Search

Carbonatites host Earth’s main REE deposits, with bastnaesite (LREE)CO3F being the main economic REE-bearing mineral. However, bastnaesite mineralisation processes are debated between hydrothermal or magmatic origin. This study aims to assess if bastnaesite can be magmatic, and to characterise the REE behaviour during carbonatite crystallisation. Crystallisation experiments have been performed from 900 to 600 °C at 1 kbar, on a REE-rich calciocarbonatitic composition. REEbearing calcite is the dominant crystallising mineral, driving the residual melt towards natrocarbonatitic compositions. Both halogens (i.e., Cl and F) and water decrease the temperature of calcite saturation. REE are slightly incompatible with calcite: for all REE, partition coefficients between carbonate melt and calcite are comprised between 1 and 11, and increase with temperature decrease. Britholite (REE, Ca)5((Si,P)O4)3(F,OH) crystallises at high temperatures (700–900 °C), while pyrochlore (Ca,Na,REE)2Nb2O6(OH,F) crystallises at low temperatures (600–700 °C), as well as REE-rich apatite (600–650 °C). No bastnaesite is found in crystallisation experiments.We thus performed a bastnaesite saturation experiment at 600 °C. The bastnaesite-saturated melt contains 20 wt% of REE: such magmatic saturation is unlikely to happen in nature. Textural evidences imply a Na, Cl, REE-rich fluid at high temperatures and hydrous conditions.We propose that fluids are the main mineralising agent for bastnaesite at hydrothermal stage (<600 °C). [ABSTRACT FROM AUTHOR]

Bastnaesite, a major economic rare earth element (REE) mineral, has been considered to be a promising geochronological tool for determining mineralization time by U–Pb dating. However, it has been found that this mineral often favours the incorporation of Th relative to U, with Th/U ratios even larger than 1000. Young samples contain considerable amounts of excess 206Pb from initial 230Th that is incorporated during formation, which could have a significant effect on U–Pb dating. In this study, we present analytical and data calibration protocols for in situ Th–Pb analyses of bastnaesite using a Cameca IMS 1280 HR SIMS. Six bastnaesite samples from the Himalayan Mianning–Dechang REE belt were analysed in this study to test our analytical procedures. The zircon from the host syenites are dated by the SIMS U–Pb method at 12.7 ± 0.2 Ma for the Dalucao deposit, 28.8 ± 0.3 Ma for the Lizhuang deposit, 28.0 ± 0.2 Ma for the Muluozhai deposit, and 26.6 ± 0.3 Ma for the Maoniuping deposit, which are interpreted as the intrusion time of the syenites. The SIMS bastnaesite Th–Pb dating yielded ages of 11.9 ± 0.2 Ma and 11.8 ± 0.2 Ma for the Dalucao deposit, 28.4 ± 0.2 Ma for the Lizhuang deposit, 26.7 ± 0.2 Ma and 26.9 ± 0.2 Ma for the Muluozhai, and 25.7 ± 0.2 Ma for the Maoniuping deposits. These bastnaesite ages are interpreted as the mineralization time. To compare the zircon age and corresponding bastnaesite age, the time span from intrusion of the host magma to REE mineralization is approximately 1 Ma or less. This work demonstrated that the Th–Pb dating method for high Th bastnaesite yields more precise ages than the U–Pb system, as the former can avoid the excess 206Pb problem. [ABSTRACT FROM AUTHOR]

In this study, an attempt to quantitatively rebuild the Early Cretaceous paleoclimate was executed by employing calcareous paleosols (fossil soils), which have a horizon of pedogenic carbonate accumulation within the profile, in the central Sichuan Basin of southwestern China and Jinlingshi-Yangshan Basin of northeastern China, using the relationships between the climate and geochemical proxies, depth to calcic horizon and oxygen stable composition of pedogenic carbonates in modern soils. During the early Valanginian and early-middle Berriasian ages in the central Sichuan Basin, the climate was dominantly cold and dry, with a mean annual temperature (MAT) within 5 and 9 °C (standard error (S.E.) = ±5.8 °C) and mean annual precipitation (MAP) within ~190 and ~320 mm. During the late Barremian age (~126 Ma), the MAP was within ~200 and ~380 mm, and MAT was between 9 and 13 °C (S.E. = ±5.8 °C) in the Jinlingsi-Yangshan Basin. Noticeably, for the paleosols which formed under the arid climate, alkali and alkaline earth cations were weakly leached from the soil profile, and even accumulated on the soil surface. Therefore, geochemical indices (such as the chemical index of alteration (KO-free) and alkaline index) to evaluate the chemical weathering, including base cations (Ca, Mg, Na and K), as the functions of precipitation and air temperature, are unsuitable for paleoclimate reconstruction of paleo-aridisols. [ABSTRACT FROM AUTHOR]

