers were carried out. In all experiments, the limit of the fluorine content in the aluminosilicate melt and saturation with respect to fluorite and topaz were reached. The change in oxygen fugacity did not qualitatively affect the phase relations in the studied samples but led to a slight increase in the solubility of topaz and a decrease in the solubility of fluorite in the silicate melt. In the composition of the silicate melt, a corresponding change occurred in the value of the agpaite coefficient Ka and the content of CaO.

Siloam’s groundwater is reportedly characterized by high fluoride. In response to the reported high incidence of dental fluorosis in the area, sources of elevated fluoride in the groundwater were investigated. Total fluoride (TF) was determined using Ion Chromatograph and Fluoride Ion Selective Electrode. The mineral composition of rocks and soils were determined using X-ray Fluorescence and X-ray diffraction, respectively. Results revealed that groundwater fluoride concentration ranged from 3.92 to 4.95 mg/L. Na-Cl water type was found to be dominant in the water samples. TF content of the rocks and soils ranged from 10 to 2000 mg/L. Leachates were obtained by making a slurry from the samples at a predetermined temperature and time. TF in leachates ranged between 0.27 and 14.88 mg/L and 0.05 to 10.40 mg/L at induced, and non-induced temperatures, respectively. The possible source of fluoride has been previously inferred to be caused by fluorite minerals occurring at greater depth. However, this study proves that fluoride decreases with depth and the elevated fluoride in the groundwater is caused by smectite-kaolinite clay, muscovite and chlorite minerals abundant in the area. Geothermal temperature exhibited by the groundwater in the area is a major factor enhancing the release of fluoride from the clay materials.

Ethanol steam reforming catalyst’s precursors, i.e., nanocomposites of complex oxides with the general formula [Pr0.15Sm0.15Ce0.35Zr0.35O2 + LaMn0.45Ni0.45Ru0.1O3] (1:1 by mass), were synthesized by three different methods. It was shown that two synthesis methods – ultrasonic dispersion and sequential polymeric method, lead to the formation of the nanocomposite perovskite–fluorite system with the specific surface area up to 50 m2/g. Reduction of samples at 400–500°C lead to the formation of Ni–Ru alloy nanoparticles strongly bound with the surface of oxide nanocomposite. Catalytic tests in ethanol steam reforming reaction at 500–600°C showed the highest specific activity of the sample prepared by the sequential polymeric method due to the location of Ni- and Ru-containing perovskite mainly on the surface of the composite providing a high concentration of active metal centers. At higher temperatures for all samples, ethanol conversion approached 100% with hydrogen yield varying in the range of 65–75%. A study of spent catalysts confirmed the absence of carbon deposits after long-term catalytic tests at 650°C.

Wolframite is often associated with calcium bearing minerals, leading to low grade, while conventional anionic collectors are hard to realize their efficient flotation separation, thus the effective structural modification of cationic collectors provides a new insight into enhancing the comprehensive development of wolframite resources. In this study, a surfactant 3-dodecyloxy propyl amidoxime (DOPA) was designed and first used as the collector in flotation separation of wolframite from fluorite and calcite. Computational calculations basically proved the correctness of predictions about the performances of DOPA. Micro-flotation tests indicated that compared to DDA and conventional anionic collector benzohydroxamic acid (BHA), DOPA possessed superior collecting ability to wolframite and excellent selectivity against fluorite and calcite at low concentration, with no frother or activator employed, which is supposed to be an efficient collector for separating wolframite from calcium bearing minerals. The separation mechanisms of DOPA between wolframite and fluorite/calcite were further probed, revealing that besides the electrostatic attraction between DOPA and wolframite, DOPA could chemisorb onto wolframite surfaces by forming five-membered ring toward the surface cation site Fe or Mn, with no significant interaction observed on fluorite/calcite surfaces, resulting to the excellent separation performance.

Using the Bridgman-Stockbarger method, crystals of triple fluoride CaF2-SrF2-BaF2 were grown in a composition range similar to that of CaSrBaF6. The crystals were 10-12 mm in diameter and 50–60 mm in length. The CaSrBaF6 crystal is a new optical material which is transparent in the mid-IR, visible and UV ranges. The uneven distribution of the components along the length of the crystal did not exceed 10 %. The edge of the absorption band in the IR range was 14.3 μm, and the optical absorption at the wavelength of 200 nm did not exceed 18 % (less than 0.2 cm–1). The refraction indices were 1.4527, 1.4488, and 1.4458 for the wavelengths of 633, 969, and 1539 nm respectively. The crystal melts in the temperature range of 1150–1210 °С. The CaSrBaF6 composition is an appropriate matrix for doping with rare-earth ions in order to obtain functional single-crystal and ceramic materials of the visible and IR ranges.

In this study, the use of eggshells was suggested as an adsorbent for fluoride removal, and their mechanism of fluoride removal was investigated. The eggshells underwent thermal treatment to improve their adsorption capacity; 800 °C was found to be the optimal temperature for treatment. Eggshells thermally treated at 800 °C (ES-800) were mainly composed of Ca (82.4%) and C (15.9%), and the peaks of ES-800 obtained from X-ray diffraction (XRD) corresponded to calcite, portlandite, and lime. Fluorine adsorption by ES-800 reached 70% of the equilibrium adsorption amount within 15 min and gradually increased until 24 h. The maximum adsorption capacity of ES-800 at pH 7 and 25 °C was 258.28 mg/g, which is 18 times larger than that of activated alumina; this is classified as the best available technology by the United States Environmental Protection Agency. Both enthalpy and entropy increased in the process of fluoride adsorption onto ES-800. Fluoride adsorption of ES-800 decreased from 59.16 to 11.85 mg/g with an increase in pH from 3 to 11. Fluoride adsorption decreased in the presence of anions, whose impact follows the order: HPO43- > HCO3- >> SO42- > Cl-. XRD, and X-ray photoelectron spectroscopy analysis revealed that fluoride removal was achieved by the formation of calcium fluorite (CaF2). Thus, it can be concluded that eggshells can function as highly efficient adsorbents for fluoride removal, replacing bone char and activated alumina; further, their adsorption capacity can be improved by thermal treatment.

