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Parshukova, K. N., E. P. Rylchenko, V. A. Muraviev, K. A. Badanina, B. A. Makeev, R. I. Korolev, and N. A. Zhuk. "SYNTHESIS OF MULTICOMPONENT COMPOUNDS WITH A PIROCHLORE-TYPE STRUCTURE." Steklo i Keramika, no. 10 (October 2022): 34–39. http://dx.doi.org/10.14489/glc.2022.10.pp.034-039.
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The possibility of solid-phase synthesis of multicomponent bismuth niobates and tantalates with pyrochlore-type structure (sp. gr. Fd-3m), containing atoms of transition 3d-elements in equimolar quantities is shown. The composition of this type pyrochlores can be described by stoichiometric formula Bi2Cr1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6Ta(Nb)2O9 ± ?. A feature of synthesis of pyrochlores is a multi-stage high-temperature heat treatment process at a temperature of 650, 850, 950, 1050 °С for 60 hours. In the case of bismuth tantalate based pyrochlore, impurity phase of triclinic bismuth orthotantalate ?-BiTaO4 (sp. gr. P-1) is formed in trace amounts (? 8 mol.%). Complex pyrochlore based on bismuth niobate is characterized by a dense, low-porous microstructure in contrast to loose, porous tantalum pyrochlore with an average grain size of ? 2 µm. The unit cell parameter for pyrochlores containing niobium and tantalum is 10.4927(10.4922) ?, respectively.
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Rushton, M. J. D., Robin W. Grimes, C. R. Stanek, and Scott Owens. "Predicted pyrochlore to fluorite disorder temperature for A2Zr2O7 compositions." Journal of Materials Research 19, no. 6 (June 2004): 1603–4. http://dx.doi.org/10.1557/jmr.2004.0231.
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In a previous publication the order–disorder pyrochlore to fluorite transformation temperatures for a series of A2Hf2O7 pyrochlores were predicted [C.R. Stanek and R.W. Grimes: Prediction of rare-earth A2Hf2O7 pyrochlore phases. J. Am. Ceram. Soc. 2002, 85, p. 2139]. This was facilitated by establishing a relationship between these temperatures and the energy required to introduce a specific defect structure into the perfect pyrochlore lattice. Here an equivalent relationship for A2Zr2O7 pyrochlores was generated, and from this the disorder temperatures for a number of compositions including Eu2Zr2O7 were predicted.
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Mitchell, Roger H., Rudy Wahl, and Anthony Cohen. "Mineralogy and genesis of pyrochlore apatitite from The Good Hope Carbonatite, Ontario: A potential niobium deposit." Mineralogical Magazine 84, no. 1 (October 4, 2019): 81–91. http://dx.doi.org/10.1180/mgm.2019.64.
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AbstractThe Good Hope carbonatite is located adjacent to the Prairie Lake alkaline rock and carbonatite complex in northwestern Ontario. The occurrence is a heterolithic breccia consisting of diverse calcite, dolomite and ferrodolomite carbonatites containing clasts of magnesio-arfvedsonite + potassium feldspar, phlogopite + potassium feldspar together with pyrochlore-bearing apatitite clasts. The apatitite occurs as angular, boudinaged and schlieren clasts up to 5 cm in maximum dimensions. In these pyrochlore occurs principally as euhedral single crystals (0.1–1.5 cm) and can comprise up to 25 vol.% of the clasts. Individual clasts contain compositionally- and texturally-distinct suites of pyrochlore. The pyrochlores are hosted by small prismatic crystals of apatite (~100–500 μm × 10–25 μm) that are commonly flow-aligned and in some instances occur as folds. Allotriogranular cumulate textures are not evident in the apatitites. The fluorapatite does not exhibit compositional zonation under back-scattered electron spectroscopy, although ultraviolet and cathodoluminescence imagery shows distinct cores with thin (<50 μm) overgrowths. Apatite lacks fluid or solid inclusions of other minerals. The apatite is rich in Sr (7030–13,000 ppm) and rare earth elements and exhibits depletions in La, Ce, Pr and Nd (La/NdCN ratios (0.73–1.14) relative to apatite in cumulate apatitites (La/NdCN > 1.5) in the adjacent Prairie Lake complex. The pyrochlore are primarily Na–Ca pyrochlore of relatively uniform composition and minor Sr contents (<2 wt.% SrO). Irregular resorbed cores of some pyrochlores are A-site deficient (>50%) and enriched in Sr (6–10 wt.% SrO), BaO (0.5–3.5 wt.%), Ta2O5 (1–2 wt.%) and UO2 (0.5–2 wt.%). Many of the pyrochlores exhibit oscillatory zoning. Experimental data on the phase relationships of haplocarbonatite melts predicts the formation of apatite and pyrochlore as the initial liquidus phases in such systems. However, the texture of the clasts indicates that pyrochlore and apatite did not crystallise together and it is concluded that pyrochlores formed in one magma have been mechanically mixed with a different apatite-rich magma. Segregation of the apatite–pyrochlore assemblage followed by lithification resulted in the apatitites, which were disrupted and fragmented by subsequent batches of diverse carbonatites. The genesis of the pyrochlore apatitites is considered to be a process of magma mixing and not simple in situ crystallisation.
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Gunn, David S. D., John A. Purton, and Ilian T. Todorov. "Simulating Radiation-Induced Defect Formation in Pyrochlores." MRS Proceedings 1514 (2013): 15–20. http://dx.doi.org/10.1557/opl.2013.197.
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ABSTRACTThe accuracy and robustness of new Buckingham potentials for the pyrochlores Gd2Ti2O7 and Gd2Zr2O7 is demonstrated by calculating and comparing values for a selection of point defects with those calculated using a selection of other published potentials and our own ab inito values. Frenkel pair defect formation energies are substantially lowered in the presence of a small amount of local cation disorder. The activation energy for oxygen vacancy migration between adjacent O48f sites is calculated for Ti and Zr pyrochlores with the energy found to be lower for the non-defective Ti than for the Zr pyrochlore by ∼0.1 eV. The effect of local cation disorder on the VO48f → VO48f migration energy is minimal for Gd2Ti2O7, while the migration energy is lowered typically by ∼43 % for Gd2Zr2O7. As the healing mechanisms of these pyrochlores are likely to rely upon the availability of oxygen vacancies, the healing of a defective Zr pyrochlore is predicted to be faster than for the equivalent Ti pyrochlore.
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Livshits, Tatiana, Sergey Yudintsev, Sergey V. Stefanovsky, and Rodney Charles Ewing. "New Actinide Waste Forms with Pyrochlore and Garnet Structures." Advances in Science and Technology 73 (October 2010): 142–47. http://dx.doi.org/10.4028/www.scientific.net/ast.73.142.
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Cubic oxides with pyrochlore and garnet structures are promising matrices for long-lived actinides immobilization. Their isomorphic capacity with respect to An and REE was determined. To predict the long-term behavior of these matrices under their underground disposal radiation stability of synthetic pyrochlores and garnets was studied. Most of titanate phases have the critical (amorphization) doses close to 0.2 displacements per atom at 298 K. This value is significantly higher for Sn- and Zr-rich pyrochlores. Corrosion behavior of the pyrochlore- and garnet-composed matrices was investigated. The lowest actinides leach rates were observed in water and alkaline solutions most typical for underground waste repositories. Amorphization of the phases has a low influence on their corrosion behavior in solutions. Possibility for joint incorporation of actinides and Tc into zirconate- and titanate-based matrices with the pyrochlore structure is discussed.
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Khanvilkar, M. B., A. K. Nikumbh, S. M. Patange, R. A. Pawar, N. J. Karale, D. V. Nighot, P. A. Nagwade, M. D. Sangale, and G. S. Gugale. "Structural, electrical and magnetic properties of substituted pyrochlore oxide nanoparticles synthesized by the co-precipitation method." Physics and Chemistry of Solid State 22, no. 2 (June 16, 2021): 353–71. http://dx.doi.org/10.15330/pcss.22.2.353-371.
