Hosts For Rare Earth Elements Research Articles

The Origin and Evolution of Rare Earth Element Mineralization in the Muluozhai Deposit (Sichuan, China): Insights from Mineralogical, Trace Element, and Sr-Nd-Pb-C-O-Ca Isotope Data

Abstract Carbonatite complexes are globally significant sources of rare earth elements (REEs); however, mechanisms governing REE deposition in various tectono-lithologic settings, encompassing host rocks, wall rocks, ore-controlling structures, and metasomatism, remain inadequately understood. The Zhengjialiangzi mining camp, situated within the extensive Muluozhai deposit (containing 0.45 million metric tons [Mt] at 4.0 wt % REE2O3) in the northern segment of the Mianning-Dechang belt, Sichuan (southwestern China), is characterized by a complex vein system that evolved within metamorphosed supracrustal rocks of the Yangxin and Mount Emei Formations. The mineralization is coeval with Oligocene intrusions of carbonatite and nordmarkite at ~27 Ma. The major gangue minerals include fluorite, barite (transitional to celestine), and calcite, with bastnäsite serving as the primary host for REEs in all analyzed orebodies. Several other accessory to minor minerals were identified in the ore veins, including some that had not previously been known to occur in the Muluozhai deposits (e.g., thorite and pyrochlore). The stable isotopic (C-O-Ca) and trace element compositions of calcite, along with whole-rock data, suggest that carbonate material was derived from the mantle and subsequently reequilibrated with the Yangxin marbles. The radiogenic isotope (Sr-Nd-Pb) compositions of vein material remained unaffected by wall-rock contamination and suggest a mantle source influenced by crustal recycling, consistent with other REE deposits hosted by carbonatite and nordmarkite in the region. The combined petrographic and geochemical evidence suggests derivation of Muluozhai mineralization from a carbonatitic source and interaction of carbonatite-derived fluids with wall rocks, xenoliths, and early-crystallizing mineral phases, particularly barite.

Microdistribution and Mode of Rare Earth Element Occurrence in the Zhijin Rare Earth Element-Bearing Phosphate Deposit, Guizhou, China

Rare-earth elements (REEs) are often highly concentrated in sedimentary phosphate deposits, and the microdistribution characteristics and occurrence state of rare earth in these deposits play a crucial role in the overall development and utilization of mineral resources. This study aims to analyze the microdistribution of REEs in REE-bearing phosphate deposits in the Zhijin region of Guizhou at the microstructural level and investigate their occurrence modes. Specifically, rock and mineral identification, X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM-EDS), and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) were utilized to analyze the samples. The correlation between the distribution of REEs and phosphorus was examined. In addition, the microdistribution of REEs in specific mineral phases and the locations of their occurrence were investigated. The analysis revealed that no REEs existed independently in the deposit. Instead, the distribution of REEs was highly consistent and significantly positively correlated with that of phosphorus. In the microarea structure, REEs were predominantly found both in particles, such as bioclasts, sand debris, and agglomerates, and in phosphate cement, where the main mineral components were collophane and apatite. Conversely, the content of REEs in dolomitized sand debris edges, sparry dolomitic cement, and siliceous cement was considerably lower. Based on these findings, it is speculated that REEs primarily occur within the lattice defects of apatite or on the surface of collophanite. There is a notable contrast in the REE content between the unaltered sand debris at the periphery and the dolomitized sand debris, indicating that the dolomitization in the diagenetic stage resulted in a depletion of REE abundance in the ore. Obviously, the dominant gangue mineral, dolomite, does not serve as the primary host for REEs. Furthermore, the highest concentration of REEs was inside organisms. This finding suggests that the high content of REEs in biological soft tissue may remain under the influence of waves and tides, and REE-bearing apatite may be preferentially separated and fill the cavities of deceased organisms. The second highest content of REEs was found in the shells of organisms, indicating that small shelly organisms absorb phosphorus materials through their life activities to construct their shells, resulting in REE enrichment. Quantitative analysis through sequential extraction procedures displayed that most REEs were present in the residual state, with a smaller portion combined with organic matter. These results confirm that REEs in the Zhijin phosphate deposits primarily exist as isomorphic substitutions in the lattice defects of apatite, with a secondary occurrence as organic matter-bound REEs.