Mono-functional chelating collectors exhibit limited selectivity in the flotation of minerals. In particular, the selective separation of calcium minerals presents a significant challenge because mono-functional chelating collectors, such as fatty acid, indistinguishably adsorb onto mineral surfaces by coordinating with the same metal cation (Ca2+). Thus, there is an urgent need to develop new-mode-functional collectors to separate calcium minerals and a need to understand the underlying chemoselectivity. Given the difference of the hydrogen bonding ability of anions with fluorite, calcite and scheelite surfaces, the introduction of additional hydrogen bonding functional groups into collector molecules is a novel strategy to improve selectivity. In this study, a hydrogen and coordination bonding (bi-functional) collector, 2-cyano-N-ethylcarbamoyl acetamide (CEA) was developed, which could form coordination bonds with the Ca2+ ions (by carbonyl groups) and hydrogen bonds with the anions (by amino groups) on calcium mineral surfaces. The results of flotation tests showed that CEA can selectively separate fluorite and calcite from scheelite at pH 7. The promising selectivity of CEA lies in both the electrical properties and the anions' hydrogen bonding ability with the three calcium minerals. The negatively charged scheelite surfaces are not conducive to coordination bonding with CEA while the positively charged fluorite and calcite surfaces are. Besides, the hydrogen bonding ability of fluorite (F-) and calcite (CO32-) with carbamido in CEA is higher than that of scheelite (WO42-), and this also plays an essential role. This coordination and hydrogen bonding based surfactant design protocol has a great potential in the development of tail-made collectors/depressants for the separation of other oxidized minerals.

Atomic-scale disorder plays an important role in the chemical and physical properties of oxide materials. The structural flexibility of pyrochlore-type oxides allows for crystal-chemical engineerin...

Single crystals of compound LiTb6O5(BO3)3 suitable for X-ray diffraction are prepared by the flux growth method. The compound crystallizes in the P21/n space group, a = 8.4350 A, b = 15.6232 A, c = 8.9025 A, β = 92.041°. The obtained compound belongs to the class of 3D-framework oxoborates. It has a similar structure to that of known LiGd6O5(BO3)3 but is not isostructural to the latter. Both structures are derivatives of fluorite, the predecessor compound in this structural series. The cationic part of the predecessor′s structural type is filled by Tb3+ and Li+ cations, while its anionic part is formed by oxide ions and oxygen atoms of borate anions. The melting point (1375±15 °C) and the band gap (4 eV) are determined for LiTb6O5(BO3)3.

The source and evolution of ore-forming fluids is important to understand the genesis of Carlin-type gold deposit. Constraints on the source and evolution of ore fluid components by the conventional geochemical methods have long been a challenge due to the very fine-grained nature and complex textures of hydrothermal minerals in these deposits. In this study, we present the crush-leach analyzed solute data of fluid inclusion extracts within quartz, calcite, realgar, and fluorite from the Shuiyindong, Nibao, and Yata gold deposits in the Youjiang Basin, providing new insights into the source and evolution of ore-forming fluids. The results show that the high molar Cl/Br ratios up to 2 508 in fluid inclusion extracts are indicative of a contribution of magmatic hydrothermal fluids. Fluids mixing between basinal and magmatic-hydrothermal fluids are evident on the plots of Cl/Br versus Na/K ratios, showing that ore-stage milky quartz near the magmatic-hydrothermal fluids reflects magma origin of the ore-forming fluids, whereas late ore-stage drusy quartz and realgar near the defined basinal fluids suggest the later input of basinal fluids in late-ore stage. Although the predominately host rocks in Shuiyindong, Nibao and Yata gold deposit are bioclastic limestone, sedimentary tuff, and calcareous siltstone, respectively, the solute data of fluid inclusion extracts records they underwent the similar fluid-rocks reactions between the Na-rich magmatic hydrothermal fluids and the Ca- and Mg-rich host rocks. This study highlights the solute data of fluid inclusion extracts obtained by crush-leach analyses have the potential to fingerprint the source and evolution of ore-forming fluids of the Carlin-type gold deposit.