An account of the synthesis, spectroscopic, thermal and structural behavior of antimony(III) bis(pyrrolidinedithiocarbamato)alkyldithiocarbonates is presented. The reaction of antimony(III) bis(pyrrolidinedithiocarbamate) chloride with potassium organodithiocarbonate in equimolar ratio yielded the corresponding mixed derivatives of the type [(CH)NCS]SbSCOR [where R = Me, Et, Pr, Pr, Bu, and Bu]. These newly synthesized complexes have been characterized by physicochemical [molecular weight determination, melting points, and elemental analysis], spectral [UV, IR, far-IR, NMR (H and C)], thermal [TG, DTA, and FAB mass], and structural [powder XRD and SEM] studies. Analytical studies leads to purity and structural properties of the synthesized complexes on the other hand powder X-ray diffraction and SEM studies show that multiphase, polycrystalline, and rod-shaped complexes have been formed having nanorange crystallite size and monoclinic crystal system. [ABSTRACT FROM AUTHOR]

Water temperature has received considerable attention as steering factor for the genesis of different types of marine carbonate sediments. However, parameters other than temperature also strongly influence ecosystems and, consequently, the carbonate grain associations in the resulting carbonate rock. Among those factors are biological evolution, water energy, substrate, water chemistry, light penetration, trophic conditions, CO concentrations, and Mg/Ca ratios in the seawater. Increased nutrient levels in warm-water settings, for example, lead to heterotrophic-dominated associations that are characteristic of temperate to cool-water carbonates. Failure to recognize the influence of such environmental factors that shift the grain associations towards heterotrophic communities in low latitudes can lead to misinterpretation of climatic conditions in the past. Modern analogues of low-latitude heterozoan carbonates help to recognize and understand past occurrences of heterozoan warm-water carbonates. Careful analysis of such sediments therefore is required in order to achieve robust reconstructions of past climate. [ABSTRACT FROM AUTHOR]

The quinuclidinolysis of 4-nitrophenyl, 2,4-dinitrophenyl and 2,4,6-trinitrophenyl O-ethyl dithiocarbonates (1, 2, and 3, respectively) are studied kinetically at 25.0 °C, ionic strength 0.2 M; the two former in water and the latter in 44 wt% ethanol–water. The Brønsted plots (log kN vs. pKa of quinuclidinium ions) for the reactions of 1 and 2 are linear with slopes β = 0.94 and 0.76, respectively, and that of 3 shows a smooth curvature. From these results and other arguments it can be concluded that the quinuclidinolysis of dithiocarbonate 1 in water is stepwise, in contrast to those of dithiocarbonates 2 in water and 3 in aqueous ethanol, which are concerted. The kinetics and mechanistic effects of the leaving and electrophilic groups and the amine nature are discussed. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The phenolysis of phenyl S-4-nitrophenyl thiolcarbonate (1), 4-chlorophenyl S-4-nitrophenyl thiolcarbonate (2), phenyl 4-nitrophenyl thionocarbonate (3), 3-chlorophenyl 4-nitrophenyl thionocarbonate (4) and 3-methoxyphenyl 4-nitrophenyl thionocarbonate (5) are studied kinetically in 44 wt% ethanol–water, at 25.0 °C, ionic strength 0.2 M. The Brønsted plots (log kNvs. pKa of phenols) for the reactions of 1 and 2 are linear and those of 3,4 and 5 are curved. The Brønsted slopes for 1 and 2 are 0.64 and 0.60, respectively, which suggest a concerted mechanism for these reactions. The curved Brønsted plots for the reactions of 3, 4 and 5 show slope values of 1.0, 1.1 and 1.1, respectively, at low pKa, and 0.39, 0.28 and 0.34, respectively, at high pKa. The shape and slope values of these Brønsted plots are in accordance with a stepwise mechanism. The reactivity towards phenoxides of the thiocarbonates increases with the increase in the Hammett σ value for the substituents in the non-leaving group. The fact that the phenolysis of 3 is stepwise, whereas that of phenyl 4-nitrophenyl carbonate is concerted can be attributed to the destabilization of the intermediate T-. A similar argument explains the stepwise phenolysis of 3, in contrast to the concerted phenolysis of 1, where there is an additional destabilization of T- caused by the change of the leaving group, from 4-nitrophenoxide in 3 to 4-nitrobenzenethiolate in 1. Thiolcarbonates 1 and 2 in 44 wt% ethanol–water are less reactive towards phenoxide anions (ca. 100-fold) than their corresponding oxycarbonates in water. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Background. Despite recognized risks associated with hyperphosphataemia in patients with chronic kidney disease (CKD) Stage 5 on dialysis, the achievement of target levels of serum phosphate is poor. It is likely that this is partly due to poor adherence by patients to their phosphate-binder treatment regimens, which often comprise large daily tablet burdens. [ABSTRACT FROM PUBLISHER]

