Fractionation of the REE during hydrothermal processes: constraints from the Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China.
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Optical, x-ray, and differential thermal methods are used to compare monazite recently discovered near Chester, New Jersey, with monazite from nine other localities. lndices of refraction, 2V and birefringence have been determined. X-ray-diffraction patterns have been measured, indexed, and their relationships determined. Micro-camera diffraction patterns have provided information on the alteration of monazite. The application of x-ray fluorescence to the quantitative analysis of rare earth elements in monazite is shown to be feasible through sensitivity to minor variations caused by crystal fractionation. Theoretical factors which interfere with precise x-ray fluorescence, quantitative analysis of rare earth elements in monazite are examined. The effect of two types of alteration, intercrystalline and intracrystalline, upon the differential thermal pattern of monazite is observed. (auth)
Differential thermal analysis
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Paragenesis
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Abstract The content of monazite in the Egyptian black beach sand and coastal sand dunes is normally equal to or below 0.01 wt.%. The obtained high grade monazite concentrate includes three minor monazite groups in addition to major canary and lemon yellow coloured monazite: (i) the colourless to pinkish white coloured monazite; (ii) the opaque light to dark resinous, reddish brown and dark brown coloured monazite; and (iii) opaque yellowish red to brownish red coloured monazite grains group. These groups represent 3%, 4% and 2%, respectively, in the high grade monazite concentrate. A negligible amount of euhedral to subhedral black to brownish black chevkinite/perrierite mineral crystals was detected in the obtained monazite concentrate. The presence of these minor mineral groups affects the chemical composition of the obtained high grade monazite concentrate. The Ce 2 O 3 is the main REE in the studied monazite. In the colourless‐pinkish monazite grains, the analyzed REE are the following, in order of abundance; Ce > La > Nd > Pr > Sm > Gd > Dy. UO 2 ranges between 0.11 and 1.74 wt.%. The contents of Eu 2 O 3 is under the limit of detection while ThO 2 ranges between 3.99 and 8.58 wt.% with an average value of 5.57 wt.%. These grains are most probably igneous monazite from a highly differentiated granite. The resinous, brown monazite grains have lower Ce 2 O 3 content (24.63 wt.%) and much lower La 2 O 3 content (6.00 wt.%) but greater content of Eu 2 O 3 (0.41 wt.%) than those of the colourless‐pinkish monazite. These monazites have the lowest contents of Th, U and Ca among the three groups. The resinous, brown monazites are most probably formed by metamorphism or alteration leading to leaching or replacement of pre‐existing minerals. The red monazite group has a lower average Ce 2 O 3 content (25.28 wt.%) than the colourless‐pinkish variety (28.02 wt.%) but slightly greater than that of the resinous, brown ones. The red monazite group has the highest ThO 2 and UO 2 contents; 5.84 wt.% and 1.24 wt.%, respectively. It has the lowest monazite component mole fraction (0.75). The red monazite seems to have been formed by hydrothermal alteration of pre‐existing monazite and other mineral species bearing for Y, REE, Ca, Th and U. The two coupled substitution mechanisms: (Th, U) 4+ + Ca 2+ 2REE 3+ , and (Th, U) 4+ + Si 4+ REE 3+ + P 5+ , are obvious in the studied colourless‐pink monazite.
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Radiometric dating
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In determining lead in monazite [(Ce,La,Th)PO4]--to be used as the basis for geologic age measurements--it was necessary to eliminate interferences due to the presences of phosphates of thorium and the rare-earth metals. The method, in which monazite samples are attacked with hot concentrated sulfuric acid, taken up with dilute nitric acid, lead extracted as the dithizonate and then determined spectrophotometrically at 520 mμ, was successfully applied to a series of monazite samples. Rapid determinations were made with good reproducibility.
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