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Uranium-rich pyrochlores from the Iímaussaq complex, South Greenland

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Uranium-bearing primary and secondary pyrochlore was found as accessory minerals in pegmatites and mineral veins in the kakortokite rocks of the Ilímaussaq complex in South Greenland. They contain between 7.70 and 17.79 wt.% UO2and have the compositional range: (Na0.08-0.88 Ca0.90-0.26 Mn0.00-0.02 REE 0.24-0.30 U0.11-0.26[]0.15-0.63)Σ2.00 (Nb1.42-1.67 Ti0.47-0.30 Zr0.00-0.02 Fe0.00-0.10 Si0.00-0.10)Σ2 O6 (F0.10-0.50 OH0.17-0.73 []0.02-0.46)Σ1 with unoccupied positions in both the A and Y-sites. Deuteric alteration has resulted in the formation of secondary pyrochlores characterized by the almost total removal of Na but without significant change in the content of the other cations present, accompanied, either by depletion of OH in the Y-site or protonation. Its compositional range is: (Na0.04-0.24 K0.00-0.05 Ca0.68-0.21 Mn0.00-0.02 REE 0.19-0.31 U0.11-0.25 []0.74-1.01)Σ2 (Nb1.42-1.69 Ti0.28-0.46 Zr0.00-0.01 Fe0.01-0.05 Si0.00-0.11 )Σ2 (O5.78-6.03 []0.35-0.01)Σ6 F0.16-0.44 · nH2 O. Complete silicification (12.7–15.4 wt.% SiO2) of some pyrochlore crystals is characterized by negative correlation between Si and (Nb+Ti) and has resulted in the following empirical composition (Na0.04 Ca0.23 Mn0.05 Al0.14 REE 0.37 U0.10)Σ0.92 (Si0.78 Nb0.95 Ti0.20 Fe3+ 0.07)Σ2.00O5.72F0.07. nH2 O. An unidentified Nb-U-Pb- REE mineral named Nb3 has the empirical composition: Ca0.9 Fe0.5 Mn0.8 Pb2.0 REE 1.4 Al0.5 Si6.2 U1.0 (Nb7.8 Ti1.9 Zr0.2)Σ10.0 F0.1 (O, OH, H2O)n.
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Keywords: ILÍMAUSSAQ COMPLEX; SILICIFIED URANIUM-RICH PYROCHLORE; SOUTH GREENLAND; UNKNOWN MINERAL NB3; URANIUM-RICH PYROCHLORE

Document Type: Research Article

Publication date: 01 December 2018

This article was made available online on 04 October 2018 as a Fast Track article with title: "Uranium-rich pyrochlores from the Iímaussaq complex, South Greenland".

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  • Neues Jahrbuch für Mineralogie - Abhandlungen: Journal of Mineralogy and Geochemistry publishes thorough, detailed studies of up to 30 printed pages as well as short and concise papers and notes (less than 15 printed pages) on mineralogy, i.e. crystallography, crystal chemistry, petrology, geochemistry and economic mineralogy (mineral deposits).

    Beginning with 2005, Neues Jahrbuch für Mineralogy (Abhandlungen) publish all papers formerly submitted to the two subjournals Abhandlungen and Monatshefte.
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