Distribution of rare earth elements and implication for Ce anomalies in the clay-sized minerals of deep-sea sediment, Western Pacific Ocean

Recent studies reveal that bioapatite and FeMn micro-nodules are predominantly hosts of rare earth elements and yttrium (REYs) in bulk deep-sea sediments by in-situ analysis and chemical extraction. However, individual clay-sized fraction (<2 μm; CSFS) could also be an important REY carriers, in which chemical species and distribution of REYs are unclear. In order to clarify the REY migration or cycle and light/heavy REE enrichment mechanism in different clay-sized minerals, this study undertook clay-sized separation, sequential chemical extraction, X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF) and inductively coupled plasma–mass spectrometry (ICP-MS) analysis of REY-rich sediment samples from two sites in the western Pacific Ocean. The results show the REY in CSFS are mainly hosted in the amorphous Fe phase (Feox1), with lesser amounts in carbonate and the crystalline Fe phase (Feox2), with average proportions of 68.0%, 27.5%, and 2.9% at site 30, and 75.2%, 16.2%, and 7.1% at site 64, respectively. The bulk sediments and CSFS show no obvious Eu anomalies, suggesting there was limited hydrothermal input. Carbonate may be an authigenic phase that inherited the REY characteristics of seawater by substituting for Ca2+, and organic degradation resulted in larger negative Ce anomalies than in seawater. The ΣREY contents increase and the heavy REEs are more readily adsorbed onto the Feox1 under oxidizing conditions. The Feox2 phase may be transformed from Feox1, which facilitates the release of light REEs. The negative Ce anomalies in Fe phase minerals are not solely caused by the depositional reduce conditions because an obvious change of Ce anomaly in different iron phase minerals.

SHRIMP U–Pb ages, mineralogy, and geochemistry of carbonatite–alkaline complexes of the Sillai Patti and Koga areas, NW Pakistan: Implications for petrogenesis and REE mineralization

Carbonatites and related alkaline igneous rocks are important hosts of rare earth elements (REEs). Carbonatite–alkaline complexes in NW Pakistan represent rare examples of carbonatite emplaced in a collisional orogenic belt. However, the precise age, magmatic source, and petrogenesis of carbonatites emplaced in collisional settings and associated REE mineralization remain uncertain. The present study focuses on the origin and REE mineralization potential of alkaline–carbonatite complexes exposed in the Koga and Sillai Patti (SP) areas, NW Pakistan.The Koga carbonatites occur as minor plug-like intrusions in syenites. SHRIMP U–Pb dating of zircons from the Koga syenites, which are spatially associated with carbonatites, yield weighted mean ages of 283.6 ± 1.7(1σ), 281.9 ± 1.6(1σ), 282.6 ± 1.6(1σ), and 306.5 ± 3.7 Ma (1σ). The Koga syenites are characterized by high SiO2 (62.79–65.58 wt%) and Al2O3 (16.92–18.55 wt%) and low TiO2 (0.10–0.59 wt%) contents and are enriched in alkalis (Na2O > K2O). Sr and Nd isotopes from the Koga carbonatite complex are characterized by positive εNd(t) values (+3.10 to + 3.47) and low (87Sr/86Sr)i values (0.70233–0.70382), which are consistent with rift-related carbonatites.The SP carbonatites yield high total REE contents (1768–1920 ppm) and are enriched in light REEs (LREEs) relative to heavy REEs (HREEs; LREE/HREE = 20.0–20.7). The results of Sr–Nd isotope analyses show that the SP carbonatite has high Sr and low Nd isotopic ratios with negative εNd(t) values ranging from − 1.71 to − 2.21, variable 207Pb/206Pb ratios, and anomalous Pb–Sr and Pb–Nd patterns, suggesting Pb contamination during ascent of the primary carbonatite melts. Values of δ18OSMOW and δ13CPDB (‰) for the SP carbonates vary from + 14.45‰ to + 18.12‰ and from − 6.57‰ to − 9.70‰, respectively, which can be attributed to low-temperature alteration.On the basis of data for bulk-rock trace elements and Sr–Nd–Pb–C–O isotopes, it is concluded that the Koga carbonatite–alkaline complex was formed during the rifting and break-up of the northern margin of Gondwanaland. In contrast, carbonatite of the SP carbonatite–alkaline complex was probably generated by partial melting of sub-continental lithospheric mantle in a post-collisional tectonic setting. Although the SP carbonatite contains abundant REEs, it does not host significant REE mineralization owing to the lack of a mature sequence of evolutionary phases and to possible phosphate mineral saturation in the primary magmas, which caused REE depletion in the later carbonatitic magmas through fractional crystallization. Therefore, we conclude that carbonatites that are relatively rich in apatite but poor in fluorite and barite may not be prospective for REE mineralization.