Huangshaping is a world-class skarn deposit hosting abundant W–Sn polymetallic mineralization in the Nanling Range, South China. Although the geochemistry of skarn-hosted mineral deposits has been extensively studied, the role of the prograde-to-retrograde stage transition in the enrichment and precipitation of metallic elements in high-temperature systems has received little study to date. Here, we analyzed garnet, scheelite, and cassiterite in high-temperature granite porphyry-related W–Sn polymetallic system of the Huangshaping deposit to investigate these processes. Three generations of garnet (Grt I, Grt II, and Grt III), two generations of scheelite (Sch I and Sch II), and two types of cassiterite (Cst I and Cst II) were distinguished with regard to their mineral associations, microscopic characteristics, and geochemical features. The results show that grossular–andradite garnet formed from Al-rich andradite (Grt I, Al2O3: 8.18 wt%) in the early prograde stage and pure Fe-andradite (Grt III, Al2O3: 0.15 wt%) in the late prograde stage. Mo-rich scheelite (Sch Ia, MoO3: 19.41 wt%) formed in the prograde stage and coexisted with Grt III, sharing the same REE patterns. A shift from HREE enrichment in Grt I to HREE depletion in Grt III reflects progressive uptake of REEs by secondary mineral phases. Grt III has the highest average contents of W (905 ppm), Mo (19 ppm), and Sn (5610 ppm), suggesting an enrichment of metallic elements at the end of the prograde stage. In contrast, Mo-poor scheelite (Sch II, MoO3: 0.63 wt%) co-crystallized with molybdenite and fluorite in the retrograde stage and shares similar REE patterns with the granite porphyry. Skarn mineralization at Huangshaping was a two-step process controlled by metamorphic stage. The prograde stage was characterized by high temperatures and fO2 and a Cl-rich fluid, and it resulted in enrichment of ore-forming elements with minor scheelite precipitation. The retrograde stage was characterized by lower temperatures and fO2 and a F-rich fluid, and it resulted in major precipitation of ore minerals (scheelite, molybdenite, and cassiterite). Dissolution–reprecipitation reactions played an important role in extraction of metallic elements from decomposing anhydrous skarn minerals and formation of ore minerals. A decrease in the high-field-strength element (HFSE) content of cassiterite from proximal skarn (Cst I) to distal skarn (Cst II) indicates declining temperature accompanied by precipitation of fluorite. This study examines the transition from the prograde stage to the retrograde stage in the Huangshaping deposit, and it provides insights into the genesis of other skarn W–Sn polymetallic deposits in the Nanling magmatic–tectonic–metallogenic belt.

Objectives To characterise the ion release, pH changes and apatite formation of a phosphate free bioactive glass. Methods A SiO2-CaO-CaF2-Na2O glass was synthesized by a melt route with a composition close to the reactive glass in the commercial Cention N® composite. The glass was characterized after immersion in three media: Artificial Saliva pH4 (AS4) Artificial Saliva pH7 (AS7) and in a high phosphate artificial saliva at pH6.5 (AS6.5). The pH and fluoride release were measured using a pH meter and an ion selective electrode. The concentration of Ca, P, Na and Si were measured by ICP-OES. The glass powders after immersion were characterized by FTIR, X-ray powder diffraction and 19F MAS-NMR. Results The glass increased the pH in all three media. Fluoride was detected in all three media but was much higher in AS 6.5. Calcium fluoride formed in AS4 with a small amount of fluorapatite at long immersion times. Fluorapatite and calcium fluoride formed in AS7, whilst in AS6.5 fluorapatite formed. The ion concentrations in solution after immersion reflected the glass composition and the immersion media with fluorapatite being favoured by higher pHs and phosphate contents in the media. Significance The results demonstrated the ability of the glass to increase the pH and to form fluorapatite in phosphate containing media. This may explain the low incidence of secondary caries found in the commercial composite. Unlike the commercial composite evidence was found for the precipitation of fluorite, which will act to reduce the release of fluoride for preventing secondary caries.

Most of the World’s large tungsten deposits are genetically related to fluorine-rich granitic magmas, their ores contain fluorine minerals, such as fluorite and topaz, and their host rocks have commonly undergone fluorine metasomatism to form greisens. In view of this evidence that tungsten ore fluids are enriched in fluorine, and the potential for fluoride to form stable complexes with hard acids (e.g., W6+), we investigated the solubility and speciation of tungsten in fluoride-bearing fluids at hydrothermal conditions. Experiments were conducted to measure the solubility of tungsten trioxide solid in fluoride-bearing aqueous fluids at temperatures between 100 and 250 °C and vapour-saturated water pressure. Based on the results of these experiments, tungsten is dissolved predominantly as H3WO4F2− in solutions containing 0.05–0.30 m HF at pH(T) values between 1.5 and 3.4. Significantly, the concentration of H3WO4F2− was up to two orders of magnitude higher than that of tungstate species in these experimental solutions. This species formed via the reaction WO3cryst + H2O + 2HF = H3WO4F2− + H+, logarithms of the equilibrium constants for which, vary from −4.39 ± 0.18 at 100 °C to −5.10 ± 0.17 at 250 °C. In solutions containing

A millimeter-sized fluorite crystal found in leucocratic syenite of the Velykovyska massif on the Ukrainian Shield became the object of study. This is a crystal of irregular shape with smoothed corners, broken by a system of contraction cracks inlaid with basnäsite-(Ce). The latter is characterized by a REE content of 57± 5 wt. %, CaO - of 3.3 ± 0.25 wt. % and Ce2O3 : La2O3 : Nd2O3 ratio of 6 : 4 : 1. A detailed study using microprobe JXA-733 and an electron microscope JSM-6700F, equipped with EDS JED-2300, showed the presence of other mineral phases in the inlays. The middle of the cracks is filled with ferrous aluminosilicate without REE. On the opposite side, a siderite rim was found around the basnäsite and single crystals of calcite were found nearby in the fluorite. Fluorite inlaid with basnäsite is considered as a product of crystallization of the carbonate-fluoride melt drop that separated from silicate melt due to their immiscibility. Subsequent stratification of the melt inside the drop led to formation of fluorite rimmed by carbonates of REE and Fe and displaced to the cracks of Fe-aluminosilicate. Crystallization of the separated carbonate-fluoride melt took place in an isolated volume without any exchange of components with the surrounding Na-K-feldspar. Fluorite, as well as carbonates and mica are less common in the Velykovyska massif than other similar syenite massifs on the Ukrainian Shield. It is assumed that this is caused by the high temperature of the syenite melt, which contributed to evaporation of volatile components (F2, CO2, H2O), and by processes of liquation. The immiscibility between salt and silicate melts contributed to formation of drops of carbonate-fluoride melt and their movement to the apical parts of the massif and outside. The discovery of inlaid fluorite increases the prospects of the Velykovyska massif for the detection of REE ore occurrences in syenites and surrounding rocks. In addition, the finding confirms the existence of carbonate-fluoride melts in nature. These melts are paternal for fluorite-carbonate veins with REE mineralization.

