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Primary phases in aluminous slags produced by the aluminothermic reduction of pyrochlore
Rare Earth Element Ore Geology of Carbonatites
Abstract For nearly 50 years, carbonatites have been the primary source of niobium and rare earth elements (REEs), in particular the light REEs, including La, Ce, Pr, and Nd. Carbonatites are a relatively rare type of igneous rock composed of greater than 50 vol % primary carbonate minerals, primarily calcite and/or dolomite, and contain the highest concentrations of REEs of any igneous rocks. Although there are more than 500 known carbonatites in the world, currently only four are being mined for REEs: the Bayan Obo, Maoniuping, and Dalucao deposits in China, and the Mountain Pass deposit in California, United States. The carbonatite-derived laterite deposit at Mount Weld in Western Australia is also a REE producer. In addition to REEs, carbonatite-related deposits are the primary source of Nb, with the Araxá deposit, a carbonatite-derived laterite in Minas Gerais state, Brazil, being the dominant producer. Other commodities produced from carbonatite-related deposits include phosphates, iron, fluorite, copper, vanadium, titanium, uranium, and calcite. Types of ores include those formed as primary magmatic minerals, from late magmatic hydrothermal fluids, and by supergene enrichment in weathered horizons. Although the principal REE-bearing mineral phases include fluorocarbonates (bastnäsite, parisite, and synchysite), hydrated carbonates (ancylite), and phosphates (monazite and apatite), the dominant mineral exploited at most mines is bastnäsite. Bastnäsite typically is coarse grained and contains approximately 75 wt % RE 2 O 3 (rare earth oxides; REOs). Processes responsible for REE enrichment include fractional crystallization of the carbonatitic magma, enrichment of REEs in orthomagmatic or hydrothermal fluids and subsequent precipitation or subsolidus metasomatic redistribution of REEs, and breakdown of primary carbonatitic mineral phases by chemical weathering and sequestration of REEs in secondary minerals or in association with clays. Carbonatites are primarily associated with continental rifting, but some carbonatites are associated with orogenic activity. Although there is debate on how carbonatite magmas are generated, the parental magma and REEs are clearly derived from mantle sources.
Rare Earth Mining and Exploration in North America
Large-Scale Development of Supergene Anatase-Rich Soils on Carbonatite-Associated Pyroxenites in Brazil (Abstract)
Abstract The climatic conditions in central and northern Brazil are conducive to the development of deep lateritic weathering in some carbonatite complexes, including the carbonatite rocks and some of their genetically and spatially related silicate rock units. Lateritic weathering of bebedourite-type pyroxenites genetically related to carbonatites in Brazil has produced a spectacular accumulation of high-grade anatase derived from the decalcification of perovskite. Carbonatite occurrences that contain vast quantities of supergene anatase-rich soils overlying weathered pyroxenites include Tapira, Serra Negra, Salitre I, and Salitre II in Minas Gerais; Cataläo I, in Goiás; and Maicuru and Maraconai in Pará. Soils overlying the weathered pyroxenites range from approximately 15 to 30 wt percent TiO 2 . At Serra Negra, an exploration drill hole gave an average of 20 wt percent TiO 2 as anatase mineralization for a depth of 150 m. The average depth of weathering in weathered pyroxenite in the northwest quadrant of the Serra Negra circular structure is 120 m and the development of anatase is reported to be 40.5 million tons (Mt) of 28.52 wt percent TiO 2 . In the Bananeira area of Salitre I, the average depth of weathering is 80 to 100 m, with reported 60.34 Mt of 24.5 wt percent TiO 2 . In the Para carbonatite, the weathering depth may exceed 300 m. Within the weatheredpyroxenites, diopside, perovskite, apatite, and calcite undergo total chemical breakdown. Calcium and magnesium are efficiently removed from the complexes in the groundwater. Decalcification of the perovskite produces polycrystalline aggregates of micrometer-size anatase platelets. Liberated REE from perovskite, apatite, and calcite are largely immobile, forming supergene monazite, rhabdophane,cerianite, and crandellite-group minerals that occur as crystallites that are inextricably associated with anatase. Direct indication of the development of anatase from perovskite is clearly shown in three ways: (1) by texture with residual cores of perovskite surroundedby polycrystalline clusters of anatase, (2) by pseudomorphsof anatase after euhedral perovskite octahedral, and (3) because chondrite-normalized REE plots for anatase and perovskite show identical slopes that serve as a geochemical proof that anatase is derived from the perovskite. Bebedourite, first described by E. Tröger in 1928, is a pyroxenite containing major amounts of perovskite. The bebedouritesof Minas Gerais and Goiás are of Cretaceous age. Similar pyroxenites occur in other carbonatites, including the Precambrian Powderhorn of Colorado, North America, and in Kovdor of the Kola Peninsula, but in these occurrences, the absence of deep-penetrating lateritic weathering produces only a very thin rock-surface coating of anatase, for which the origin from the decalicification of perovskite is virtually identical to the anatase found in the Brazil carbonatites.