Complex REE systematics of carbonatites and weathering products from uniquely rich Mount Weld REE deposit, Western Australia

Carbonatite intrusive complexes are important hosts for Rare Earth Elements (REE) deposits and are commonly affected by surficial weathering processes. Mount Weld REE deposit of Western Australia is one of the world’s richest REE deposits and here we present whole-rock and REE mineral geochemical data on compositions of primary and weathered carbonatites. The REE concentrations in magmatic carbonatites at Mount Weld vary by two orders of magnitude, suggesting a significant role of magmatic processes in REE enrichment in the carbonatite complex. The Th-Pb age 2056±67 (2σ) Ma of monazite-(Ce) from the carbonatite obtained by in situ LA-ICP-MS analyses confirm Paleoproterozoic age of the mineralization. The regolith samples preserve a detailed record of evolution from carbonatite to products of intensive weathering. The silica-cemented regolith (silcrete) contains monazite-(Ce) and apatite chemically identical to their carbonatite-hosted equivalents. The high-REE regolith containing 51.8 wt% ∑REE2O3 has some of the highest REE concentrations found to date, negative Ce anomaly and contains lithogenic monazite within matrix of florencite-(Ce) and rhabdophane-(Nd) and rhabdophane-(Nd) tubes resembling casts of plant material. The pronounced negative Ce anomalies in the ferruginous cap rock (ferricrete) and the high-REE regolith indicate intensive weathering, where Ce4+ was preferential removed relative to the REE3+. The presence of paleo-plants in the high-REE regolith implicates bio-assisted processes involved in the extreme REE fractionation. The diverse REE enrichment processes, which occurred in supergene environment of Mount Weld, have implications for the understanding of the genesis of REE deposits worldwide.

First find of merrillite, Ca3(PO4)2, in a terrestrial environment as an inclusion in lower-mantle diamond

Abstract Merrillite, ideally Ca18Na2Mg2(PO4)14 (Dana No: 38.03.04.04; Strunz No: 08.AC.45), an analog to synthetic tricalcium phosphate β-Ca3(PO4)2, was identified as an inclusion in lower-mantle diamonds from the Rio Soriso area, Brazil. It was associated with former bridgmanite, CaSi- and CaTi-perovskites, and ferropericlase. This is the first report of merrillite in a terrestrial environment; previously, it was known only in meteorites and lunar rocks. The compositions of merrillite vary in different localities; the Rio Soriso sample was enriched in SO3 (2.03 wt%). Merrillite from lower-mantle diamonds may be a retrograde phase of the tuite [γ-Ca3(PO4)2]. Owing to their crystal structures, both merrillite and tuite may be important potential hosts for rare earth elements (REE) and large ion lithophile elements (LILE), including Sr and Ba, in the deep Earth. The find of merrillite suggests a larger variety of mineral species in the lower mantle than previously assumed.

Mineralogical and Geochemical Characteristics of Lithium and Rare Earth Elements in High-Sulfur Coal from the Donggou Mine, Chongqing, Southwestern China

Coal and coal by-products are considered as the potential raw materials for critical elements (e.g., rare earth elements, Li, Ga, Ge, etc.), which have attracted much attention in recent years. The purpose of this study is to investigate the mineralogical and geochemical characteristics, and controlling geological factors of lithium and rare earth elements in the Lopingian (Wujiaping Formation) coal from the Donggou Mine, southeastern Chongqing Coalfield, China. Results indicate that lithium and rare earth elements are significantly enriched in the Donggou coals, which could be new potential alternative sources for critical elements. Concentrations of lithium and rare earth elements in the Donggou coals gradually increase from top to bottom. Lithium is mainly associated with kaolinite, while rhabdophane, florencite, goyazite, and xenotime are the main hosts of rare earth elements. The controlling geological factor is the groundwater leaching of underlying tuff, and to a lesser extent, the terrigenous clastic materials input from the top layer of the Kangdian Upland. This study provides mineralization information for lithium and rare earth elements exploration in coal measures.