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Five substituted pyrochlore nanooxides such as Nd1.9Ho0.1Zr1.8Ce0.2O7, La1.95Ce0.05Zr0.29Ce1.71O7, Y1.79Pr0.21Ru1.99Pr0.01O7, Dy1.9Yb0.1Mn1.93Cu0.07O7 and Dy1.99Sr0.01Sn2O7 were synthesized by coprecipitation method. These precursors were monitored by thermal studies (TGA-DTA). The prepared nanosized substituted pyrochlore oxides were characterized by EDS, XRD, SEM, TEM, d. c. electrical conductivity, Thermoelectric power, Hall effect measurement, dielectric properties and magnetization measurements. XRD confirmed the formation of a single phase crystalline substituted pyrochlores with a cubic nature of nanoparticles. All substituted compounds were adopted a stable pyrochlore structure with rA3+/rB4+ = 1.395 except La1.95Ce0.05Zr0.29Ce1.71O7 compound, which has rA3+/rB4+ = 1.175 indicate disorder pyrochlore structure (i.e. fluorite structure). The temperature dependence of d. c. electrical conductivity for all substituted pyrochlores exhibits two distinct slopes with a break. This discontinuity can be attributed to extrinsic to intrinsic semiconducting properties. The thermoelectric power and Hall effect measurements for all compounds were confirmed the p-type semiconductivity except Y1.79Pr0.21Ru1.99Pr0.01O7 compound and which showed n-type semiconductivity. The dielectric constant (ε’) and dielectric loss (tan δ) i. e dissipation factor decreases with an increase in frequencies and reaching constant at particular frequencies. The applied field dependence of magnetization curve at room temperature (300 K) for Nd1.9Ho0.1Zr1.8Ce0.2O7, Y1.79Pr0.21Ru1.99Pr0.01O7 and Dy1.9Yb0.1Mn1.93Cu0.07O7, showed hysteresis loop with a small kink around the origin and which can be attributed to small but definite ferromagnetic ordering along with significant paramagnetic and superparamagnetic components. The magnetization at 2K showed a clear hysteresis loop for Dy1.9Yb0.1Mn1.93Cu0.07O7 and Dy1.99Sr0.01Sn2O7 pyrochlores are soft (weak) ferromagnets.
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Hase, I., Y. Higashi, and T. Yanagisawa. "Quasi-Flat-Band in s1/s2 Pyrochlore Oxides and the Effect of Spin-Orbit Interaction." Journal of Physics: Conference Series 2164, no. 1 (March 1, 2022): 012063. http://dx.doi.org/10.1088/1742-6596/2164/1/012063.
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Abstract An isotropic tight-binding model with the nearest-neighbour hopping on a pyrochlore lattice gives a rich variety of physical properties due to the emergence of the flat-band. Moreover, by introducing spin-orbit coupling into this model, the topological properties of the system changes significantly. This model is well applicable to some pyrochlore oxides called s1/s2 pyrochlores. In this paper we apply this model to an s2 pyrochlore oxide Pb2Ta2O7 and found a characteristic quadratic touching of the quasi-flat band and dispersive band. Furthermore, when a ferromagnetic order appears due to this quasi-flat band, a pair of Weyl points appears in that direction.
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Xue, Yuan, Ningyue Sun, and Guowu Li. "Evolution of Nb–Ta Oxide Minerals and Their Relationship to the Magmatic-Hydrothermal Processes of the Nb–Ta Mineralized Syenitic Dikes in the Panxi Region, SW China." Minerals 11, no. 11 (October 29, 2021): 1204. http://dx.doi.org/10.3390/min11111204.
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Previous geochemical and petrological studies have concluded that initially magmatic Nb–Ta mineralization is often modified by post-magmatic hydrothermal fluids; however, there is still a lack of mineralogical evidence for the syenite-related Nb–Ta deposit. From the perspective of Nb–Ta minerals, the pyrochlore supergroup minerals have significance for indicating the fluid evolution of alkaline rock or related carbonatite type Nb–Ta deposits. The Panzhihua–Xichang (Panxi) region is a famous polymetallic metallogenic belt in southwestern China, abound with a huge amount of Nb–Ta mineralized syenitic dikes. This study focuses on the mineral textures and chemical compositions of the main Nb–Ta oxide minerals (including columbite-(Fe), fersmite, fergusonite-(Y), and especially pyrochlore group minerals) in samples from the Baicao and Xiaoheiqing deposits, in the Huili area, Panxi region, to reveal the magma evolution process of syenitic-dike-related Nb–Ta deposits. The Nb–Ta oxides in the Huili syenites are commonly characterized by a specific two-stage texture on the crystal scale, exhibiting a complex metasomatic structure and compositional zoning. Four types of pyrochlore group minerals (pyrochlores I, II, III, and IV) formed in different stages were identified. The euhedral columbite-(Fe), fersmite, and pyrochlores I and II minerals formed in the magmatic fractional crystallization stage. Anhedral pyrochlore III minerals are linked to the activity of magma-derived hydrothermal fluids at the late stages of magma evolution. The pyrochlore IV minerals and fergusonite-(Y) tend to be more concentrated in areas that have undergone strong albitization, which is a typical phenomenon of hydrothermal alteration. These mineralogical phenomena provide strong evidences that the magmatic-hydrothermal transitional stage is the favored model for explaining the Nb–Ta mineralization process. It is also concluded that the changes in chemical composition and texture characteristics for pyrochlore group minerals could serve as a proxy for syenite-related Nb–Ta mineralization processes.
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Wang, Yuhao, Chong Jing, Zhao-Ying Ding, Yun-Zhuo Zhang, Tao Wei, Jia-Hu Ouyang, Zhan-Guo Liu, Yu-Jin Wang, and Ya-Ming Wang. "The Structure, Property, and Ion Irradiation Effects of Pyrochlores: A Comprehensive Review." Crystals 13, no. 1 (January 13, 2023): 143. http://dx.doi.org/10.3390/cryst13010143.
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Since the beginning of the use of nuclear energy, humans have been faced with the problem of radionuclide disposal. At present, a large amount of waste is stored in pools or dry tanks at reactor sites. With the development of the nuclear power generation industry worldwide, the high storage cost (including building, maintaining, and operating storage pools) is overwhelming and serious, and urgent radionuclide disposal problems have become increasingly difficult. Safe and economical strategies are urgently needed for long-term storage and disposal of nuclear waste, which has become among the core issues in the utilization of nuclear energy. Pyrochlore ceramics are able to immobilize a variety of radionuclides and have excellent irradiation stability, so they have received extensive attention as hosts of radionuclides waste. This review summarizes the structure, composition, synthesis process, properties, and irradiation stability of pyrochlore ceramics, focusing on the ion irradiation effect of pyrochlore. In general, the cation radii ratio rA/rB is a key parameter related to various properties of pyrochlores. Zirconate pyrochlore is more easily transformed from pyrochlore to defective fluorite, and leads to better irradiation resistance.
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Talanov, Mikhail V., and Valeriy M. Talanov. "Formation of breathing pyrochlore lattices: structural, thermodynamic and crystal chemical aspects." CrystEngComm 22, no. 7 (2020): 1176–87. http://dx.doi.org/10.1039/c9ce01635j.
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Zhou, Haidong, and Christopher Wiebe. "High-Pressure Routes to New Pyrochlores and Novel Magnetism." Inorganics 7, no. 4 (April 2, 2019): 49. http://dx.doi.org/10.3390/inorganics7040049.
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The pyrochlore structure (A2B2O7) has been an object of consistent study by materials scientists largely due to the stability of the cubic lattice with respect to a wide variety of chemical species on the A or B sites. The criterion for stability under ambient conditions is controlled by the ratio of these cations, which is empirically 1.36 < RA/RB < 1.71. However, under applied pressure synthesis conditions, the pyrochlore lattice is stable up to RA/RB ∼ 2.30, opening up possibilities for new compounds. In this review, we will highlight recent work in exploring new rare-earth pyrochlores such as the germanates RE2Ge2O7 and platinates RE2Pt2O7. We highlight recent discoveries made in these pyrochlores such as highly correlated spin ice behavior, spin liquid ground states, and exotic magnetic ordering.
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Starikova, Anastasia E., Ekaterina P. Bazarova, Valentina B. Savel’eva, Eugene V. Sklyarov, Elena A. Khromova, and Sergei V. Kanakin. "Pyrochlore-Group Minerals in the Granite-Hosted Katugin Rare-Metal Deposit, Transbaikalia, Russia." Minerals 9, no. 8 (August 15, 2019): 490. http://dx.doi.org/10.3390/min9080490.