A new cerite group mineral species, taipingite-(Ce), ideally (Ce73+, Ca2)Σ9Mg(SiO4)3[SiO3(OH)]4F3, has been found in the Taipingzhen rare earth element (REE) deposit in the North Qinling Orogen (NQO), Central China. It forms subhedral grains (up to approximately 100 ​μm ​× ​200 ​μm) commonly intergrown with the REE mineral assemblages and is closely associated with allanite-(Ce), gatelite-(Ce), tornebohmite-(Ce), fluocerite-(Ce), fluocerite-(La), fluorite, bastnasite-(Ce), parisite-(Ce) and calcite. Taipingite-(Ce) is light red to pinkish brown under a binocular microscope and pale brown to colorless in thin section, and it is translucent to transparent with a grayish-white streak and vitreous luster. This mineral is brittle with conchoidal fracture; has a Mohs hardness value of approximately 5½ and exhibits no cleavage twinning or parting. The calculated density is 4.900(5) g/cm3. Optically, taipingite-(Ce) is uniaxial (+), with ω ​= ​1.808(5), e ​= ​1.812(7), c ​= ​e, and a ​= ​b ​= ​ω. Furthermore, this mineral is insoluble in HCl, HNO3 and H2SO4. Electron microprobe analysis demonstrated that the sample was relatively pure, yielding the empirical formula (with calculated H2O): (Ce4.02La1.64Nd1.49Pr0.41Sm0.10Gd0.02Ho0.02Tm0.01Lu0.02Y0.03Ca0.66Mg0.05Th0.01–0.51)Σ9(Mg0.75Fe0.253+)Σ1(SiO4)3{[SiO3(OH)]3.98[PO3(OH)]0.02}Σ4(F1.81OH1.17Cl0.02)Σ3. Taipingite-(Ce) is trigonal and exhibits space group symmetry R3c with unit cell parameters a ​= ​10.7246(3) A, c ​= ​37.9528(14) A, V ​= ​3780.39(20) A3 and Z ​= ​6. The strongest eight lines in the X-ray diffraction pattern are [d in A(I)(hkl)]: 4.518(50)(202), 3.455(95)(122), 3.297(85)(214), 3.098(35)(300), 2.941(100)(02,10), 2.683(65)(220), 1.945(40)(238) and 1.754(40)(30,18). The crystal structure has been refined to a R1 factor of 0.025, calculated for the 2312 unique observed reflections (Fo ​≥ ​4σ). The mineral is named after its discovery locality and is characterized as the F-dominant analogue of cerite-(Ce).

Sluggish oxygen reduction reaction (ORR) activity and poor CO2-tolerance has been the long-standing limitations for the application of alkaline earth metal oxide cathode for solid oxide fuel cells (SOFCs). Here we report this situation can be ameliorated with a composite cathode based on Ba0.9Co0.7Fe0.3O3-δ (B90CF) by surface-decorated Pr6O11 (PO) particles. A halved polarization resistance is obtained by B90CF-15PO (PO of 15 wt%) cathode (0.033 Ω cm2 at 700 °C) compared to blank B90CF, suggesting boosted oxygen reduction reaction activity owing to the accelerated oxygen surface exchange kinetics introduced by PO particles. PO protective layer also brings up desirable CO2-tolerance for B90CF cathode due to the more stable fluorite cubic structure of PO and higher acidity of Pr3+/Pr4+ than Ba2+, which ensures the stable operation of cells. This work demonstrates the positive potential of surface-decoration with PO in developing cathodes with high performance and CO2-tolerance.

The deposit of Bayan Obo in Inner Mongolia is the world's largest rare earth element (abbreviated as REE) resource. The exploration of the theory of mineral formation of Bayan Obo is an important foundation for mineralogical research, and is the scientific basis for mining, industrial beneficiation, smelting and extraction, and processing and utilization. With the rapid development of science and technology, the demand for the utilization of rare earth elements is increasing, and the separation process between rare earth elements needs to be developed. The purpose of this paper is to provide high temperature experimental information for the formation and application of rare earth minerals. To this end, the mineral evolution of high-grade rare earth concentrates with increasing temperature and the migration of rare earths at different stages and their reaction mechanisms were studied. According to Thermogravimetric analysis and differential scanning calorimetry (TG-DSC), the calcination was carried out at different temperature ranges, and the calcined products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope and energy dispersive spectrometer (SEM-EDS) and other analytical techniques. The results are shown in this process, the rare earth phase is first converted into rare earth oxide and rare earth oxyfluoride. As the temperature increases, Ca5(PO4)3F and a large number of self-shaped spherical Ca-RE-OF and Ca-RE-PO4 particles are formed, and the separation of La and Ce elements is discovered. According to the phase diagram analysis, the production of Ca5(PO4)3F is due to the reaction of monazite and fluorite, and the phases CeF2 and CeF3 are formed during the reaction. When it reaches 1500°C, barium ferrite is produced and a new substance containing Ba2+ is formed.

Bayan Obo ore contains mainly bastnaesite, monazite and associated iron, fluorite, calcite, dolomite, barite, and trace amount of columbite. Due to its complex mineral compositions and similar mineral properties, it is difficult to separate and obtain rare earth and fluorite concentrates with high purity by conventional magnetic separation and flotation. In this paper, a stepwise magnetic separation process comprised of wet permanent magnetic separation (PMS), wet electromagnetic separation (EMS), and superconducting magnetic separation (SCMS) was proposed to pre-concentrate iron, rare earth, and fluorite minerals from Bayan Obo ore. The results showed that iron, rare earth, and fluorite preconcentrates with high purity and low impurity content could be achieved, which might be helpful to shorten the technological process and improve the separation efficiency for the subsequent flotation of iron, rare earth, and fluorite minerals.