Adsorption of REEs on kaolinite and halloysite: A link to the REE distribution on clays in the weathering crust of granite

The clay minerals, kaolinite and halloysite are dominant hosts for rare earth elements (REEs) in the weathered crust elution-deposited REE deposits. However, the accumulation of REEs on kaolinite and halloysite has not been systematically compared. In this study, the adsorption of complete REE series on kaolinite and halloysite was investigated under different pH levels and ionic strengths, which was linked to the enrichment and fractionation of REEs within the clay fraction of a typical weathered crust elution-deposited REE deposit. At low ionic strength, the adsorption of REEs on kaolinite and halloysite increased with increase in pH, with some noticeable fluctuation observed on halloysite at high pH. All the REEs were adsorbed to a similar extent without apparent fractionation, except for the slight enrichment of heavy REEs (HREEs) at high pH. At high ionic strength, REE adsorption exhibited a linear increase with the increase in pH, particularly at high pH, with HREEs being preferentially adsorbed. Compared to halloysite, kaolinite possessed a higher specific surface area (SSA) normalized adsorption capacity towards REEs. These observed adsorption characteristics could possibly explain the distribution of REEs in the clay fraction along the REE deposit. The decrease of ion-exchangeable REE content with depth was significant for kaolinite, while the REE fractionation was ascribed to the selective adsorption of HREEs on both kaolinite and halloysite. The enrichment and fractionation mechanism of REEs on kaolinite and halloysite were also discussed in terms of the surface chemistry and morphology of the clay minerals and the variations of chemical properties across the REE group.

Crystal chemistry of Eu-bearing tuite synthesized at high-pressure and high-temperature conditions

Tuite, γ-Ca3(PO4)2, is a potential host for rare-earth elements in the deep mantle. Eu-bearing tuite single crystals with chemical formulas of Ca2.93Eu0.04(PO4)2 and Ca2.88Eu0.07Na0.06(PO4)2 were synthesized at 16 GPa and 1600 °C, and their structures were investigated at room temperature with single-crystal X-ray diffraction. The structure refinements of Eu-bearing tuite indicate the preference of Eu3+ for the smaller 6-coordinated Ca1 site with a coupled vacancy due to the substitution of 2Eu3+ + □ → 3 Ca2+. In NaEu-bearing tuite, both Eu3+ and Na+ occupy the smaller 6-coordinated Ca1 site according to the coupled substitution of Eu3+ + Na+ → 2 Ca2+. The presence of Na results in more Eu2O3 in NaEu-bearing tuite, because the coupled substitution is energetically more favorable. REE-bearing tuite could be found on the Moon and the deep Earth’s mantle as a consequence of REE-bearing apatite or/and merrillite transformation under high-pressure and high-temperature conditions.

Trace element distribution in mineral inclusions in zoned garnets from eclogites of the Atbashi Range (South Tianshan)

Ion microprobe data for minerals from the eclogites of the Atbashi Range (South Tianshan) constrain the distribution of trace (Rb, Sr, Ba, Cr, V, Zr, Hf, Nb, Ta, U, Th, and Y) and rare-earth elements (REE) in zoned garnets and mineral inclusions in them. This study showed that garnets from the Atbashi eclogites are the main hosts for heavy REE; epidotes are important hosts for REE, Y, Sr, Th, and U; and omphacites are depleted in almost all trace elements compared with the bulk-rock compositions. Garnet, as well as epidote and omphacite inclusions exhibit systematic rimward depletion in a number of trace elements, which is related to the depletion of the rock matrix in these elements during crystallization. Deviations from this trend, including the enrichment of garnet rims in HREE and strong variations in the REE contents of garnets and mineral inclusions, can be explained by metamorphic reactions involving the destabilization of REE-bearing minerals. Our data suggest that the mobility of trace elements under eclogites-facies conditions is mainly controlled by the stability of certain minerals.