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Pyrochlore group minerals are the main raw phases in granitic rocks of the Katugin complex-ore deposit that stores Nb, Ta, Y, REE, U, Th, Zr, and cryolite. There are three main types: Primary magmatic, early postmagmatic (secondary-I), and late hydrothermal (secondary-II) pyrochlores. The primary magmatic phase is fluornatropyrochlore, which has high concentrations of Na2O (to 10.5 wt.%), F (to 5.4 wt.%), and REE2O3 (to 17.3 wt.%) but also low CaO (0.6–4.3 wt.%), UO2 (to 2.6 wt.%), ThO2 (to 1.8 wt.%), and PbO (to 1.4 wt.%). Pyrochlore of this type is very rare in nature and is limited to a few occurrences: Rare-metal deposits of Nechalacho in syenite and nepheline syenite (Canada) and Mariupol in nepheline syenite (Ukraine). It may have crystallized synchronously with or slightly later than melanocratic minerals (aegirine, biotite, and arfvedsonite) at the late magmatic stage when Fe from the melt became bound, which hindered the crystallization of columbite. Secondary-I pyrochlore follows cracks or replaces primary pyrochlore in grain rims and is compositionally similar to the early phase, except for lower Na2O concentrations (2.8 wt.%), relatively low F (4 wt.%), and less complete A- and Y-sites occupancy. Secondary-II pyrochlore is a product of late hydrothermal alteration, which postdated the formation of the Katugin deposit. It differs in large ranges of elements and contains minor K, Ba, Pb, Fe, and significant Si concentrations but also low Na and F. Its composition mostly falls within the field of hydro- and keno-pyrochlore.
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Viladkar, Shrinivas G., and Natalia V. Sorokhtina. "Evolution of pyrochlore in carbonatites of the Amba Dongar complex, India." Mineralogical Magazine 85, no. 4 (June 7, 2021): 554–67. http://dx.doi.org/10.1180/mgm.2021.50.
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AbstractPyrochlore-group minerals are common accessory rare-metal bearing minerals in the calcite and ankerite carbonatites of the Amba Dongar complex (India). Pyrochlore from the Amba Dongar carbonatites differs from that in other Indian complexes in Ta, Zr, Ti, rare earth element (REE) and Pb contents, but is similar with respect to Ca, Ba and Sr abundances. The evolution of pyrochlore composition was studied to understand the alteration processes and the formation of late-stage pyrochlores enriched in REE and Pb. The early magmatic pyrochlore are calcio- and niobium-dominant types and were replaced by secondary cation-deficient varieties as a consequence of the action of hydrothermal fluids and supergene weathering. These processes produce changes mainly at the A site, rarely at the B site, and the original F is replaced by OH– groups. Calcium and Na can be extracted from the structure at the alteration stage and charge balance is achieved by the introduction of REE, Th, U, Ba or Sr. At the latest supergene stages, marginal and fractured zones of pyrochlore grains are altered to Pb-rich, Si-rich and cation-deficient hydrated varieties. The magmatic pyrochlore was crystallised in a highly alkaline environment at a high activity of Ca and at temperatures near 600°C, the alteration of pyrochlore began in a hydrothermal environment at temperatures below 350°C. The major compositional changes that are associated with the alteration are summarised by the following reactions: Ca2+ + Nb5+→ REE3+ + Ti4+; Nb5+ + Fe3+ → Ti4+ + Zr4+; and 2Nb5+ + Ca2+ → Ti4+ + Si4+ + U4+.
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HARRIS, M. J., and M. P. ZINKIN. "FRUSTRATION IN THE PYROCHLORE ANTIFERROMAGNETS." Modern Physics Letters B 10, no. 10 (April 30, 1996): 417–38. http://dx.doi.org/10.1142/s021798499600047x.
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In an antiferromagnet with the pyrochlore structure, the magnetic ions are situated on a framework of corner-sharing tetrahedra. This lattice results in a high degree of geometrical frustration, and theoretical studies predict that the magnetic ground state is continuously disordered. The majority of pyrochlore compounds are observed to remain disordered down to the lowest temperatures, and susceptibility measurements show that they often behave as conventional spin glasses. Neutron diffraction studies have discovered strong magnetic diffuse scattering at low temperatures. This reveals the presence of very short-ranged ferro- and antiferromagnetic correlations, similar in nature to the short-range correlations in a liquid. In this article, we review the current theoretical and experimental understanding of the pyrochlores, and present the results of recent experiments which have investigated the static and dynamic properties of this short-range order.
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Austin, D. A., M. Cole, M. C. Stennett, C. L. Corkhill, and N. C. Hyatt. "A preliminary investigation of the molten salt mediated synthesis of Gd2TiO5 ‘stuffed’ pyrochlore." MRS Advances 6, no. 4-5 (April 2021): 149–53. http://dx.doi.org/10.1557/s43580-021-00057-6.
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Abstract Refractory ‘stuffed’ pyrochlores such as Gd2TiO5 are of interest for nuclear applications, including as matrices for actinide disposition and as neutron absorbers in control rods. Here, we report the results of a preliminary comparative investigation of the synthesis of Gd2TiO5 by molten salt and conventional solid-state synthesis. We show that synthesis of Gd2TiO5 proceeds from the pyrochlore phase Gd2Ti2O7 which is first formed as a kinetic product. Molten salt synthesis afforded single phase Gd2TiO5 at 1300 °C in 2 h, via a template growth mechanism, and is effective for the synthesis of these refractory materials. This work demonstrates molten salt mediated synthesis of ‘stuffed’ pyrochlore for the first time. Graphic abstract
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Muravyov, Vitaliy A., Maria G. Krzhizhanovskaya, Boris A. Makeev, Andrey N. Nizovtsev, Sergey V. Nekipelov, Viktor N. Sivkov, Danil V. Sivkov, and Nadezhda A. Zhuk. "Features of the Preparation of Ni-Doped Bismuth Tantalate Pyrochlore." Crystals 13, no. 3 (March 9, 2023): 474. http://dx.doi.org/10.3390/cryst13030474.
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The possibility of high-temperature solid-phase synthesis of Ni-doped bismuth tantalate pyrochlore, not only from the oxide compounds BiTaO4 and NiO but also using nickel chloride NiCl2 as a precursor, was shown for the first time. The concentration range of the formation of nickel pyrochlore Bi2NixTa2O9−δ (0.85 ≤ x ≤ 1.0) at an equal molar ratio of bismuth(III)/tantalum(V) ions was determined. This indicates that the pyrochlore structure may be stable at 20–33% vacant bismuth sublattice. The influence of non-stoichiometric composition relative to bismuth and tantalum, by the example of Bi2±xNiTa2O9−δ and Bi2NiTa2−xO9−δ (x ≤ 0.5) compositions, on the phase composition of ceramics was studied. X-ray powder diffraction phase analysis of Bi2Ni1+yTa1.75−yO9−δ samples made it possible to determine the solubility limit of nickel ions in the tantalum sublattice. It was shown that the molar ratio of metal atoms (Ta, Ni) in the B2O6 (B-Ni(II), Ta(V)) octahedral sublattice of mixed pyrochlores is also limited.
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Gukasov, Arsen. "Polarized neutron diffraction and anisotropic exchange in pyrochlores." Acta Crystallographica Section A Foundations and Advances 70, a1 (August 5, 2014): C1462. http://dx.doi.org/10.1107/s2053273314085374.
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Rare earth frustrated pyrochlore magnets are among the materials that exhibit no long range order down to the lowest explored temperatures. To better understand the many complex phenomena observed in the pyrochlores, knowledge of the local interactions present in these materials is necessary. In the pyrochlore lattice, selection between Ising, Heisenberg, or XY types of the anisotropy cannot be based on the analysis of the macroscopic properties because of the presence of four equivalent local anisotropy (<111> -type) axes. Then only an average over the four axes can be measured by classical methods. Polarized neutron diffraction based on the `site susceptibility approach' (A Gukasov and P. J. Brown, J. Phys.: Cond. Matt., 14, 8831, 2002) allows to determine the local susceptibility tensor on the magnetic site. Its temperature dependence in the paramagnetic phase can be accounted for by the crystal field anisotropy and a molecular field tensor that encompasses exchange and dipolar interactions. It was found that for rare earth pyrochlores an isotropic exchange model is not suitable to explain their magnetic behavior and that, instead, the exchange interaction appear anisotropic to large extent (H. Cao et al., PRL 103, 056402, 2009). The experimental evaluation of the anisotropic exchange in pyrochlores can provide a clue to the richness of the phase diagram observed in rare earth frustrated magnets.
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Lian, Jie, Rodney C. Ewing, L. M. Wang, and K. B. Helean. "Ion-beam irradiation of Gd2Sn2O7 and Gd2Hf2O7 pyrochlore: Bond-type effect." Journal of Materials Research 19, no. 5 (May 2004): 1575–80. http://dx.doi.org/10.1557/jmr.2004.0178.