Thermal decomposition and phase transformation for the mixture of Bayan Obo rare earth concentrate (BORC) and sodium carbonate (Na2CO3) roasted at different temperatures with weight ratio of 100:20 were studied in detail in our study. The aim of our study is to reveal the nature of roasting reaction between BORC and Na2CO3 and thus providing a new method for processing BORC. The results indicate that BORC can be decomposed completely with Na2CO3 at around 600 °C after 3 h. During the calcination process, Ce0.5Nd0.5O1.75, NaF, Na3PO4, and a rare earth double phosphate phase Na3RE(PO4)2 are formed after the decomposition of BORC with Na2CO3. In addition, the thermal decomposition mechanism is determined in the paper. Based on these facts, a clean technique processing BORC was developed. And a CeF3 powder, whose composition was measured and stability was also evaluated, was obtained for some potential application from the new technique. This research is of significance in terms of the Na2CO3-roasting BORC solid reaction study and sheds a light on a potential clean technique for BORC.

Anionic nonstoichiometry in fluorides (replacement of F1– with O2–) deteriorates the quality of optical materials. Two reviews have been devoted to the initial stage of anionic nonstoichiometry: fluorides MF2 (M = Ca, Sr, Ba) are considered in this paper, and fluorides RF3 (R are 16 rare-earth elements) will be considered in the next one. These 19 fluorides play an important role in the fluoride materials science, comprising more than 70% of 27 MFm that are used to design two-component fluoride crystalline materials. The initial stage of anionic nonstoichiometry in MF2 is the only one in which oxofluorides MF2 – 2xOx with low oxygen content are formed. Partial replacement of F1– with O2– in the fluorite structure is accompanied by thermal stabilization of the structure type when moving up the temperature scale with a maximum in the melting curves of the oxofluoride phase, which decomposes upon cooling. No other intermediate phases containing fluorine and oxygen were found in the MF2–MO systems studied.

Single-atom catalysts (SACs) have attracted considerable attention in the catalysis community. However, fabricating intrinsically stable SACs on traditional supports (N-doped carbon, metal oxides, etc.) remains a formidable challenge, especially under high-temperature conditions. Here, we report a novel entropy-driven strategy to stabilize Pd single-atom on the high-entropy fluorite oxides (CeZrHfTiLa)Ox (HEFO) as the support by a combination of mechanical milling with calcination at 900 °C. Characterization results reveal that single Pd atoms are incorporated into HEFO (Pd1@HEFO) sublattice by forming stable Pd–O–M bonds (M = Ce/Zr/La). Compared to the traditional support stabilized catalysts such as Pd@CeO2, Pd1@HEFO affords the improved reducibility of lattice oxygen and the existence of stable Pd–O–M species, thus exhibiting not only higher low-temperature CO oxidation activity but also outstanding resistance to thermal and hydrothermal degradation. This work therefore exemplifies the superiority of high-entropy materials for the preparation of SACs., Fabricating intrinsically stable single-atom catalysts (SACs) on traditional supports remains a formidable challenge in catalysis. Here, the authors propose a new strategy to construct a sintering-resistant Pd SAC on a novel equimolar high-entropy fluorite oxide.

CeO2 is a typical fluorite oxide with desirable lattice oxygen conductivity and can be applied as an active support for the Fe-based oxygen carrier in chemical looping hydrogen generation (CLHG). H...

The homologous series of MF2 (M = Ca2+, Sr2+, Ba2+, Cd2+, Pb2+, Sm2+, Eu2+, Yb2+, Ra2+, and Hg2+) crystals having the fluorite structure (CaF2, space group $$Fm\bar {3}m,$$ Z = 4) holds an important place in fluoride materials science. A correlation between the enthalpy of anti-Frenkel defect formation (Hf) and the enthalpy of melting (Hm) was elucidated for CaF2, SrF2, BaF2, and PbF2 based on the most reliable experimental data set: Hf (kJ/mol) = 8.6Hm (kJ/mol) or Hf (eV) = 0.089Hm (kJ/mol). Experimentally determined Hm values were used to calculate the enthalpies of defect formation Hf = 2.0 and 2.6 eV for CdF2 and EuF2 crystals, respectively. The correlation is prognostic for the whole series of homologous MF2 fluorite crystals and for SrCl2.

The effects of melt composition, temperature and pressure on the solubility of fluorite (CaF2), i.e., fluorine concentration in silicate melts in equilibrium with fluorite, are summarized in this paper. The authors present a statistic study based on experimental data in literature and propose a predictive model to estimate F concentration in melt at the saturation of fluorite (C F in melt Fl-sat ). The modeling indicates that the compositional effect of melt cations on the variation in C F in melt Fl-sat can be expressed quantitatively as one parameter FSI (fluorite saturation index): FSI=(3AlNM+Fe2++6Mg+Ca+1.5Na-K)/(Si+Ti+AlNF+Fe3+), in which all cations are in mole, and AlNF and AlNM are Al as network-forming and network-modifying cations, respectively. The dependence of C F in melt Fl-sat on FSI is regressed as: C F in melt Fl-sat =1.130−2.014·exp (1 000/T)+2.747·exp (P/T)+0.111·C melt H2O +17.641·FSI, in which T is temperature in Kelvin, P is pressure in MPa, C melt H2O is melt H2O content in wt.%, and C F in melt Fl-sat is in wt.% (normalized to anhydrous basis). The reference dataset used to establish the expression for conditions within 540–1 010 °C, 50–500 MPa, 0–7 wt.% melt H2O content, 0.4 to 1.7 for A/CNK, 0.3 wt.%–7.0 wt.% for C F in melt Fl-sat . The discrepancy of C F in melt Fl-sat between calculated and measured values is less than ±0.62 wt.% with a confidence interval of 95%. The expression of FSI and its effect on C F in melt Fl-sat indicate that fluorine incorporation in silicate melts is largely controlled by bonding with network-modifying cations, favorably with Mg, AlNM, Na, Ca and Fe2+ in a decreasing order. The proposed model for predicting C F in melt Fl-sat is also supported by our new experiments saturated with magmatic fluorite performed at 100–200 MPa and 800–900 °C. The modeling of magma fractional crystallization emphasizes that the saturation of fluorite is dependent on both the compositions of primary magmas and their initial F contents.