REE geochemical characteristics of titanium-rich bauxites: the Permian Kanigorgeh horizon, NW Iran

The Permian Kanigorgeh bauxite horizon is one of the typical titanium-rich bauxite deposits in the northeast of Bukan, West Azerbaijan Province, NW Iran. It is a part of the Irano-Himalayan karst bauxite belt and was developed as discontinuous stratified layers and lenses in Upper Permian carbonates. Mineralogically, diaspore and kaolinite are 2 major mineral phases accompanied by boehmite, smectite, illite, rutile, anatase, hematite, goethite, chlorite, quartz, and plagioclase as minor phases. Geochemical considerations for a selected profile indicate that the concentration values of rare earth elements (REEs) in the bauxite ores range from 8.9 to 200.4 ppm. Eu and Ce anomalies show varying values within the range 0.42-0.65 and 0.12-3.23, respectively. On the basis of Ce behavior, the studied profile can be divided into 2 parts by redox boundary. The most pronounced geochemical characteristics of the studied profile are the occurrences of positive Ce anomalies in the ores of low REE content in the upper part of the profile and negative Ce anomalies in the ores of high REE content in the lower part of the profile. The occurrence of positive and negative Ce anomalies are due to the change of oxidation state of Ce (from Ce3+ to Ce4+) in the upper part and the complexing of Ce4+ with carbonate ligands in the lower part of the profile. The concordance of the analytical values of Na and Ca with those of Eu revealed that the degree of plagioclase alteration is the most important controlling factor for Eu anomalies in the ores. The obtained results revealed that the variations in the chemistry of weathering solutions (e.g., pH), function of carbonate bedrocks as a geochemical barrier, and discrepancy in degree of stability of lanthanide-bearing primary minerals are the major factors influencing the mobilization, distribution, and fractionation of REEs. The correlation coefficients among elements suggest that Mn-oxides are the principal hosts for REEs in the ores at Kanigorgeh.

P-V-T relations of -Ca3(PO4)2 tuite determined by in situ X-ray diffraction in a large-volume high-pressure apparatus

Tuite, γ-Ca3(PO4)2, is regarded as an important phosphate mineral in the deep mantle playing a crucial role as a host for rare earth elements, large ion lithophile elements, and phosphorus. In this study we report the thermoelastic properties of synthetic γ-Ca3(PO4)2 at simultaneously high pressures and temperatures of up to 35.4 GPa and 1300 K, respectively, as determined by means of in situ energydispersive X‑ray diffraction in a large-volume multi-anvil apparatus. The pressure-volume-temperature data obtained for γ-Ca3(PO4)2 were fitted by the high-temperature Birch-Murnaghan equation of state to yield V2 = 447.4(4) Å3, KT0 = 100.8(18) GPa, K′T0 = 5.74(13), (∂KT/∂T)P = -0.020(1) GPa/K, and αT = 3.26(18) × 10-5 + 1.76(24) × 10-8 T. In addition, fitting the present data to the Mie-Grüneisen-Debye equation of state gives γ0 = 1.35(6), Θ0 = 944(136) K, and q = 0.37(29). Based on the thermoelastic properties obtained in our study, the density profiles of γ-Ca3(PO4)2 tuite along typical cold and hot slab geotherms were calculated and are compared with those of the coexisting silicate minerals in subducting mid-ocean ridge basalt.

Provenance and tectonic setting of the metapelites deposits in the Bulfat Complex, NE-Iraq

Major and trace elements including rare earth elements (REEs) chemistry of the metapelitic rocks of Bulfat Complex (Iraqi Zagros Suture Zone) indicate their enrichment in large-ion Lithophile, light rare earth (LREE) elements, and relative depletion of high field strength and heavy rare earth (HREE) elements. The linear correlation coefficients between TiO2, K2O, and Al2O3 and total REE reveal that phyllosilicates (e.g., mica) and accessory minerals mainly Ti-bearing phases (e.g., ilmenite) are likely the dominant hosts for REEs. Chondrite-normalized REE patterns typical of continental margin settings with significant enrichment of LREE, prominent negative Eu anomalies, and nearly flat HREE are positively correlated with post-Archean Australian shale (PAAS) and upper continental crust (UCC) patterns. Additionally, their consistent elemental La/Sc, Th/Sc, La/Co, Th/Co, Cr/Th, and Eu/Eu* values suggest that sediments may have been originally derived from an old post-Archean upper continental crust composed chiefly of granitic component. It seems most likely that the felsic source rocks were originated by a process of intra-crustal differentiation such as partial melting and/or fractional crystallization involving fractionation of Ca-plagioclase. The geochemical evidences particularly REEs evaluation show that deposition of clasts occurred in an active continental margin setting during lower–upper Cretaceous period contemporaneous with the igneous activities. It is evident therefore that the clasts source is from the north–northeast side, i.e., from the active margin of Iranian microcontinent (Sanandaj–Sirjan Zone).

