Mineralogy and occurrence of europium-rich dark monazite
1983; United States Government Publishing Office; Linguagem: Inglês
10.3133/pp1181
ISSN2330-7102
Autores Tópico(s)Radioactive element chemistry and processing
ResumoEuropium (Eu)-rich dark monazite has been found in 64 alluvial concentrates from 14 areas across Alaska between the Canadian border and the west end of the Seward Peninsula.This monazite is characterized by gray to black color, pelletlike form, high-Eu and low-thorium (Th) contents, turbidity caused by microscopic clouds of amorphous carbon and sagenitic rutile rods, and many inclusions of siltsized detrital minerals.Density (D) of Alaskan dark monazite ranges from 4.25 to 4.70, mainly due to the presence of inclusions.Otherwise, physical properties are similar to those of yellow mona zite.Refractive indices, a and 7, are similar to those of yellow mona zite, but interference figures are generally diffuse and show small (0°-5°) positive 2V.X-ray diffraction patterns and cell parameters are similar to those of yellow monazites.The average composition of 11 dark monazites from Alaska is (in percent): La-jOs 13.54, Ce2O3 29.00, Pr2O3 3.34, Nd2O3 14.00, Sm2O3 1.92, Eu2O3 0.31, Gd2O3 0.98, Y2O3 0.47, P2O6 22.36, ThO2 0.88, SiO2 8.35, TiO2 0.66, A1 2O3 1.98, Fe2O3 1.77, CaO 0.28, MgO 0.23, H2O 0.94=101.01.Amor phous carbon ranged from 0.22 to 1.22 percent in four samples and, in seven samples, minor elements were (in ppm): Ba 100-500, Cr 200-2,000, Cu 50, Pb 20-70, Sn 150, and U 29-260.Comparison of average analyses of dark monazites from several countries shows generally small variations of all oxides.Major com positional differences between worldwide averages of dark and yellow monazites are (in percent): Eu2O3 0.36 (dark), 0.05 (yellow); ThO2 0.84 (dark), 7.15 (yellow); and SiO2 9.65 (dark), 1.70 (yellow); but totals for (La-to-Gd) 2O3 are similar: 60.23 for 31 dark monazites, and 59.01 for 64 yellow monazites.In dark monazite, D varies directly with Ln2O3 (total lanthanide oxides) and inversely with SiO2 contents.REE (rare-earth element) distributions in dark monazites from Alaska, France, Taiwan, Zaire, and the U.S.S.R. follow the trend shown by REE distributions in yellow monazites in a graph of La/Nd vs. £ (=La+Ce+Pr).The same data plotted on log-normal paper fall along a straight line.CNR (chondrite-normalized ratios) and SNR (shale-normalized ratios) of La through Gd in dark monazite from Alaska are remarkably similar to these ratios in dark monazites from France, Taiwan, Zaire, and Montana; but most of the patterns for dark monazites from the U.S.S.R. and Spain are dissimilar.CNR and SNR patterns for La to Gd in dark monazites differ from the patterns found for yellow monazites from igneous and metamorphic rocks, but the CNR and SNR patterns for La to Gd in dark monazites are similar to CNR and SNR patterns for La to Gd in sedimentary rocks, especially shales.Yellow monazite has been shown to form mainly through igneous and high-grade regional metamorphic processes.Origin of dark monazite with low thorium and high europium has been attributed to several processes including precipitation from seawater, metamorphism of diagenetic rhabdophane, and as an accessory mineral in tin-bearing granites; but none of these concepts has general application.The origin of dark monazite by igneous, high-grade regionalmetamorphic, or diagenetic processes is rejected, because of the dissimilarity in the distributions of the REE in dark monazite and in yellow monazite of igneous, regional-metamorphic, or diagenetic origin.We presume that the REE distribution in dark monazite is inherited from the REE distribution in a former mineral, rock, or en vironment.Thus, rhabdophane cannot be the precursor mineral, nor can seawater be the source of the REE distribution found in dark monazite.Contact metamorphism is proposed as the mode of origin of dark monazite.Geologic relations of the Alaskan occurrences of dark monazite show that the preferred source rock is weakly metamor phosed shale or phyllite intruded by biotite granite.Commonly the shale is black and phosphatic.Under low-grade regional metamor phism, dark monazite should be widespread, but our limited data indicate mainly local associations of this mineral with granitic intru sions.From these observations, the prediction was made that dark monazite poor in Th and rich in Eu should be present in baked shales of the Phosphoria Formation.A test of this hypothesis in Montana was successful in finding dark monazite in sediments of streams that drain phyllitic rocks of the Phosphoria Formation.Dark monazite is an important source of Eu as well as other REE.Assuming that the contact-metamorphic origin of dark monazite is correct, then the presence of dark monazite indicates an area that should be prospected for metalliferous ores and phosphatic layers.The upper stability limit of about 300 °C for dark monazite may be useful in geothermometry applications.Recommendations are made for mineral exploration in overseas localities, based on the occur rence of dark monazite.70J174°I
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