The paper reports the results of melting experiments on the fluorite-enriched rhyolites of the Nyalga Basin (Central Mongolia). The fluoride–calcium (F–Ca) melt was obtained in a wide range of P-T parameters (1250–750°С, 5.5–1 kbar). Fluoride–silicate liquid immiscibility was observed at F > 2.5 wt % and CaO > 5.3 wt % in the initial system. An increase of temperature and pressure was accompanied by a significant increase in the concentrations of REE, Y, Sr, P, Th, U, Nb, Co, Cu, Sn, Sb, and Mo in the F–Ca melt. The peculiarity of the DREE distribution coefficients between F–Ca and silicate melts can lead to the formation of M-type tetrad effects for the first, third, and fourth tetrads in the chondrite-normalized REE patterns of silicate melt. The F–Ca melt has existed up to subsolidus temperatures of the rhyolitic melt. None of the models of magmatic crystallization of fluorite in haplogranitic melts or subsolidus postmagmatic and hydrothermal fluoritization can explain the origin of fluorite-enriched rhyolites. It is suggested that these rocks were formed from magma containing the emulsion of rhyolitic and F–Ca melts. The fluoride–silicate liquid immiscibility resulted in the redistribution of trace elements (REE, Y, Sr, P, Zr, Hf, Ta, Nb, Sc, Li, Be, and Rb) between melts. The formation of rock matrix was accompanied by degassing of the rhyolitic melt. An effective viscosity of the magma (melts emulsion with fluid bubbles) is comparable with the viscosity of a liquid. The F–Ca melt was quenched into F–Ca phase consisting of submicron fluorite particles, while solidification of the rhyolitic melt and silicate glass devitrification resulted in the formation of quartz–sanidine symplectites. The F–Ca phase has the elevated contents of O, Sr, LREE, Y, Si, sometimes Sc, P, and Al. The isomorphous substitution of O2– → F– in the fluorite structure led to the formation of aggregates of oxygen-vacancy centers, which are responsible for the laser-induced luminescence of the F–Ca phase in the rhyolite matrix. The wide variations of the REE, Y, Sr, Th, Nb, Ta, Zr, and Hf contents in the F–Ca phase are related to its recrystallization under the effect of fluid, which was released during degassing of rhyolitic melt. The submicron-sized fluorite particles in the F–Ca phase during the interaction with fluid were gradually liberated from impurities (besides Sr) and transformed into larger crystalline segregations. It is suggested that the oxygenated F–Ca melt exists in a metastable supercooled state under oxidizing conditions during eruption of rhyolitic magma. This is inconsistent with previously obtained experimental data on crystallization of fluorite from CaF2- and H2O-saturated haplogranitic melts at < 950°С and 1–2 kbar. Using rhyolites as example, it is shown that fluorite and associated ore mineralization (monazite-group minerals, cerianite) were derived from F–Ca melt with elevated REE and Y contents. In many igneous rocks and magmatogenic ores, fluorite could be a product of transformation of the F–Ca melt.

Protactinium pentoxide (Pa2O5) represents the fluorite and the layered structures of the protactinium-oxygen system and the least studied oxide in the group. Our DFT work explores some possible structures of Pa2O5 stoichiometry by starting from the fluorite PaO2 structure and adding oxygen in octahedral position of protactinium. For all simulated structures, the structural, thermodynamic and electronic properties including lattice parameters, bulk moduli, entropy, zero point energy (ZPE) and band gaps were predicted. The fluorite Pa2O5 in the b- and g-Pa2O5 structures were found to be the most stable overall. Indeed, the doping of oxygen atoms in the octahedral position of Pa is significantly better than other doping positions for the fluorite PaO2 structure. In particular, we found that the PaO2.5 structure is the most thermodynamically stable under ambient conditions. Furthermore, while x from 0 increases to 0.5 for protactinium oxides (PaO2+x, x = 0, 0.25, 0.5, 0.75, 1), the decreasing trend with increasing x of the optimized volume is in reasonable agreement with experimental findings.

Groundwater is only the primary source for drinking water in the northern Telangana, South India, where a number people suffers from fluorosis. With this concern a 34 groundwater samples were collected and studied to identify the occurrence, hydrochemical distribution of fluoride groundwater, using geo-statistical tool such as principal component analysis (PCA), saturation indices (SI), and correlation analysis were executed in this study. The concentration of fluoride ranges from 0.06 to 4.33 mg/L, with a mean of 1.13 mg/L and 30% of groundwater samples having above the maximum acceptable limit of 1.2 mg/L fluoride for drinking purposes. Fluoride shows a considerable relation with pH, and TDS, while fluoride also demonstrations an insignificant correlation with Ca2+. Moreover, alkaline nature, elevated HC$${\text{O}}_{3}^{ - }$$, Na+ and Na+–HC$${\text{O}}_{3}^{ - }$$ water type were also influenced to enhance the fluoride concentration in the groundwater. The two components from the principal components (PC) analysis reveals that chemical variable accounts for above 67% of the total variance of the groundwater chemistry. The PC-1 and PC-2 have high positive loadings reveals that the dissolution of fluoride bearing minerals like apatite and biotite are the chief source to larger concentration of fluoride in the study region groundwater. Further, groundwater also obviously approves the over-saturated with respect to calcite, fluorite, and dolomite are the major factors to upholds the enrichment of fluoride concentration, while the geogenic activities are also a principal controlling factors to influence the groundwater chemistry in the study region.