Two new bryozoan species are described from the Upper Ordovician Sassito Formation of the Argentinean Precordillera: Moyerella  spinata sp. nov. and Phylloporina  sassitoensis sp. nov. The bryozoans are found in cool-water carbonates. The Silurian genus Moyerella is reported the first time in the Ordovician, showing palaeobiogeographic connections with Estonia and Siberia. [ABSTRACT FROM AUTHOR]

A CO 2-fixation reaction of 4-(benzylamino)-2-butynyl carbonates and benzoates, carried out in the presence of DBU, provides substituted 5-vinylideneoxazolidin-2-ones. The reaction has been successfully applied to the CO 2-recycling process and fixation of atmospheric CO 2. [ABSTRACT FROM AUTHOR]

Suzuki-Miyaura coupling of diarylmethyl carbonates with arylboronic acids proceeded in the presence of Pd(3-C3H5)Cl2−DPPPent (1,5-bis(diphenylphosphino)pentane) catalyst, yielding a variety of triarylmethanes. [ABSTRACT FROM AUTHOR]

Far-from-equilibrium dissolution experiments with diopside and basaltic glass in mixed-flow reactors at 70°C and pH 8.2 show that solute concentrations do not reach steady state over the experimental duration of 45–60 days. Chemical modelling indicates that during the dissolution experiments, solutions have become supersaturated with respect to carbonates in the case of diopside, and carbonates, clay minerals and zeolites in the case of the basaltic glass. Decreasing dissolution is therefore interpreted as a result of secondary surface precipitates blocking the reactive surface area. Calcite formation was supported in both experiments by a significant increase in Ca (and Sr) concentrations as pH was abruptly lowered from 8.2 to 7 because this change increased carbonate solubility and caused all potential carbonate precipitates to re-dissolve. The reduction in pH also led to an increase in Si concentration for diopside and a decrease in Si concentration for basaltic glass. This observation is in accordance with previous experiments on the pH-dependent dissolution rates of pyroxenes and basaltic glass. [ABSTRACT FROM AUTHOR]

Storage of CO2 as solid Ca, Mg, and Fe carbonates in basaltic rocks may provide a long-lasting solution for reduction of industrial CO2 emissions. Here, we report on the underground stratigraphy of the chemical composition and crystallinity of rocks and their alteration state at a targeted field site for injection of CO2-charged waters, the Hellisheidi area in SW Iceland. [ABSTRACT FROM AUTHOR]

