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Precipitation and alteration of coffinite (USiO (sub 4) .nH (sub 2) O) in the presence of apatite

Artur P. Deditius, Satoshi Utsunomiya, Veronique Pointeau and Rodney C. Ewing
Precipitation and alteration of coffinite (USiO (sub 4) .nH (sub 2) O) in the presence of apatite
European Journal of Mineralogy (January 2010) 22 (1): 75-88


A mineral assemblage of coffinite, USiO (sub 4) .nH (sub 2) O, n=0-2, carbonate-fluorapatite (CFAp) and (Ca, Sr)-(meta)autunite (M-Aut) from the Woodrow Mine, Grants uranium region, New Mexico, has been investigated in order to understand the influence of a P-rich micro-geochemical environment on precipitation of coffinite and its subsequent alteration under oxidizing conditions. Fine-grained coffinite (< or =10 mu m) precipitated under reducing conditions replacing CFAp, pyrite and aluminosilicates. Electron-microprobe analyses (EMPA) of coffinite indicate limited incorporation of P (sub 2) O (sub 5) and CaO, <2.7 and <3.0 wt%, respectively, into the coffinite structure during replacement of CFAp. The chemical formula of coffinite is (U (sub 0.95 + or - 0.09) Ca (sub 0.15 + or - 0.02) ) (sub Sigma 1.10+ or -0.1) (Si (sub 0.84 + or - 0.08) P (sub 0.06 + or - 0.02) ) (sub sigma 0.90+ or -0.08) . Analysis by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) revealed that coffinite initially formed as crystals as large as 100 nm at the edges of altered CFAp. Subsequently, infiltration of (Na, Ba, Sr)-rich oxidizing fluids into fractures resulted in precipitation of Sr-rich M-Aut (up to 4 wt% of SrO) at the expense of coffinite and CFAp. High-resolution TEM reveals that Na-rich fluids caused a distortion of the ideal coffinite structure and stabilized amorphous domains that formed due to alpha-decay event radiation damage. Subsequently, the Na-enriched amorphous areas of coffinite were preferentially altered, and secondary porosity formed at the scale of approximately 1 mu m. Porosity also was formed during alteration of CFAp to M-Aut, which facilitated the migration of oxidizing fluids over distances of approximately 150 mu m into CFAp, as evidenced by precipitation of M-Aut. We report, for the first time, the precipitation of coffinite at the expense of apatite and the subsequent alteration of coffinite under P-rich, oxidizing conditions. These results show that micro-scale dissolution of apatite can create conditions conducive to the precipitation of U(IV)- and U(VI)-minerals, leading to the reduced mobility of U-species under both reducing and oxidizing conditions.

ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 22
Serial Issue: 1
Title: Precipitation and alteration of coffinite (USiO (sub 4) .nH (sub 2) O) in the presence of apatite
Affiliation: University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States
Pages: 75-88
Published: 201001
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 44
Accession Number: 2010-033128
Categories: General geochemistryMineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., 3 tables, geol. sketch map
N35°15'00" - N35°15'00", W107°15'00" - W107°15'00"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201019
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