The Sanjiu uranium ore field, located in the central of Zhuguangshan granitic batholith, is a newly discovered granite-related uranium ore field in South China. The main sulfide in the ore field is pyrite, which is closely related to uranium mineralization. The textures, major and trace elements, S–He–Ar isotopes compositions of pyrites in ores of different grade were observed and/or analyzed by optical microscope, scanning electron microscope, electron microprobe, laser ablation inductively coupled plasma mass spectrometry, and noble gas mass spectrometer (Helix-SFT). It is observed that these U-related pyrites are generally euhedral–subhedral with dissolution textures, anhedral variety with colloform texture, veinlet and fine particles, and the color of the associated minerals is mostly dark hue, such as purple–black fluorite, dark-red hematite, and dark-green chlorite, etc. The analytical results show that the average compositions of major elements in pyrite are FeS1.944. Pyrites are characterized by S-deficiency, low content of Co and Th, and Co/Ni > 1, which indicate that these ores are of low-temperature hydrothermal origin. We found that the higher the grade of ore, the more deficient in S, the more obvious negative δ34S, and the higher REE content (close to U-rich granitic pluton) of pyrite. The S–He–Ar isotopic compositions of various varieties of pyrites indicate that the ore-forming fluids mainly come from crust-derived fluids and mixed with mantle-derived fluids.

The effects of TaO2.5 additions to Y4Zr3O12 (YZO) on its interactions with calcium-magnesium alumino-silicate (CMAS) melts are investigated. YZO is a candidate material for the protection of novel ZrO2-YO1.5-TaO2.5 based thermal barrier coatings (TBCs) against CMAS attack, but thermochemical considerations dictate that YZO contain TaO2.5 for interfacial compatibility. Oxides based on Y4Zr3-xTaxO12+0.5x (x = =0, 0.7, 1.4) were mixed with various ternary and quinary silicate compositions ranging in Ca:Si ratio from 0.35 to 1.13 and equilibrated at 1300 °C. The results show that all combinations form the desired apatite phase except for the mixture of the higher Ta5+ content with the lowest Ca:Si melt. It is found that the addition of Ta5+ increases the capture of Y3+ by the reprecipitated fluorite, decreasing the amount available to form apatite and eventually suppressing it at low Ca:Si. The effect is ascribed to the attractive interactions between Y3+ and Ta5+ in solid solution, similar to that found in tetragonal zirconia, but with concurrent incorporation of anion vacancies associated with excess Y3+ that stabilize the cubic form. As the Ca2+ content of the melt increases, so does the amount of Ca2+ incorporated in fluorite. The latter also interacts attractively with Ta5+ and leads to a reduction of captured Y3+ in fluorite as the Ca content in the reacting deposit increases. This serves to increase the amount of Y3+available in the melt to form apatite in TBC-CMAS interactions.

Hydrogeochemical investigations were carried out in the eight villages of Tosham Block; district Bhiwani, Haryana to recognize the mechanism and sources of enrichment of fluoride in the groundwater. The results specify that concentrations of fluoride attain up to 1.9 mg/l in groundwater samples. In the soils, the overall fluorine contents vary between 1.1 and 2.7 mg/kg, which could have sufficient potential to released fluorine into the groundwater. In groundwater, the fluoride enrichment is primarily controlled by solubility of fluorite, intensity of evapotranspiration, residence time and the processes of weathering prevailing in Tosham area. Moreover, various other water quality parameters such as pH, electrical conductivity, total hardness, and total alkalinity as well as calcium, magnesium, carbonate, bicarbonate and chloride concentrations were also calculated. A logical calculation of correlation coefficients between different physico-chemical parameters was done. The 67% of groundwater samples do not comply with WHO standards of fluoride for drinking purposes. The excessive fluoride concentration in the groundwater of villages under study causes dental fluorosis among people especially the children. Except few of villages, without any prior treatment, the overall quality of water was found unacceptable for drinking purposes.

A key challenge in the catalytic conversion of CH4 and CO2 into a synthesis gas (CO and H2) via the dry reforming of methane (DRM) is the development of stable catalysts. We demonstrate that the re...

Present study deals with the identification of geochemical processes and the probable mobilization mechanism responsible for the release of fluoride (F−) and nitrate (NO3−) in groundwater. For this purpose 50 groundwater samples are collected from the Gharbar and its adjacent villages of Jharkhand. The order of abundance of anion and cation in the groundwater sample are HCO3– > Cl− > SO42- > NO3− > F− > Br− and Ca2+ > Na+ > Mg2+ > K+ respectively. F− concentration ranges from 0.01 to 18.55 mg/L and the NO3− concentration ranges from 34.10 to 319.10 mg/L where majority of the water samples exceeds the permissible limit of F− (1.5 mg/L) and NO3− (45 mg/L). Alkaline aquifer condition accelerates the mobilization of F− in groundwater due to enhanced rate of weathering of fluoride-bearing minerals whereas anthropogenic activities play a major role in NO3− contamination of the study area. Saturation index (SI) indicates under-saturation of fluorite and calcite in the majority of water samples. Principal component analysis reveals 3 principal components explaining 72.33% of the data variance and indicates anthropogenic sources of contamination of NO3− and geogenic sources of contamination for F−. Keywords: Fluoride, Nitrate, Hydrogeochemistry, Weathering, Saturation index (SI), Gharbar village, Jharkhand