Spherical 10 nm rhabdophane (LaPO4·H2O) particles were made by controlled precipitation in water using lanthanum citrate chelate and phosphoric acid (La-Cit+/H3PO4) at a temperature of 30°C. Rod-shaped 10 nm × 100 nm rhabdophane particles were made by direct precipitation from lanthanum nitrate and phosphoric acid (La(NO3)3/H3PO4). The lanthanum nitrate to phosphoric acid molar ratios (La:P) were fixed at 1:1 and 1:5 for both methods to study their effect on particle size and shape. The particles were characterized with DTA/TGA, XRD, TEM, SEM, Fourier Transform infrared spectroscopy, and ζ potential measurements. The spherical particle surfaces had absorbed citric acid. The point of zero charge of the rod-shaped and spherical particles was pH 5.4 and 4.3, respectively. Formation mechanisms for the spherical particles are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Studies of commercial heats of AF1410 steel suggest that under appropriate conditions additions of rare-earth elements can significantly enhance fracture toughness. This improvement in toughness is not due to an extremely low inclusion volume fraction but is apparently due to the formation of larger and more widely spaced inclusions. The purpose of this work is to discuss our experience in using rare-earth additions to laboratory scale vacuum induction melted and subsequently vacuum arc remelted heats of ultra-high strength steels to achieve inclusion distributions similar to those observed in commercial heats modified with lanthanum additions. The results indicate that lanthanum additions of 0.015wt.% to low sulfur steels which have been well deoxidized using carbon-vacuum deoxidation can result in lanthanum rich inclusions which are similar in size, volume fraction and spacing to those obtained in commercially produced heats of ultra-high strength steel to which lanthanum has been added. The heat of steel to which lanthanum additions of 0.015wt.% were made had significantly higher toughness than did the heat of the same steel in which the sulfur had been gettered as small and closely spaced particles of MnS and which had an inclusion volume fraction similar to that of the heat modified by the addition of 0.015wt.% lanthanum. This improvement in toughness was attributed to an increase in inclusion spacing. An addition of 0.06wt.% lanthanum was excessive. Such an addition of lanthanum resulted in a huge volume fraction of large cuboidal inclusions which primarily contain lanthanum and oxygen and which are extremely detrimental to toughness. [Copyright &y& Elsevier]

Abstract: Strontium doped lanthanum vanadates, La1−x Sr x VO3 (denoted LSV with x =0, 0.3, 0.5, 0.7 and 1.0), were synthesized using a solid-state reaction method. The oxidation and reduction behaviors of LSV were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The chemical stability of LSV in a reducing atmosphere containing H2S up to 10 vol.% at 950 °C was investigated and their electrical conductivity at 500–1000 °C in reducing atmospheres measured with a 4-electrode technique. The coefficient of thermal expansion (CTE) was determined by dilatometry. The electrochemical catalytic activity of LSV as anodes in solid oxide fuel cells (SOFCs) was characterized using impedance spectroscopy. It was found that the La1−x Sr x VO3 materials with x in the range of 0.3–0.7 exhibit good resistance to H2S, high electrical conductivity and acceptable catalytic activity at elevated temperatures in reducing atmosphere. However, the volume change associated with oxidation/reduction cycle may limit their tolerance to re-oxidation when used as anode materials for SOFCs. [Copyright &y& Elsevier]

Abstract: The isotopic composition of lanthanum has been measured with high precision using a thermal ionization mass spectrometer, equipped with a Daly collector, whose linearity was verified by measuring an isotopically certified reference material for potassium (NIST 985), whose isotopes span a wide range of isotope ratios. The abundance sensitivity of the mass spectrometer in the vicinity of the measured LaO+ ion beams was examined to ensure the absence of tailing effects and interfering isotopes. The isotope fractionation of the lanthanum isotopes was estimated by reference to the isotopically certified reference material for lead (NIST 981) and the fractionation of oxygen isotopes determined from LaO+ measurements. These procedures are essential because of the extremely low isotope abundance of 138La. An accurate determination of the abundance of 138La is required in order to calculate the atomic weight, and because it is the parent nuclide of two geochronometers, 138La–138Ba and 138La–138Ce. The magnitude of the rare odd–odd neutron-deficient isotope 138La is a key nuclide in p-process nucleosynthetic calculations. The measured isotopic composition has been corrected for isotope fractionation to give 139La/138La=1125±3, which gives isotope abundances for 138La of 0.000888±0.000002 and 139La of 0.999112±0.000002. The isotope abundances and relative atomic masses of the two isotopes give an atomic weight of La of 138.905461±0.000003. [Copyright &y& Elsevier]

An effective interionic interaction potential (EIOP) approach is employed for the description of phase transitions and equations of state of lanthanum monochalcogenides LaX {X=S, Se, Te} compound semiconductors. The long-range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction up to second neighbor ions within the Hafemeister and Flygare approach with modified ionic charge are properly incorporated in an EIOP. We have obtained a reasonably good agreement between the present theoretical and available experimental values on the phase transition pressures ( P t =25.5, 12.4 and 16.8 GPa) and elastic properties in all materials under consideration. The associated volume collapses at the phase transitions have been found to be 8.1%, 10.4% and 7.2% for lanthanum monochalcogenides. The vast volume discontinuity in pressure volume phase diagram identifies the structural phase transition from NaCl type (B1) to CsCl type (B2) structure. The variations of elastic constants and their combinations with pressure follow a systematic trend identical to that observed in other compounds of NaCl type structure family. The present approach has also succeeded in predicting the Born and relative stability criteria. [ABSTRACT FROM AUTHOR]