Recognizing juvenile and relict lithospheric mantle beneath the North China Craton: Combined analysis of H2O, major and trace elements and Sr–Nd isotope compositions of clinopyroxenes

Recognizing juvenile and relict lithospheric mantle is crucial in unraveling the mechanism of lithospheric thinning (delamination vs. thermal/mechanic erosion). The H2O contents and Sr–Nd isotopic compositions of juvenile lithospheric mantle are expected to be similar to the MORB source (asthenospheric mantle), whereas those of the relict lithospheric mantle should be of typical cratonic character. Consequently, combined analysis of H2O contents and Sr–Nd isotope compositions could be an effective way to distinguish the juvenile and relict lithospheric mantle. Among the peridotite minerals, clinopyroxene is the major host for rare earth elements as well as H2O contents, making it the most suitable target sample for such analyses.We collected fresh peridotite xenoliths hosted by Cenozoic basalts from Beiyan, Shandong province and Yangyuan, Hebei province to carry out combined analyses of major elements, trace elements, Sr–Nd isotopes and H2O contents for clinopyroxene. At both Beiyan and Yangyuan, pyroxene from peridotite xenoliths shows homogenous H2O contents within individual grains, and equilibrium distribution of H2O contents between clinopyroxene and orthopyroxene has been achieved. There is a positive correlation between H2O contents and Al contents in clinopyroxene and orthopyroxene, these features imply that the pyroxenes largely preserve the H2O contents of their mantle source. The variations of H2O contents in clinopyroxene are controlled by partial melting rather than the later episode of mantle metasomatism, because there is a correlation between H2O contents of clinopyroxene and degree of partial melting index (Yb content of clinopyroxene and Mg# of olivine).Based on the correlation between H2O contents and Sr isotope ratios of clinopyroxene, the estimated H2O contents and 87Sr/86Sr ratios of mantle source of peridotites in Beiyan (450 to less than 600ppm, and ~0.7028 respectively) are similar to the MORB source, thereby implying that the lithospheric mantle beneath Beiyan is juvenile. In contrast, the variation of H2O contents and Sr–Nd isotope compositions of clinopyroxene from the Yangyuan peridotites is best explained as relict mantle (H2O contents less than 300ppm and EM1-type Sr–Nd isotope ratios) coexisting with juvenile lithospheric mantle (H2O contents more than 600ppm and 87Sr/86Sr about 0.7030). These conclusions are in agreement with previous studies which have demonstrated that the lithospheric mantle beneath Beiyan is made up of juvenile material accreted from the asthenosphere after the North China Craton had undergone thinning.

Geochemistry of Major, Trace, and Rare Earth Elements in Biglar Permo-Triassic Bauxite Deposit, Northwest of Abgarm, Ghazvin Province, Iran

Biglar Permo-Triassic bauxite deposit is located in ~15 km northwest of Abgarm, southwest of Ghazvin province, west of central Iran. It consists of 8 stratiform and discontinuous bauxite lenses lying along the contact of Ruteh (Permian) and Elika (Triassic) carbonate formations. Petrographically, the bauxite ores exhibit collomorphic-fluidal, pseudo-breccia, pseudo-porphyritic, panidiomorphic-granular, nodular, and skeletal textures indicative of authigenic origin. Weathering of andesitic parent rocks led to the formation of Ferruginous laterite, bauxitic clay, and siliceous bauxite. Mass change calculations of elements indicate that Si, and Ba were depleted during bauxitization and Al, Ti, Zr, Nb, Hf, Ga, U, Th, V, and Cr were enriched. However, Fe, Y, Rb, Sr, Co, Ni, LREEs, and HREEs experienced leaching-fixation mechanism during the development of the residual system. Based upon obtained data, the available organic matters, pH variations in weathering solutions, adsorption process, functioning of carbonate bedrock as a geochemical barrier, existing in resistant minerals, and fixation in the neomorphic phases have been shown to play significant role in distribution of trace and rare earth elements. Further geochemical considerations indicate that minerals such as monazite, rhabdophane, belovite, churchite, and xenotime are the potential hosts for rare earth elements in Biglar bauxite deposit.