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Ceramics with III-IV pyrochlore compositions, A3+2B4+2O7 (A = Y and rare earth elements; B = Ti, Zr, Sn, or Hf), show a wide range of responses to ion-beam irradiation. To evaluate the role of the B-site cations on the radiation stability ofthe pyrochlore structure-type, Gd2Sn2O7 and Gd2Hf2O7 have been irradiated by1 MeV Kr+. The results are discussed in terms of the ionic size and type ofbonding of Sn4+ and Hf4+ and compared to previous results for titanate andzirconate pyrochlores. Gd2Sn2O7 is sensitive to ion beam–induced amorphizationwith a critical amorphization dose of approximately 3.4 displacements per atom(dpa) (2.62 × 1015 ions/cm2) at room temperature and a critical amorphization temperature of approximately 350 K. Gd2Hf2O7 does not become amorphous at adose of approximately 4.54 displacement per [lattice] atom (3.13 × 1015 ions/cm2) at room temperature, but instead is transformed to a disordered fluorite structure upon ion-beam irradiation. Although the radius ratio of the A- to B-site cations provides a general indication of the type of radiation response of different pyrochlore compositions, the results for Gd2Sn2O7 emphasize the importance of bond type, particularly the covalency of the 〈Sn–O〉 bond in determining the radiation response.
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Tarassov, Mihail, and Eugenia Tarassova. "Conditions and Mechanism of Crystallization of Hydrous W-Fe Oxides with a Pyrochlore-Type of Structure (Elsmoreite Group) in the Oxidation Zone of Ore Deposits." Minerals 14, no. 4 (April 19, 2024): 422. http://dx.doi.org/10.3390/min14040422.
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The purpose of the article is to elucidate the conditions and mechanism of crystallization of W-Fe oxide minerals with a pyrochlore-type structure (formerly ferritungstite, now elsmoreite group minerals) based on (1) a study of representative samples of secondary tungsten minerals from the oxidation zone of the Grantcharitsa tungsten deposit (Bulgaria) and (2) laboratory experiments under conditions suggested by the study of natural samples. It has been shown that crystallization of W-Fe pyrochlores occurs easily and effectively when treating WO3·xFe2O3·nH2O compounds (meymacite and tungstite) with W-Fe-containing solutions with pH 2.5–6.5 (70 °C); at the same time, direct crystallization (direct low-temperature hydrothermal synthesis) of these phases from aqueous solutions is unlikely. The crystallization of W-Fe pyrochlores under natural and laboratory conditions occurs through the oriented attachment of their nanocrystals to the {111} faces of growing crystals. The nucleation of such nanocrystals occurs in the bulk of the solution as a result of the interaction of the W-Fe solution with the W-(Fe) oxide hydrate precursor. The Fe/W ratio in the resulting W-Fe pyrochlore phase depends on the Fe/W ratio in the solution and precursor, as well as on the pH of the solution.
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Lumpkin, G. R., and R. C. Ewing. "Transmission electron microscopy of alpha-decay damage and alteration of betafite." Proceedings, annual meeting, Electron Microscopy Society of America 45 (August 1987): 376–77. http://dx.doi.org/10.1017/s0424820100126676.
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The pyrochlore structure type occurs as a constituent of crystalline nuclear waste form assemblages proposed for the long term isolation of actinides. Betafite, the Ti-rich end-member of the pyrochlore group, is a natural analogue similar in composition to waste form pyrochlores. This study examines the effects of alpha-recoil damage and geochemical alteration on a specimen of betafite from Antanifotsy, Madagascar (Harvard no. 87876, UNM no. 205).Electron microprobe analyses were performed using a JEOL 733 Superprobe operated at 15 kV and 20 nA sample current crushed fragments of the sample were dispersed on holey-carbon grids and examined with a JEOL 2000 EX TEM operated at 200 kV. Phase compositions were checked using a Tracor TN5500 EDS.The 1.5 cm diameter betafite crystal displays an unusual zonal sequence of alteration. The core consists of relatively unaltered betafite containing ca. 27 wt.% U02 and 2.5 wt.% CaO (TABLE 1, analysis 1).
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Wang, Hong, Yong Tang, Yu-Sheng Xu, Hui Zhang, Zheng-Hang Lv, Shan-Xian Qin, and Ying-Wei Song. "Textures and Chemical Compositions of Nb-Bearing Minerals and Nb Mineralization in the Shuangshan Nepheline Syenite Pluton, East Qinling, China." Minerals 11, no. 11 (October 21, 2021): 1163. http://dx.doi.org/10.3390/min11111163.
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The Shuangshan alkaline complex located in the Henan province of China is a newly discovered, potentially giant niobium (Nb) deposit. A variety of Nb-bearing minerals including pyrochlore, zircon, and titanite have been identified in this deposit. Distinct textural and chemical differences of pyrochlore and zircon indicate that both have different origins. The magmatic pyrochlore and zircon both have euhedral grains with small sizes. On the other hand, hydrothermal pyrochlore is mainly intergrown on the edge or inside of hydrothermal zircon in the form of an aggregate. Compared with magmatic pyrochlore, the contents of F, Ca, and Na in hydrothermal pyrochlore are obviously high. The texture and composition of hydrothermal pyrochlore and zircon indicate that Ca-bearing hydrothermal alteration resulted in the migration of Nb from Nb-bearing zircon and the reprecipitation of Nb to form aggregate pyrochlore. However, the quantitative calculation shows that the amount of Nb migrated from zircon is very small. Therefore, this study suggests that hydrothermal alteration plays a certain role in the redistribution of Nb, but the enrichment of Nb is limited.
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Goh, Gregory K. L., Carlos G. Levi, and Fred F. Lange. "Hydrothermal epitaxy of KtaO3 thin films." Journal of Materials Research 17, no. 11 (November 2002): 2852–58. http://dx.doi.org/10.1557/jmr.2002.0414.
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Heteroepitaxial KTaO3 thin films were synthesized by the hydrothermal method below 175 °C in highly alkaline aqueous KOH solutions. KTaO3 films synthesized in 7 M KOH solutions contained a defect pyrochlore phase that had (400) and (111) out-of-plane orientations. The (400) oriented pyrochlore islands were epitaxially related to the (100) SrTiO3 substrate while the (111) oriented pyrochlore islands had four in-plane variants. Pyrochlore-free KTaO3 films could be synthesized in a 15 M KOH solution. Pyrochlore-free films were also grown at 7 M KOH by delaying the introduction of the (100) SrTiO3 substrate into the synthesis solution until the conditions favoring the formation of the pyrochlore phase had dissipated.
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Hogarth, D. D., C. T. Williams, and P. Jones. "Primary zoning in pyrochlore group minerals from carbonatites." Mineralogical Magazine 64, no. 4 (August 2000): 683–97. http://dx.doi.org/10.1180/002646100549544.
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AbstractPyrochlore group minerals have the general formula A16–xB16O48Z8–y·nH2O, with A mainly Na, Ca, Sr, REE, Th, U; B mainly Nb, Ta, Ti and Z being O, F, OH. In this study, pyrochlore specimens were examined from carbonatites at Argor, Carillon Dam, Chilwa Island, Fen, Lueshe, Oka, Mbeya, Meech Lake and Verity. Primary features include a background with little compositional variation, from core to rim, upon which are commonly superimposed narrow oscillatory zones, parallel to {111}. These zones are usually characterized by high Ta, in many cases coupled with U (here Argor, Meech Lake and Verity specimens), but Chilwa Island and Fen pyrochlores have little Ta and zonation is mainly by enrichment in Ce and Nb. Primary zonation may persist through high-temperature metamorphism (Meech Lake and Verity) and metamictization (Meech Lake). Oscillatory zones were generated by a disequilibrium system that cooled under tranquil conditions, signalling absence of magma turbulence and, in many cases, the end of crystal growth. Some fresh crystals (Oka, Fen) have no oscillatory zones, possibly the product of magma turbulence in space or time. Low-temperature effects may mimic those of primary high temperature and are especially characterized by replacement rims, pyrochlore-inpyrochlore veinlets and low A-ion totals (Carillon Dam, Lueshe, Myeba).
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Lumpkin, Gregory R., and Rodney C. Ewing. "Geochemical alteration of pyrochlore group minerals; pyrochlore subgroup." American Mineralogist 80, no. 7-8 (August 1, 1995): 732–43. http://dx.doi.org/10.2138/am-1995-7-810.
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Yoon, Seok-Hyun, Young-Sun Park, Jeong-Oh Hong, and Dong-Sook Sinn. "Effect of the pyrochlore (Y2Ti2O7) phase on the resistance degradation in yttrium-doped BaTiO3 ceramic capacitors." Journal of Materials Research 22, no. 9 (September 2007): 2539–43. http://dx.doi.org/10.1557/jmr.2007.0326.