Background/Aims: Hyperphosphatemia is an important clinical consequence of renal failure, and its multiple adverse systemic effects are associated with significantly increased risks of morbidity and mortality in dialysis patients. Existing oral phosphate binders have not permitted control of serum phosphate within currently accepted guidelines. This study compares lanthanum carbonate with calcium carbonate for control of serum phosphate in hemodialysis patients. Methods: In this European multicentre study, 800 patients were randomised to receive either lanthanum or calcium carbonate and the dose titrated over 5 weeks to achieve control of serum phosphate. Serum levels of phosphate, calcium and parathryoid hormone were followed over the following 20 weeks. Results: Around 65% of patients in each group achieved phosphate control, but in the calcium carbonate group this was at the expense of significant hypercalcemia (20.2% of patients vs. 0.4%). Consequently, calcium x phosphate product tended to be better controlled in the lanthanum group. Conclusion: This 6-month study demonstrates that serum phosphate control with lanthanum carbonate (750–3,000 mg/day) is similar to that seen with calcium carbonate (1,500–9,000 mg/day), but with a significantly reduced incidence of hypercalcemia. Lanthanum carbonate is well tolerated and may be more effective in reducing calcium x phosphate product than calcium carbonate. Copyright © 2005 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

It is demonstrated by ESR measurements that O(CO + O2) radical anions result from CO + O2 adsorption on the oxidized surface of CeO2. These radical anions are stabilized in the coordination sphere of Ce4+ cations located in isolated and associated anionic vacancies. This reaction shows an activation behavior determined by CO adsorption. The variation of O(CO + O2) concentration with CO adsorption temperature suggests that surface carbonates and carboxylates participate in this reaction. In the (0.5– 10.0)%CeO2/ZrO2 system, O forms on supported CeO2 and is stabilized on Ce4+ and Zr4+ cations. The stability of O-Ce4+ complexes is lower on supported CeO2 than on unsupported CeO2, indicating a strong interaction between the cerium cations and the support. [ABSTRACT FROM AUTHOR]

It is established by ESR that the adsorption of an NO + O2 mixture at 20°C on oxidized CeO2 (O2, T = 400–700°C) produces radical anions O located both on isolated Ce4+ cations (O(1)) and in associated anionic vacancies (O(2)). These species differ in thermal stability. For example, O(2) decomposes at 20°C, while O(1) decomposes at 50°C. Only O (1) species are observed at −196°C in ZrO2-supported CeO2. In the case of NO + O2 adsorption at 20°C, O is stabilized on Zr4+ cations and decomposes at 270°C. Increasing the cerium oxide content of the ZrO2 surface from 0.5 to 10% only partially inhibits the formation of O-Zr4+. The Zr4+ cation is shown to possess a higher Lewis acidity than the Ce4+ cation, and the ionic bond in O-Zr4+ complexes is stronger than that in O-Ce4+ complexes. ESR, temperature-programmed desorption, and IR spectroscopic data for various adsorption complexes of NO on CeO2 suggest that, in the key step of O formation, free electrons appear on the surface owing to the conversion of adsorbed NO molecules into nitrito chelates on coordinately unsaturated ion pairs Ce4+-O. [ABSTRACT FROM AUTHOR]

The ETS-l 0 molecular sieve was modified with lanthanum by ion-exchange method. The lanthanum modified ETS-10 shows best thermal and hydrothermal stability in comparison with the as-synthesized ETS-10 and ammonium-exchanged ETS-10. Moreover, the lanthanum modified ETS-10 has an increase in the conversion of n-hexadecane by more than 3%, compared with the ammonium-exchanged ETS-10. And the resonable speculation was presented based on the detailed characterization of these samples by DSC FT-IR, XRD, XRF, N2 physisorption. [ABSTRACT FROM AUTHOR]

