Precipitation and alteration of coffinite (USiO 4 nH 2 O) in the presence of apatite ARTUR P. DEDITIUS 1,2, * ,SATOSHI UTSUNOMIYA 1,3 ,VE ´ RONIQUE POINTEAU 1,4 and RODNEY C. EWING 1 1 Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA 2 CSIRO, Division of Exploration and Mining, 26 Dick Perry Avenue, Kensington, WA 6151, Australia *Corresponding author, e-mail: artur.deditius@csiro.au 3 Present address: Department of Chemistry, Kyushu University, Ropponmatsu, 4-2-1, Chuou-ku, Fukuoka-shi, 810–8560, Japan 4 CEA Saclay, DPC/SECR, 91191 Gif-Sur-Yvette cedex, France Abstract: A mineral assemblage of coffinite, USiO 4 nH 2 O, n ¼ 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 (10 mm) precipitated under reducing conditions replacing CFAp, pyrite and aluminosilicates. Electron-microprobe analyses (EMPA) of coffinite indicate limited incorporation of P 2 O 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 0.950.09 Ca 0.150.02 ) P 1.100.1 (Si 0.840.08 P 0.060.02 ) s0.90 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 1 mm. Porosity also was formed during alteration of CFAp to M-Aut, which facilitated the migration of oxidizing fluids over distances of 150 mm 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. Key-words: coffinite, (Ca,Sr)-(meta)autunite, carbonate-fluorapatite, reducing conditions, oxidizing conditions. 1. Introduction The stability of coffinite, USiO 4 nH 2 O, n ¼ 0–2, at the nano- scale under reducing and oxidizing conditions received con- siderable attention recently ( e.g., Robit-Pointeau et al. , 2006; Cai et al. , 2007; Deditius et al. , 2008). Coffinite and other U(IV)-silicates are considered to be potentially important phases for U-sequestration during the early stages of spent nuclear fuel (SNF) ( 95% of UO 2 ) alteration in Si-rich ground water under reducing conditions ( e.g., Janeczek & Ewing, 1992a and b; Amme et al. , 2005; Grambow & Giffaut, 2006). However, there has been only limited inves- tigation of the precipitation and alteration of coffinite at the nano-scale under both reducing and oxidizing conditions. Most of the previous work has been focused on the analysis of variations in the composition of coffinite ( e.g., Hansley & Fitzpatrick, 1989; Janeczek & Ewing, 1996). However, Deditius et al. (2008) have shown that the presence of other phases, such as organic matter, can have an important effect on the crystal size and composition of coffinite. In this study we present the results of extensive nano-scale analysis, utilizing a variety of electron microscopy techniques (Utsunomiya & Ewing, 2003), for which coffinite occurs with carbonate-fluorapatite (CFAp) and (Ca,Sr)-(meta)autunite (M-Aut). This provides the basis for understanding the role of P, Ca and Na during the formation and alteration of coffinite. We demonstrate the importance of nano-scale geo- chemical environments in the formation of coffinite and its subsequent alteration to the U(VI)-phosphates. 2. Coffinite Coffinite is an orthosilicate, with the general formula of ABO 4 nH 2 O, n ¼ 0–2, (I4 1 /amd, Z ¼ 4). Coffinite may 0935-1221/10/0022-1990 $ 6.30 DOI: 10.1127/0935-1221/2010/0022-1990 # 2009 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart Eur. J. Mineral. 2010, 22, 75–88 Published online November 2009