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Degradation of insulation resistance due to the formation of the Y2Ti2O7 (pyrochlore) phases in yttrium(Y)-doped BaTiO3 ceramics was investigated. The addition of Y2O3 and sintering aid SiO2 to the BaTiO3 ceramics caused the formation of barium-related BaSiO3 and titanium-related Y2Ti2O7 second phases. The appearance of the abnormally grown large pyrochlore grains with the increase of sintering temperature critically degraded insulation resistance at high temperatures. When the volume fraction of the pyrochlore phase increased, the resistance degradation was also observed although the grain size of the pyrochlore phase became smaller. The highly oxygen ionic conductive nature of the Y2Ti2O7 pyrochlore phase is supposed to accelerate electromigration of oxygen vacancies resulting in the resistance degradation. The formation of pyrochlore phase should be prevented to guarantee the high-temperature reliability of BaTiO3 bases ceramic capacitors.
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Bambi, A. C. J. M., A. Costanzo, A. O. Gonçalves, and J. C. Melgarejo. "Tracing the chemical evolution of primary pyrochlore from plutonic to volcanic carbonatites: the role of fluorine." Mineralogical Magazine 76, no. 2 (April 2012): 377–92. http://dx.doi.org/10.1180/minmag.2012.076.2.07.
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AbstractThree Angolan carbonatites were selected to evaluate the change in composition of pyrochlores during magmatic evolution: the Tchivira carbonatites occur in a plutonic complex, the Bonga carbonatites represent hypabyssal carbonatites and the Catanda carbonatites are volcanic in origin. In Tchivira pyrochlore, zoning is poorly developed; fluorine is dominant at the Y site; chemical zoning may arise as a result of substitutions for Nb in the B site; and the rare earth element (REE), U, Th and large-ion lithophile element (LILE) contents are very low. Pyrochlores from Bonga show oscillatory zonation; the F and Na contents are lower than those in the pyrochlores from Tchivira; and as substitution of Na at the A site increases, the Th, U, REE contents and inferred vacancies also increase. Pyrochlores from Catanda display complex textures. They generally have a rounded corroded core, which is mantled by two or three later generations. The core composition is similar to the Bonga pyrochlores. The rims are enriched in Zr, Ta, Th, Ce and U, but depleted in F and Na. In pyrochlores from the Angolan carbonatites, the F and Na contents decrease from plutonic to volcanic settings and there is enrichment of Th, U and REE in the A site and Ta and Zr in the B site. Zoning may be explained by changes in the activity of F, due to the crystallization of fluorite or apatite in the plutonic and hypabyssal carbonatites, or to volatile exsolution in the volcanic carbonatites.
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Teterin, Anton, Konstantin Maslakov, Yury Teterin, Kirill Ivanov, Sergei Yudintsev, Sergei Stefanovsky, Tatyana Livshits, and Mariya Lapina. "The XPS study of pyrochlore matrixes for the radioactive waste disposal." Nuclear Technology and Radiation Protection 25, no. 3 (2010): 157–63. http://dx.doi.org/10.2298/ntrp1003157t.
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Two pyrochlore ceramic samples were studied in this work. The X-ray diffraction and the scanning electron microscopy showed that the ceramics with the calculated composition CaThSn2O7 was formed by the dominating pyrochlore phase with the traces of thorianite and hematite, while the CaThZr2O7 ceramics - by the dominating pyrochlore phase with the minor admixtures of thorianite and perovskite. The real compositions of pyrochlore phases determined by the scaning electron microscopy are Ca0.88Th0.92Sn2O6.72 and Ca0.84Th0.80Zr2O6.44. On the basis of the X-ray photoelectron spectral parameters of the outer and core electrons in the binding energy range of 0-1250 eV it was found that tin, zirconium and thorium in pyrochlore are at least 93%-94% tetravalent. Sn-O and Zr-O interatomic distances in BO6-octahedrons in the pyrochlore were found to be 0.210 nm and 0.220 nm, respectively, and these octahedrons are possible to be tetragonaly distorted.
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Tiwari, V. S., Arun Kumar, V. K. Wadhawan, and Dhananjai Pandey. "Kinetics of formation of the pyrochlore and perovskite phases in sol-gel derived lead zirconate titanate powder." Journal of Materials Research 13, no. 8 (August 1998): 2170–73. http://dx.doi.org/10.1557/jmr.1998.0303.
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Lead zirconate titanate (PZT) powder is prepared by the sol-gel method. The formation of pyrochlore and perovskite phases is investigated by high temperature x-ray diffraction (XRD) and thermal analysis techniques. The pyrochlore phase first appears in x-ray amorphous form, and then gets converted to crystalline state on annealing in air. We show that vacuum annealing of the pyrolyzed amorphous PZT gel suppresses the formation of the crystalline pyrochlore phase. This, in turn, enhances the kinetics of conversion of pyrochlore to perovskite, such that a pyrochlore-free perovskite phase can be obtained by annealing at about 500 °C. On the other hand, if annealing is carried out in air, a crystalline pyrochlore phase is formed, which requires annealing temperatures higher than 600 °C for transformation to the perovskite phase. These observations are explained tentatively in terms of the oxygen stoichiometry of the two phases.
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Wall, F., C. T. Williams, A. R. Woolley, and M. Nasraoui. "Pyrochlore from Weathered Carbonatite at Lueshe, Zaire." Mineralogical Magazine 60, no. 402 (October 1996): 731–50. http://dx.doi.org/10.1180/minmag.1996.060.402.03.
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AbstractA detailed study of weathered pyrochlore in the laterite above carbonatite at Lueshe, NE Zaire, has been made in order to determine its chemical and textural variations. Pyrochlore in fresh carbonatite at Lueshe is close to an ideal formula of (Ca,Na)2Nb2O6(OH,F) (where a general formula is A2−xB2O6(OH,F)1−y·zH2O. The first and principal change on weathering occurs at the base of the profile and involves the leaching and partial exchange of A cations together with hydration. This change appears common to weathered pyrochlore worldwide. As a result weathered pyrochlore at Lueshe has a large apparent A cation deficiency with A totals between 0.25 and 0.59. The B cations remain stable. Abundant kalipyrochlore is unique to Lueshe and is thought to be related to the abundance of potassium feldspar in the fresh carbonatite, showing that the actual composition of weathered pyrochlore is a characteristic of a particular deposit. Weathered profiles at Lueshe are not simple trends from the least to most leached compositions. Further factors including variation in whole rock mineralogy and chemistry, and cation exchange and uptake are responsible for local concentrations of strontio-, bario- and calcium-rich, sodium-poor pyrochlore in the ore body, as well as rims of ceriopyrochlore on kalipyrochlore. The most important textural relationship in the Lueshe pyrochlore is the intimate intergrowth with crandallite in the most weathered parts of the laterite. Although pyrochlore persists throughout the weathering profile, niobium-beating goethite is thought to represent the final product of pyrochlore breakdown.
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Torró, L., C. Villanova, M. Castillo, M. Campeny, A. O. Gonçalves, and J. C. Melgarejo. "Niobium and rare earth minerals from the Virulundo carbonatite, Namibe, Angola." Mineralogical Magazine 76, no. 2 (April 2012): 393–409. http://dx.doi.org/10.1180/minmag.2012.076.2.08.
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AbstractThe Virulundo carbonatite in Angola is one of the largest in the world and contains pyrochlore as an accessory mineral in all of the carbonatite units (calciocarbonatites, ferrocarbonatites, carbonatite breccias and trachytoids). The primary magmatic pyrochlore is fluorine dominant and typically contains about equal molar quantities of Ca and Na at the A site. High-temperature hydrothermal processes have resulted in the pseudomorphic replacement of the primary pyrochlore by a second generation of pyrochlore with less F and Na. Low-temperature hydrothermal replacement of the first and second generation pyrochlore, associated with quartz-carbonate-fluorite vein formation in the carbonatite, has produced a third generation of pyrochlore, with a high Sr content. The Sr appears to have been released by low-temperature hydrothermal replacement of the primary magmatic carbonates. Finally, supergene alteration processes have produced late-stage carbonates, goethite, hollandite and rare earth element (REE) minerals (mainly synchysite-(Ce), britholite-(Ce), britholite-(La), cerite-(Ce)). Cerium separated from the other REE s in oxidizing conditions and Ce4+ was incorporated into a late generation of supergene pyrochlore, which is strongly enriched in Ba and strongly depleted in Ca and Na.
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Rodriguez, Mark A., Timothy J. Boyle, Bernadette A. Hernandez, Catherine D. Buchheit, and Michael O. Eatough. "Formation of SrBi2Ta2O9: Part II. Evidence of a bismuth-deficient pyrochlore phase." Journal of Materials Research 11, no. 9 (September 1996): 2282–87. http://dx.doi.org/10.1557/jmr.1996.0290.