Gallium Oxide as a Host for Rare Earth Elements

AbstractPulsed Electron Beam Deposition (PEBD) and Pulsed Laser Deposition (PLD) were used to grow Gallium Oxide (Ga2O3) thin films on double sided polished sapphire substrates. At 850°C substrate temperature, smooth single crystal β-Ga2O3 films were obtained, which were confirmed with measurements by AFM of RMS surface roughness of about 1 nm. When characterized under electron beam excitation, the films exhibited different responses. For example: Europium doped films emitted intense red emission from 5D0 to 7Fj transitions while exhibiting weak broad emission from 300 to 500 nm. In contrast, Erbium doped films emitted strong emission from 300 to 500 nm peaked at 360 nm that was attributed to defects in the host matrix. Green emission from the Erbium transitions was observed at 528 and 550 nm. Films with different rare earth compositions varying from 0.1 % to 0.4 % were also prepared.High quality natural waveguides were formed with the deposited Ga2O3 films on the lower refractive index substrate sapphire. This was confirmed by measuring the refractive index by prism coupling and sharp coupling spectra.

Optical properties of Pr implanted GaN epilayers and Al xGa 1− xN alloys

It has been shown recently that III–V nitrides serve as a good host for rare earth elements and have many potential applications in optical communications. Most work in rare earth implanted III-nitride materials so far has been focused on GaN, while AlGaN alloys should have advantages over GaN due to wider energy band gap. In this work, photoluminescence (PL) spectroscopy was used to investigate praseodymium (Pr) related transitions in Pr-implanted GaN and Al x Ga 1− x N (0.15< x<0.33). The GaN epilayers and Al x Ga 1− x N alloys were rapid thermally annealed in nitrogen ambient to facilitate recovery from implantation related damage. We observed narrow PL emission bands near 526, 650, 950, 1100 and 1300 nm. The dependence of PL emission including line width, peak position and emission intensity on sample temperature, excitation intensity, aluminum concentration and annealing conditions were systematically studied. We found that PL intensity increases with annealing time and temperature. In contrast to GaN epilayers, different behaviors have been observed in the AlGaN host alloys.

Metasomatism in the North Qôroq centre, South Greenland: apatite chemistry and rare-earth element transport

AbstractThe North Qôroq syenite centre forms part of the Gardar Province of South Greenland. Extensive metasomatism, associated with the evolution of syenitic magmas, has resulted in redistribution of the rare-earth elements (REE), originally concentrated by magmatic processes, in both the syenites and surrounding granite-gneiss and quartzite country rocks. An important host for REE is apatite which can occur in significant quantities. Metasomatic apatites show complex, concentric, but irregular patterns of zonation, clearly seen using CL and BSE imaging. This zonation is related to successive pulses of metasomatising fluids. Electron microprobe analysis confirms the presence of significant quantities of REE in the apatites. The dominant cation exchange mechanism proposed is Ca2+ + P5+ ⇌ REE3+ + Si4+. In contrast to apatites from the nearby Ilímaussaq intrusion, there is no significant Na present in the structure and exchange reactions involving Na+ and REE3+ for Ca2+ have not occurred.Apatites from the quartzite are fluor-apatites, while those from the granite-gneiss are more Cl-rich. These differences reflect the fact that granite-gneiss apatites are original and modified by metasomatism, whereas, those in the quartzite are metasomatic in origin. REE were probably transported as carbonate, fluoride or fluor-carbonate complexes, and reflect the activity of a F−-rich, CO2−3-rich fluid phase.

Rare earth elements in chondritic phosphates—implications for 244Pu chronology

In stony meteorites of the chondrite class, phosphates (comprising about 0.6% of the whole) act as host for rare-earth elements (REE) and for Pu which was present at the time of formation. For the purpose of Pu–Xe dating it has been suggested that, in the absence of a stable Pu isotope, measured Xe abundances may be related to REE concentrations, on the grounds of presumed similarity in the geochemical behaviour of Pu and REE1. There are two phosphate phases in chondrites, apatite and merrillite (or whitlockite), which are difficult to separate from each other for analysis. Recent results based on grain-size discrimination2 indicate similar REE concentrations in both phosphates, whereas Pu favours merrillite3. Direct in situ analyses of individual grains by the ion-probe technique, however, show REE concentrations to be about five times higher in merrillite. The use of REE in the role of Pu analogue thus appears a better prospect than suggested by the previous indirect evidence.