Uranium-rich pyrochlores from the Iímaussaq complex, South Greenland
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 U0.11-0.260.15-0.63)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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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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