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A bismuth-deficient pyrochlore phase has been observed in both powder and film samples fired at 775 °C. The estimated stoichiometry of this pyrochlore (based on calculated diffraction patterns) was Sr0.2(Sr0.5Bi0.7)Ta2O6.75. This bismuth-deficient pyrochlore phase may be considered deleterious to the formation of the SrBi2Ta2O9“SBT” ferroelectric compound since a significant presence of this pyrochlore compound implies a large deviation from the desired cation ratios. Additionally, films prepared on platinized silicon substrates indicate the SBT phase formation may be encouraged by the substrate; there appears to be some 00l preferential orientation for stoichiometric SBT thin films.
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Radha, A. V., Sergey V. Ushakov, and Alexandra Navrotsky. "Thermochemistry of lanthanum zirconate pyrochlore." Journal of Materials Research 24, no. 11 (November 2009): 3350–57. http://dx.doi.org/10.1557/jmr.2009.0401.
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A thermodynamic study was carried out to resolve discrepancies in the enthalpy of formation and related parameters for lanthanum zirconate pyrochlore. The homogeneity field for single phase pyrochlore formation was determined to be ∼33–35 mol% La2O3 at 1500 °C. High-temperature oxide melt drop solution calorimetry was performed in sodium molybdate and lead borate solvents on three compositions ranging from La1.98Zr2.01O7 to La2.07Zr1.95O7. The enthalpy of formation from oxides at 25 °C, ΔH0f,ox, for stoichiometric lanthanum zirconate pyrochlore is −107.3 ± 5.1 kJ/mol, and the standard enthalpy of formation from elements, ΔH0f,el, is −4102.2 ± 6.0 kJ/mol. La2Zr2O7 pyrochlore was found by differential thermal analysis to be stable up to its melting point. The melting point and the fusion enthalpy of La2Zr2O7 pyrochlore were measured as 2295 ± 10 °C and ∼350 kJ/mol, respectively.
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Lumpkin, G. R., and R. C. Ewing. "Alpha-decay damage and the aperiodic structure of pyrochlore." Proceedings, annual meeting, Electron Microscopy Society of America 46 (1988): 470–71. http://dx.doi.org/10.1017/s0424820100104418.
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Alpha-decay damage in minerals which contain U and Th involves two separate damage processes: a high energy (4-5 MeV) alpha particle with a range of 10,000 nm displaces several hundred atoms creating Frenkel defects near the end of die track; and a recoil atom (0.1 MeV) with a range of 10 nm produces several thousand atomic displacements creating “tracks” of disordered material. Pyrochlore group minerals are suitable for the study of alpha-decay damage because they contain variable amounts of U and Th such that the accumulated dose spans the transition from the crystalline to the fully aperiodic, “metamict” state.More than 50 natural pyrochlores were characterized by a number of experimental techniques. Electron microprobe analyses were obtained using a JEOL 733 Superprobe operated at 15 KV and 20 nA. X-ray diffraction studies were completed using an automated Scintag powder diffractometer.
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Jung, Se-Yeon, Seung-Joon Hwang, and Yun-Mo Sung. "Suppression of pyrochlore phase formation in Sr0.7Bi2.4Ta2O9 thin films using nanoseed layer." Journal of Materials Research 18, no. 8 (August 2003): 1745–48. http://dx.doi.org/10.1557/jmr.2003.0240.
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A Sr0.7Bi2.4Ta2O9 (SBT) seed layer approximately 40 nm thick was formed on Pt/Ti/SiO2/Si substrates, and SBT thin films with the same chemical composition were deposited on the substrates with and without seed layer using sol-gel and spin coating methods. The influence of seed layer on the phase formation characteristics of SBT thin films was investigated using x-ray diffraction and scanning electron microscopy analyses. Formation of pyrochlore as well as Aurivillius phase was observed in both the unseeded and seeded SBT films heated at 740 °C. However, it was revealed that Aurivillius phase formation was enhanced in seeded SBT thin films and pyrochlore phase formation was highly suppressed. In this study, two possible mechanisms for the suppression of pyrochlore phase formation were proposed from the perspectives of activation energy difference between Aurivillius and pyrochlore phase formation, and Bi-ion diffusion to pyrochlore phase.
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Teng, Zhen, Yongqiang Tan, and Haibin Zhang. "High-Entropy Pyrochlore A2B2O7 with Both Heavy and Light Rare-Earth Elements at the A Site." Materials 15, no. 1 (December 24, 2021): 129. http://dx.doi.org/10.3390/ma15010129.
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A novel class of high-entropy pyrochlore ceramics (HEPCs) with multiple heavy and light rare-earth elements at the A site were successfully synthesized via solid-state reaction. Both the XRD patterns and Raman spectroscopy demonstrated the single pyrochlore structure feature of seven kinds of HEPCs. Electron microscopic images revealed the typical morphology and the homogeneous distribution of all rare-earth elements. It can be concluded that the significance of configuration entropy in the HEPC system has promoted the tervalent lanthanide nuclides to form a single pyrochlore structure. This work is expected to provide guidance for the further design of high-entropy pyrochlore/fluorite ceramics.
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Yin, Rong, Xiaoming Sun, Shengwei Wang, and Bin Wu. "Mineral Chemistry of Pyrochlore Supergroup Minerals as Records of Nb Mineralization Processes in NYF-Type Pegmatites: A Case Study of the Emeishan Large Igneous Province, SW China." Minerals 14, no. 1 (December 21, 2023): 13. http://dx.doi.org/10.3390/min14010013.
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Alkaline igneous rocks have become a potentially important source of Nb, except for the carbonatites. The metallogenetic mechanism of Nb during the magmatic-hydrothermal evolution of alkaline rocks remains ambiguous. From the perspective of Nb minerals, the mineral chemistry of pyrochlore supergroup minerals provides the mineralogical evidence for indicating the respective contributions of magma and hydrothermal fluids to Nb mineralization. In the Miyi County of the Panzhihua-Xichang (Pan-Xi) area, the central zone of the Permian Emeishan large igneous province (ELIP) in SW China, hundreds of Nb-Y-F mineralized pegmatites (NYF-type) are exposed. This study conducted in situ mineral chemistry analyses on four types of pyrochlores to elucidate the Nb mineralization process. Both Pcl-I and Pcl-II exhibit well-developed oscillatory zoning (OZ), representing magmatic pyrochlore formed through disequilibrium crystallization in an oscillatory environment. Pcl-III, with a homogeneous and less variable composition, is also considered of magmatic origin due to its coherent chemical evolution with Pcl-II. Pcl-IV is considered of hydrothermal origin based on its irregular zoning texture, extremely high Na2O contents, and compositional gap compared with magmatic types. The decrease in TiO2 contents, coupled with the increase in Na2O, F, and Nb2O5 contents from Pcl-I to Pcl-III and from the core to the rim of zoned Pcl-II, indicates that fractional crystallization facilitates the crystallization of albite and the enrichment of volatiles, as well as the precipitation of Nb from the early to late stages. During the magmatic-hydrothermal transition stage, the reductive, Na- and F-enriched fluid transports Nb more effectively, leading to further Nb enrichment in pyrochlore. Consequently, there are two-stage Nb mineralization processes associated with the magmatic-hydrothermal evolution in the Miyi pegmatite, with the primary magmatic ore assemblages being the dominant Nb mineralization, which may be a general model for the mineralization of NYF-type pegmatites.
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Matsunami, M., T. Hashizume, and A. Saiki. "Ion-Exchange Reaction Of A-Site In A2Ta2O6 Pyrochlore Crystal Structure." Archives of Metallurgy and Materials 60, no. 2 (June 1, 2015): 941–44. http://dx.doi.org/10.1515/amm-2015-0234.
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Abstract Na+ or K+ ion rechargeable battery is started to garner attention recently in Place of Li+ ion cell. It is important that A+ site ion can move in and out the positive-electrode materials. When K2Ta2O6 powder had a pyrochlore structure was only dipped into NaOH aqueous solution at room temperature, Na2Ta2O6 was obtained. K2Ta2O6 was fabricated from a tantalum sheet by a hydrothermal synthesize with KOH aqueous solution. When Na2Ta2O6 was dipped into KOH aqueous solution, K2Ta2O6 was obtained again. If KTaO3 had a perovskite structure was dipped, Ion-exchange was not observed by XRD. Because a lattice constant of pyrochlore structure of K-Ta-O system is bigger than perovskite, K+ or Na+ ion could shinny through and exchange between Ta5+ and O2− ion site in a pyrochlore structure. K+ or Na+ ion exchange of A2Ta2O6 pyrochlore had reversibility. Therefore, A2Ta2O6 had a pyrochlore structure can be expected such as Na+ ion rechargeable battery element.