We examined the thermal stabilization of a catalyst support by surface modification of γ-alumina using the homogeneous precipitation reaction of aqueous lanthanum species with hexamethylenetetramine. The phase transition of alumina, its surface area loss and the solid state compounds with lanthanum oxides at elevated temperatures were examined in the temperature range of 600–1400 °C. At 1200 °C, the surface area was as high as 88–92 m2/g for 0.5–1.5 mol% La addition compared with 45 m2/g for pure alumina. At 1300 °C, 1.5 mol% La-modified Al2O3 showed the 27 m2/g, which was higher than 6 m2/g for pure alumina. At temperatures higher than 800 °C, major crystalline lanthanum species was present as LaAlO3, however, over 1300 °C crystalline LaAl11O18 appeared. In the wide temperature range of 800–1300 °C, the surface area of the alumina with the homogeneous precipitation of lanthanum nitrate was larger than the impregnated support. The effect of less aggregate state of La-modified alumina was assumed for the present process. [Copyright &y& Elsevier]

A new lanthanum antimony chalcogenide compound has been synthesized by the combination of ZnO–Sb2S3–La2O3 at 1150 °C and characterized by single-crystal X-ray diffraction. The compound La6Sb4O12S3 crystallizes in the tetragonal space group I41/amd with a=10.860(3) Å, c=15.316(5) Å, and Z=4. The structure comprises a three-dimensional network of lanthanum oxysulfide polyhedra connected by antimony(III) oxide psi-tetrahedra. The structure forms around channels that are filled with a disordered sulfide anion that appears to be loosely held in the solid. Diffuse reflectance measurements showed that this compound has an optical band gap of 2.2 eV. Solid state Raman spectroscopy of this compound showed the vibrations expected for the [SbIIIO3]3- unit. According to thermal analysis data, this material decomposes at temperatures over 1050 °C. [Copyright &y& Elsevier]

Slowly heating LaI2 under 1 atm hydrogen to 650 °C leads to LaI2H, analyzed as LaI2H0.95(3), accompanied by a structural change from tetragonal to hexagonal. Sharp reflections in the XRD pattern can be indexed in P63/mmc with a=4.2158(7) Å and c=15.508(3) Å, however, diffuse reflections indicate a polytypic intergrowth of MoS2 and NbS2 type structures. The earlier determined miscibility gap between LaI2 and LaI2Hx (x≈0.5) is closely related to the change of the structure. The Pauli paramagnetism for LaI2, χP = 43(2)×10-6 cm3/mol, derived from measurements on LaI2 and LaI2H is in good agreement with the value estimated from band structure calculations. [Copyright &y& Elsevier]

Interaction of [Ce(LOEt)2(NO3)2] (LOEt− = [Co(η5-C5H5){P(O)(OEt)2}3]−) with (NH4)6[Mo7O24] in water affords the cerium(iv)-containing oxomolybdenum cluster [H4(CeLOEt)6Mo9O38], which exhibits a unique Ce6Mo9O38 core structure. [ABSTRACT FROM AUTHOR]

The tetravalent organometallic cerium complex [CeL4] is readily accessible from the oxidation of the trivalent [CeL3], L = a bidentate N-heterocyclic carbene alkoxide ligand, [C{(NPri)CHCHN}CH2CMe2O]. The [CeL4] complex should behave like the [UL4] analogue, but the two complexes show significantly different structures, highlighting the differences between 4f and 5f metals. [ABSTRACT FROM AUTHOR]

The article reports selective synthesis of cubic cerium oxide nanoparticles by an alkoxide-primary amine surfactant method in a solution system at the near room temperature of 353 Kelvin. The prepared cerium oxide cubic nanoparticles had diameters of 2.7-3.8 nm, characteristics of a mono-dispersed and highly crystalline structure, and the cerium oxide calcined at 673 Kelvin using freeze-dried cerium oxide and formed a rod-like structure with diameters of 20-50 nm and lengths of 100-200 nm.