Petrographic and mineralogical observations are corroborated with the hydrochemistry data to identify the exact lithological source of fluoride contamination in the central part of India. Groundwater samples (n = 57) representing both shallow and deep aquifers and lithological formations ranging from Archean to Late Cretaceous from central part of India were studied, where fluoride contamination in drinking water is causing a serious environmental health hazard. About 90% samples show high fluoride content than the maximum permissible limit of 1.5 mg/L. Fluoride concentrations in different rock formations such as granite gneisses, limestones, sandstones, and basalts were studied which shows maximum fluoride values of up to 4.9 mg/L. Several parameters (including Saturation Index and Gibbs’s plots) were evaluated to understand the factors which controlled the groundwater quality in the area. All signifies water–rock interaction as a major factor influencing composition of groundwater in both the seasons as well as support geogenic enrichment of fluoride. Low-temperature hydrothermal mineralization of fluorite in the limestone aquifers is responsible for the dissolution of fluorine in groundwaters with highest concentration. Moderate concentrations of fluoride up to 2.7 mg/L are observed in the basement granite gneisses. Our mineralogical studies recognized abundant fluor-apatite in granite gneiss aquifers, with high F−content ranging from 2.5 to 4.2 wt%, as a major contributor of fluoride in groundwaters. Rock-water interactions amplified in alkaline environments played an important role in the dissolution of fluorine in both the aquifers. Identification of main geogenic source of fluoride will be helpful for proper management of drinking water sources in the highly affected areas.

Litho-geochemical characteristics of low and high fluoride (F−) groundwater along with hydrological processes were investigated to delineate its genesis and enrichment mechanism in a watershed sedimentary basin. In this study, groundwater F− concentration ranged from 0 to 20 mg/L with a mean and standard deviation of 2.8 and ± 3.7 mg/L, respectively. Out of N = 87, 63% of samples exceeded the World Health Organization (WHO) limit of 1.5 mg/L. The order of cationic and anionic dominance in groundwater samples with mean was found in decreasing order as Na+ > Mg2+ > Ca2+ > K+ and HCO3− > SO42− > Cl− > PO43− > NO3− measured in milligrams per liter. Groundwater chemistry changed from Ca-HCO3 to Na-HCO3 type and low to high fluoride as we moved from mountain foot towards the synclinal basin. Low fluoride groundwater reflected weathering, recharge, and reverse ion exchange processes with Ca–HCO3– and Ca–Mg–Cl–type water while high fluoride groundwater revealed base ion exchange, mixing, and desorption as dominant hydrological processes with Na-HCO3 and Na-Cl types of water. Gibb’s diagram showed rock weathering and mineral dissolution as the major geochemical processes controlling water chemistry with an insignificant role of evaporation in the semi-arid area. Fluoride was undersaturated with mineral fluorite, indicating fluoride in groundwater is released by secondary minerals. However, due to complex geological features, groundwater fluoride enrichment was affected by a broad-scale process across a wide area such as depth, residence time, and most important geomorphological units hosting the aquifer.

The article represents results of the study aimed on sensitivity of balsam poplar (Populus balsamifera L.) leaves to hydrogen fluoride airborne emissions in the vicinity of aluminum smelter. Widespread occurrence of poplar in urban land was one of the purposes to choose this plant as a research object. Reaction of poplar leaves on influence of hydrogen fluoride (HF) was investigated by ionometric technique, scanning electron microscopy-related method (SEM–EDS), X-ray diffraction analysis (XRD), and the thermodynamic modelling. The fluoride content in poplar leaves decreases with distance from the emission source. Leaves have signs of deterioration, e.g., chemical burns and necrosis, inside 2-km zone of the plant. In stomatal cells of poplar leaves, neogenic mineral fluorite (CaF2) was discovered. In sieve-tube elements of leaf phloem, a large quantity of secondary calcite (CaCO3) crystals was found. Ca2+ ions in enchylema react with F− that has released as part of airborne emissions from aluminum smelter. Result of their interaction is fluorite formation in stomatal cells of poplar leaves. Such fluorite is a mineral indicator of fluoride pollution in ground-level ambient air. Thermodynamic conditions of interaction of Ca2+ and F–, HCO3– and Ca2+ were estimated. The model of maintenance of phytochemical calcium barrier in the poplar leaves stomata in HF contamination was proposed and described.

Anode carbon residue is produced in the production of electrolytic aluminum. Its properties need to be studied for secondary utilization. In this paper, mineralogy of anode carbon residue from an electrolytic aluminum plant in Guizhou was studied. The anode residue chemical composition, structure, mineral composition, occurrence state of main elements, etc, was investigated. The results show that: Anode carbon residue is mainly composed of 14 minerals such as cryolite, cryolithionite, elpasolite and graphite. Among them, the opaque minerals are mainly graphite and the transparent minerals are mainly cryolite. Carbon in the form of independent mineral occurrence in graphite; fluoride in the form of independent mineral occurrence in cryolite, cryolithionite, elpasolite and fluorite; aluminum in the form of independent mineral occurrence in cryolite, cryolithionite, elpasolite, aluminium oxide and magnesium aluminate; sodium in the form of independent mineral occurrence in cryolite, cryolithionite, elpasolite. The mineralogical characteristics and occurrence state of carbon and electrolyte were studied, which provided a basis for the separation and recovery of carbon and electrolyte in anode carbon residue.