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Stennett, Martin C., Matthew L. Hand, and Neil C. Hyatt. "Decontamination of Molten Salt Wastes for Pyrochemical Reprocessing of Nuclear Fuels." MRS Proceedings 1518 (2013): 97–102. http://dx.doi.org/10.1557/opl.2013.162.
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ABSTRACTPyrochemical reprocessing of nuclear fuels, in which electrochemical separation of actinides and fission products is mediated by a molten alkali chloride salt (typically a LiCl-KCl eutectic) is of interest for future nuclear energy cycles. A key challenge in the management of pyrochemical reprocessing wastes is decontamination and recycling of the molten salt medium to remove entrained actinides and radioactive lanthanide fission products. Since pyrochlore oxides are promising candidates for the immobilisation of lanthanides and actinides, we sought to use the “problematic” molten salt to our advantage as a reaction medium for low temperature synthesis of titanate pyrochlores. Through control of reaction time and temperature, we demonstrated the synthesis of lanthanide pyrochlores at temperatures as low as 700 °C in 1 h, compared to 1350 °C in 36 h for conventional solid state synthesis. The importance of this study is in demonstrating the potential feasibility for decontamination of pyrochemical reprocessing wastes by simple addition of TiO2 to form lanthanide and actinide pyrochlores by rapid molten salt assisted reaction at moderate temperature.
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Christy, A. G., and D. Atencio. "Clarification of status of species in the pyrochlore supergroup." Mineralogical Magazine 77, no. 1 (February 2013): 13–20. http://dx.doi.org/10.1180/minmag.2013.077.1.02.
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AbstractAfter careful consideration of the semantics of status categories for mineral species names, minor corrections and disambiguations are presented for a recent report on the nomenclature of the pyrochlore supergroup. The names betafite, elsmoreite, microlite, pyrochlore and roméite are allocated as group names within the pyrochlore supergroup. The status of the names bindheimite, bismutostibiconite, jixianite, monimolite, partzite, stetefeldtite and stibiconite is changed from 'discredited' to 'questionable' pending further research.
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Zhang, Fu Xiang, and Maik Lang. "Pressure-Induced Cationic Disordering in Pyrochlore Oxides (La1-xCex)2Zr2O7 and Enhancement of Compressibility." Advanced Materials Research 1033-1034 (October 2014): 583–87. http://dx.doi.org/10.4028/www.scientific.net/amr.1033-1034.583.
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Pyrochlore oxides La2Zr2O7,Ce2Zr2O7and their solid solutions were studied by in situ x-ray diffraction (XRD) measurement at high pressures. Pressure dependence of cationic disordering was derived from Rietveld refinement of the XRD patterns. The results indicated that the mixed occupancy in the cation site enhanced the compressibility obviously. All the pyrochlore structure became unstable at ~20 GPa and an orthorhombic high-pressure phase formed. The cations changed from the ordered state in pyrochlore to a disordered state in the high-pressure phase. The high-pressure phase is about 8% denser than the pyrochlore phase and not stable at ambient conditions and a disordered defect-fluorite structure was quenched for all the samples.
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Wang, Hong, and Xi Yao. "Structure and dielectric properties of pyrochlore–fluorite biphase ceramics in the Bi2O3–ZnO–Nb2O5 system." Journal of Materials Research 16, no. 1 (January 2001): 83–87. http://dx.doi.org/10.1557/jmr.2001.0016.
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The structure and dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics with pyrochlore–fluorite biphase structure were investigated. Mixed-sintered ceramics were prepared by two precalcined constituents in the system of x[Bi1.5Zn0.5(Zn0.5Nb1.5)O7]−(1 − x)Bi3/4Nb1/4O7/4 (0.05 ≤ x ≤ 0.35), where Bi1.5Zn0.5(Zn0.5Nb1.5)O7 is a cubic pyrochlore (α) and Bi3/4Nb1/4O7/4 is a defect cubic fluorite (F). The phase composition of the mixed-sintered ceramics were characterized as a biphasic structure with a distorted pyrochlore (β) and a fluorite (F) coexisting. The phase ratio of β pyrochlore and F fluorite is related to the chemical composition x. The dielectric properties of the ceramics gradually change with their structure.
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Kim, Woo Jin, Jeongkeun Song, Yangyang Li, and Tae Won Noh. "Perspective on solid-phase epitaxy as a method for searching novel topological phases in pyrochlore iridate thin films." APL Materials 10, no. 8 (August 1, 2022): 080901. http://dx.doi.org/10.1063/5.0097608.
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Pyrochlore iridates exhibit various novel topological phenomena due to their topology and electron correlation. Notably, pyrochlore iridate was the first proposed material system shown to host the time-reversal symmetry broken Weyl semimetal phase. After this profound theoretical proposal, extensive experimental attempts have been made to synthesize high-quality samples in both bulk and film forms. In particular, adjusting thin film geometry represents a practical way to tune the U/ t ( U: Coulomb interaction and t: for hopping parameter) and identify the various topological phases in the regime U ∼ λ ( λ: spin–orbit interaction). However, the instability of pyrochlore iridates at high temperature and low oxygen pressure has long been a barrier to growing pyrochlore iridate thin films by conventional film growth methods. To overcome this, pyrochlore iridate films have often been grown by the solid-phase epitaxy method, which uses a metastable amorphous layer grown at low temperature. During a high temperature post-annealing process, the layer in contact with a single crystalline substrate will crystallize epitaxially in the solid state by rearranging atoms at the interface. Here, we present a perspective on the solid-phase epitaxy as a method to synthesize epitaxial pyrochlore iridate thin films and a way to search for novel correlated phenomena.
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Gardner, Jason S., Michel J. P. Gingras, and John E. Greedan. "Magnetic pyrochlore oxides." Reviews of Modern Physics 82, no. 1 (January 26, 2010): 53–107. http://dx.doi.org/10.1103/revmodphys.82.53.
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Henley, Christopher L. "Depleted pyrochlore antiferromagnets." Journal of Physics: Conference Series 145 (January 1, 2009): 012022. http://dx.doi.org/10.1088/1742-6596/145/1/012022.
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Matsumoto, Aika, Ze-Xing Cai, and Takeshi Fujita. "Synthesis of Pyrochlore Oxides Containing Ir and Ru for Efficient Oxygen Evolution Reaction." Materials 15, no. 17 (September 2, 2022): 6107. http://dx.doi.org/10.3390/ma15176107.
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A versatile synthesis method for pyrochlore oxides containing Ir and Ru with lanthanides Pr, Nd, Eu, Gd, Tb, and Ho is herein presented. Based on the systematic synthesis and Rietveld refinement results, the lattice constants were tunable depending on the ionic radius of the lanthanide used. Subsequently, Pr-based pyrochlore oxides containing Ru and Ir in different ratios were fabricated for the oxygen evolution reaction (OER) in alkaline media, and the OER activity of these catalysts increased when the content of Ir and Ru was the same. Thus, the co-substitution of Ir and Ru in pyrochlore oxides is a novel synthesis strategy for electrocatalysts, which provides great potential for the fabrication of other pyrochlore oxides with various architectures and compositions for application in electrocatalysis.
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Shlyakhtina, Anna V., Nikolay V. Lyskov, Galina E. Nikiforova, Anna V. Kasyanova, Galina A. Vorobieva, Igor V. Kolbanev, Dmitry N. Stolbov, and Dmitry A. Medvedev. "Proton Conductivity of La2(Hf2−xLax)O7−x/2 “Stuffed” Pyrochlores." Applied Sciences 12, no. 9 (April 25, 2022): 4342. http://dx.doi.org/10.3390/app12094342.