The bastnasite of Baotou (China) was roasted in concentrated sulfuric acid at 250–300 °C and the calcined products were leached by water. Almost all rare earths (RE) were moved into solutions in trivalent along with some radioactive impurity thorium(IV) (Th(IV))which accounts for 0.4% of RE and other impurities such as Fe(III), Ca, F, P, etc. Through fractional extraction (seven stages for extraction and nine for scrubbing), the mass ratio of Th(IV) and RE (ThO2/REO) in solution has decreased to 5×10-6. The purity of ThO2 product recovered from organic phase is above 99%. The iron(III) in solutions can be removed in the form of precipitation by adding some magnesia into the solutions. Then RE can be concentrated by solvent extraction with 2-ethylhexyl phosphinic acid 2-ethylhexylester (P507). The results of fractional extraction show that the concentration of total RE in aqueous solutions stripped by hydrochloric acid is over 200 g REO/l with the yield of RE above 99%. Individual RE can be attained by solvent extraction with P507 in the following process. [Copyright &y& Elsevier]

Reflection-absorption electronic spectroscopy was applied to measuring the electronic absorption spectra of the Ce3+ ions, corresponding to the 4f → 5d transition, in 2CsCl-NaCl, 2CsCl-NaCl-NaF, LiF-NaF, and LiF-NaF-ZrF4 melts. The variations in the absorption spectra of the cerium ion in these systems are due to the changes in the composition of the complex species and their symmetry. The dissolution of Ce2O3 in fluoride melts follows a chemical mechanism. The solubility of Ce2O3 in the LiF-NaF melt at 940 K was estimated. [ABSTRACT FROM AUTHOR]

Hydroxyl-bästnasite-(Ce), ideally CeCO3(OH), had been regarded isostructural with bästnasite-(Ce), CeCO3F, the dominant member of the bästnasite family that produces ~70% of the world's supply of rare-earth elements. Using single-crystal X-ray diffraction and Raman spectroscopy, our structural analysis on hydroxyl-bästnasite-(Ce) shows that the previous assumption is incorrect. The crystal structure of hydroxyl-bästnasite-(Ce) possesses P6̄ symmetry with unit-cell parameters a = 12.4112(2), c = 9.8511(3) Å, and V = 1314.2(1) ų, in contrast to the space group P6̄2c and a ≈ 7.10, c ≈ 9.76 Å, and V ≈ 430 ų for bästnasite-(Ce). Moreover, there are 6, 3, and 5 symmetrically-distinct CO3 groups, Ce cations, and (OH/F) ions, respectively, in hydroxyl-bästnasite-(Ce), but 1, 1, and 2 in bästnasite-(Ce). The two structures, nevertheless, are similarly characterized by the layers of CO3 groups alternating with the Ce-(OH/F) layers along the c direction. The Raman spectrum of hydroxyl-bästnasite-(Ce) is dominated by three strong bands at 1080, 1087, and 1098 cm-1 in the CO3 symmetrical stretching region, along with at least four bands in the OH stretching region. Our study further suggests that natural hydroxyl-bästnasite-(Nd) is most likely isotypic with hydroxyl-bästnasite-(Ce), rather than with bästnasite-(Ce), as previously proposed. [ABSTRACT FROM AUTHOR]

Background: Control of serum phosphate over the long term is essential in patients with end-stage renal disease. Six-month and 2-year extensions to a 6-month study evaluated the long-term safety, tolerability and efficacy of the new phosphate binder lanthanum carbonate. Methods: Patients who participated in a 6-month, randomized trial comparing lanthanum carbonate with calcium carbonate were eligible for a 24-week, open-label extension. Lanthanum carbonate-treated patients continued taking their established maintenance dose (‘continued-lanthanum group’) and calcium carbonate-treated patients switched to lanthanum carbonate, 375–3,000 mg/day (‘switch group’). Patients could also enter a further 2-year extension. Efficacy parameters, including serum phosphate, were monitored. Results: Mean serum phosphate was ∼1.80 mmol/l throughout the trial. The percentage of patients with controlled serum phosphate (≤1.80 mmol/l) after the 6-month extension was 63.3 and 58.4% in the continued-lanthanum and switch groups, respectively; after the 2-year extension, 54.4% of patients had controlled serum phosphate. After discontinuation of calcium carbonate and initiation of lanthanum carbonate, the hypercalcemia incidence was 2.7%, compared with 20.2% during the double-blind phase. Calcium × phosphate product was maintained at an acceptable level. Lanthanum carbonate was well tolerated; adverse events were mild/moderate and mainly gastrointestinal. Conclusions: Lanthanum carbonate maintains effectiveness with continued tolerability for up to 3 years. Copyright © 2006 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]