Rare earth elements in synthetic zircon: Part 1. Synthesis, and rare earth element and phosphorus doping
Rare-earth-element-activated cathodoluminescence in apatite
Crystal chemistry of the monazite and xenotime structures
The atomic arrangement of bastnäesite-(Ce), Ce(CO 3 )F, and structural elements of synchysite-(Ce), röntgenite-(Ce), and parisite-(Ce)
Zirconium-bearing minerals of the Strange Lake intrusive complex, Quebec-Labrador
Dielectric constants of diaspore and B-, Be-, and P-containing minerals, the polarizabilities of B 2 O 3 and P 2 O 5 , and the oxide additivity rule
Rare-earth-element ordering and structural variations in natural rare-earth-bearing apatites
Abstract: Luminescence in calcite and dolomite is governed by physical phenomena that are common to all oxygen-dominated crystalline substances, including other carbonates and silicates. Absorption of excitation energy, energy transfer, and emission involve predictable transitions between electronic energy levels. Strong emission in various colors is always caused by impurities which function as activators of luminescence. Visible luminescence is not expected from pure, undistorted insulators, including carbonates. However, a faint blue ‘intrinsic’ luminescence, with a broad emission peak (band) around 400 nm, presumably caused by lattice defects, occurs in pure calcite and dolomite, and even in some samples containing impurities. The most important activators in carbonates are transition elements and rare earth elements. Luminescence spectra can be used for activator identification. These spectra are largely independent of the type of excitation, e.g., electron beam (cathodoluminescence = CL), photon (photoluminescence = PL), X-Ray (radioluminescence = RL) excitation, and others. Emission intensities depend on activator, sensitizer, and quencher concentrations, and on the method of excitation. At a given activator concentration, the luminescence intensity generally increases with an increase in excitation energy from PL (relatively weak) to CL (strong). Changes in visual luminescence color between different excitation methods are caused by relative changes in emission peak heights. Mn 2+ appears to be the most abundant and important activator in natural calcite and dolomite. Substituting for calcium in both minerals, its emission is orange-red to orange-yellow, with a fairly broad band between 570-640 nm (maximum between 590-620 nm). The emission band maximum of Mn 2+ substituting for Mg 2+ (in dolomite) is located around 640-680 nm. As little as 10-20 ppm Mn 2+ in solid solution are sufficient to produce visually detectable luminescence, if total Fe contents are below about 150 ppm. Sm 3+ activated luminescence can be visually indistinguishable from that activated by Mn 2+ . The spectrum of Sm 3+ emission, however, is quite distinct from that of Mn 2+ and consists of three narrow bands at 562 nm, 604 nm, and 652 nm. Tb 3+ and Dy 3+ activate green and cream-white luminescence, respectively. The main emission of Tb 3+ is at 546 nm. The emission of Dy 3+ consists of three bands, located at 484 nm, 578 nm, and 670 nm. Emission from Eu-containing calcite is red or blue. Narrow spectral bands of 590 nm, 614 nm, and 656 nm are caused by Eu 3+ and correspond to the red emission. A broad emission spanning a large range of shorter wavelengths is caused by Eu 2+ and corresponds to the blue emission. As in the case of Sm 3+ -activated luminescence, the red Eu 3+ luminescence can be mistaken visually for Mn 2+ -activated luminescence. Visual luminescence detection limits for rare earths are on the order of 10 ppm. Pb 2+ is an activator, with an emission band around 480 nm, but it also is a sensitizer of Mn 2+ -activated luminescence in carbonates. Another recognized sensitizer for Mn 2+ in carbonates is Ce 3+ . Sensitizers appear to be effective at concentrations as low as 10 ppm in calcite. Quenchers of Mn 2+ -activated luminescence in carbonates are Fe 2+ , Co 2+ , Ni 2+ , and Fe 3+ . The concentrations at which quenchers appear to be effective may vary from element to element and with host mineralogy. Effective minimum concentrations as low as 30-35 ppm have been reported for calcite. The interplay of Mn 2+ and Fe 2+ , commonly regarded to be the most important activator and quencher, respectively, in determining the luminescence characteristics of natural carbonates is not well understood because the available data are partially inconsistent. The Mn/Fe ratio may exert a control on luminescence intensity. Mn and Fe concentrations at which ‘bright’ CL changes to ‘dull’ can be determined only semi-quantitatively. The available data on the concentration of Mn 2+ at which quenching starts are partially inconsistent. Consequently, the Mn 2+ concentration at which concentration extinction occurs has not been determined unequivocally. The data presented and summarized in this paper can be used as a basis for the interpretation of luminescence of geological materials. In particular, knowledge of the possibilities and complexities of activation, sensitization, and quenching has great potential for the interpretation of diagenetic carbonate cements.