Full textAbstract:
The design of new oxygen- and proton-conducting materials is of paramount importance for their possible utilization in solid oxide fuel cells. In the present work, La2(Hf2–xLax)O7–x/2 (x = 0, 0.1) ceramics were prepared using ball milling of oxide mixtures (La2O3 and HfO2) followed by high-temperature annealing at 1600 °C for 10 h in air. La2Hf2O7 ceramics exhibit an ordered pyrochlore-type structure, whereas La2(Hf1.9La0.1)O6.95 has a defect pyrochlore structure type with oxygen vacancies at the 48f positions. The oxygen ion and proton conductivity of La2(Hf1.9La0.1)O6.95 “stuffed” pyrochlore ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) and four-probe DC measurements in a dry and a wet atmosphere (air and nitrogen). The use of two distinct conductivity measurement techniques ensured, for the first time, the collection of reliable data on the proton conductivity of the La2(Hf1.9La0.1)O6.95 “stuffed” hafnate pyrochlore. La2Hf2O7 was found to be a dielectric in the range 400–900 °C, whereas the La2(Hf1.9La0.1)O6.95 “stuffed” pyrochlore had both oxygen ion and proton conductivities in this temperature range. The proton conductivity level was found to be equal to ~8 × 10−5 S/cm at 700 °C. Clearly, the proton conductivity of the La2(Hf1.9La0.1)O6.95 “stuffed” hafnate pyrochlore is mainly due to the hydration of oxygen vacancies at 48f positions.
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Shlyakhtina, Anna V., Nikolay V. Lyskov, Galina E. Nikiforova, Anna V. Kasyanova, Galina A. Vorobieva, Igor V. Kolbanev, Dmitry N. Stolbov, and Dmitry A. Medvedev. "Proton Conductivity of La2(Hf2−xLax)O7−x/2 “Stuffed” Pyrochlores." Applied Sciences 12, no. 9 (April 25, 2022): 4342. http://dx.doi.org/10.3390/app12094342.
Full textAbstract:
The design of new oxygen- and proton-conducting materials is of paramount importance for their possible utilization in solid oxide fuel cells. In the present work, La2(Hf2–xLax)O7–x/2 (x = 0, 0.1) ceramics were prepared using ball milling of oxide mixtures (La2O3 and HfO2) followed by high-temperature annealing at 1600 °C for 10 h in air. La2Hf2O7 ceramics exhibit an ordered pyrochlore-type structure, whereas La2(Hf1.9La0.1)O6.95 has a defect pyrochlore structure type with oxygen vacancies at the 48f positions. The oxygen ion and proton conductivity of La2(Hf1.9La0.1)O6.95 “stuffed” pyrochlore ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) and four-probe DC measurements in a dry and a wet atmosphere (air and nitrogen). The use of two distinct conductivity measurement techniques ensured, for the first time, the collection of reliable data on the proton conductivity of the La2(Hf1.9La0.1)O6.95 “stuffed” hafnate pyrochlore. La2Hf2O7 was found to be a dielectric in the range 400–900 °C, whereas the La2(Hf1.9La0.1)O6.95 “stuffed” pyrochlore had both oxygen ion and proton conductivities in this temperature range. The proton conductivity level was found to be equal to ~8 × 10−5 S/cm at 700 °C. Clearly, the proton conductivity of the La2(Hf1.9La0.1)O6.95 “stuffed” hafnate pyrochlore is mainly due to the hydration of oxygen vacancies at 48f positions.
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Muraviev, V. A., B. A. Makeev, M. G. Krzhizhanovskaya, R. I. Korolev, and N. A. Zhuk. "SYNTHESIS OF Bi2NiTa2O9 WITH A PYROCHLORE-TYPE STRUCTURE." Steklo i Keramika, no. 2 (February 2022): 40–46. http://dx.doi.org/10.14489/glc.2022.02.pp.040-046.
Full textAbstract:
Nickel pyrochlore of the composition Bi2NiTa2O9 was synthesized by the solid-phase reaction method (sp. gr. Fd-3m, a = 10.5238 Å). The process of phase formation of nickel-containing bismuth tantalate Bi2NiTa2O9 with a pyrochlore structure is investigated. The pyrochlore phase synthesis reaction is described by the interaction of orthorhombic bismuth orthotantalate and nickel oxide. Synthesis proceeds mainly at temperatures above 900 °С. The duration of calcination of the preparations leads to the coalescence of grains with the formation of larger particles. The calcination temperature did not significantly affect the microstructure of the samples. The samples are characterized by a porous dendritic microstructure with a grain size of 0.5 – 1.0 µm. The optimal conditions for obtaining pure pyrochlore can be considered a temperature of 1050 °С and calcination time of 15 hours.
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Sun, Zhenghao, Kezhang Qin, Yajing Mao, Dongmei Tang, Fangyue Wang, Noreen J. Evans, and Qifeng Zhou. "Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation." Minerals 12, no. 7 (June 21, 2022): 785. http://dx.doi.org/10.3390/min12070785.
Full textAbstract:
Alkaline rocks are generally enriched in rare metals (e.g., Nb, Ta, and Zr) and rare earth elements (REE), but the key factors controlling Nb-Ta-REE enrichment remain unclear. The Boziguoer Nb (Ta-Zr-Rb-REE) deposit in Southwest Tianshan (northern margin of Tarim Basin) is China’s largest, with reserves of 0.32 Mt Nb2O5 and 0.02 Mt Ta2O5. It is an alkaline felsic complex 4.45 km in length and 0.5–1.3 km in width, composed of alkalic granite and syenite, which can be subdivided into syenite I and syenite II. The main minerals in each lithofacies are the same (albite, K-feldspar, quartz, arfvedsonite and aegirine). The Nb in the deposit is mainly hosted in pyrochlore supergroup minerals, ubiquitous in alkalic granite and syenite of the Boziguoer deposit. The wide variation in cations (Ca, Na, REE, U, Th) in the A-site further classifies the Boziguoer pyrochlore supergroup minerals as fluornatropyrochlore, fluorcalciopyrochlore and fluorkenopyrochlore. All Boziguoer pyrochlore supergroup minerals are Nb-rich and Ta-poor at the B-site and dominated by F at the Y-site. These cation occurrence illustrate a new mechanism of substitution in the Boziguoer pyrochlore supergroup minerals (2Ca2+ +Ti4+ +4Ta5+ = REE3+ +A-V + 5Nb5+, where A-V is the A-site vacancy). This substitution mechanism is different from that in the pyrochlore supergroup minerals from other rocks such as carbonatite and nepheline syenite, which are dominated by the replacement of Ba (Rb, Sr) with Ca+ Na + A-V. In addition, the substitution of REE (mainly La, Ce) for Ca in the Boziguoer pyrochlore supergroup minerals is likely a result of either REE enrichment or a change in the REE partition coefficient during the evolution of the alkaline magma. Both the pyrochlore supergroup minerals and their host rocks display negative large ion lithophile element (LILE; K, Rb, Sr, and Ba) anomalies, positive high-field-strength element (HFSE) anomalies and light rare earth element (LREE) enrichment with negative Eu anomalies. This is consistent with the crystallization of the pyrochlore supergroup minerals from the magma rather than from hydrothermal fluids, suggesting a magmatic origin. These findings indicate that the mechanisms of pyrochlore supergroup minerals crystallization in alkaline magma may be significantly different from those in carbonatite and nepheline syenite, and that magmatic differentiation processes may have played a role in the enrichment of the Boziguoer deposit by Nb.
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Amores-Casals, Melgarejo, Bambi, Gonçalves, Morais, Manuel, Neto, Costanzo, and Molist. "Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)." Minerals 9, no. 10 (September 30, 2019): 601. http://dx.doi.org/10.3390/min9100601.
Full textAbstract:
The Bonga complex is composed of a central carbonatite plug (with a ferrocarbonatite core) surrounded by carbonatite cone sheets and igneous breccias of carbonatitic, fenitic, phoscoritic and lamprophyric xenoliths set in a carbonatitic, lamprophyric or mingled mesostase. To reconstruct the dynamics of the complex, the pyrochlore composition and distribution have been used as a proxy of magmatic-hydrothermal evolution of the complex. An early Na-, F-rich pyrochlore is disseminated throughout the carbonatite plug and in some concentric dykes. Crystal accumulation led to enrichment of pyrochlore crystals in the plug margins, phoscoritic units producing high-grade concentric dykes. Degassing of the carbonatite magma and fenitization reduced F and Na activity, leading to the crystallization of magmatic Na-, F- poor pyrochlore but progressively enriched in LILE and HFSE. Mingling of lamprophyric and carbonatite magmas produced explosive processes and the formation of carbonatite breccia. Pyrochlore is the main Nb carrier in mingled carbonatites and phoscorites, whereas Nb is concentrated in perovskite within mingled lamprophyres. During subsolidus processes, hydrothermal fluids produced dolomitization, ankeritization and silicification. At least three pyrochlore generations are associated with late processes, progressively enriched in HFSE, LILE and REE. In the lamprophyric units, perovskite is replaced by secondary Nb-rich perovskite and Nb-rich rutile. REE-bearing carbonates and phosphates formed only in subsolidus stages, along with late quartz; they may have been deposited due to the release of the REE from magmatic carbonates during the hydrothermal processes.