Alteration of Coffinite (USiO 4 ) Under Reducing and Oxidizing Conditions
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0985-NN01-08
Alteration of Coffinite (USiO4) Under Reducing and Oxidizing Conditions Artur Piotr Deditius, Satoshi Utsunomiya, and Rodney C. Ewing Geological Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI, 48109
ABSTRACT Samples of natural coffinite (USiO4•nH2O) from Grants uranium region, New Mexico were investigated in order to understand the alteration process of coffinite under reducing and oxidizing conditions. Alteration of the primary coffinite under reducing conditions was promoted by organic acids, and as a result, secondary coffinite precipitated. Subsequently oxidizing fluids altered the coffinite, and (Na,K)-boltwoodite [(Na,K)(UO2)(SiO3OH)(H2O)1.5] and jáchymovite [(UO2)(SO4)(OH)14(H2O)13] precipitated with no rare earth elements. Based on the charge balance calculation, we suggest that the amount of U6+ in the coffinite is less than 0.2 [apfu] and U6+ is accommodated in the structure via substitution: U4+ + Si4+ ⇔ U6+ + 2(OH)-. The high and variable totals for electron microprobe analyses indicate that H2O is not an essential component in coffinite structure. The U-Pb ages of coffinite formation vary from 36.6-0 Ma suggesting that the coffinite has precipitated continuously in this period and organic matter can preserve reducing conditions even when oxidizing conditions dominate. INTRODUCTION Coffinite (USiO4•nH2O, I41/amd, Z=4) is major alteration phase of uraninite, UO2+x, under reducing conditions in natural uranium deposits. Because spent nuclear fuel (SNF) contains ~95% of UO2, it is suggested that coffinite will be also a dominant alteration phase in reducing and silica-rich conditions of repositories. Additionally, the temperatures of coffinite precipitation in sedimentary rocks vary from 80 to 130oC, which is in good agreement with estimated postdepositional temperatures, 80-120oC. Indeed, the previous experimental studies reported that coffinite precipitated during UO2 corrosion under reducing conditions at temperatures up to 180oC [1, 2-5]. On the other hand, a study on natural coffinite from Bangombé, Gabon has shown that the coffinite can incorporate a trace amount of lanthanides and actinides [6]. Coffinite belongs to the orthosilicate group described as a general formula, ABO4, where AVIII position can be occupied by U4+, Th4+, Zr4+, Np4+, Pu4+, Am4+, Hf4+, U6+, Ca2+, Y3+, REE3+, while the tetragonal B-site can be occupied by Si4+, P5+, S6+ and As5+. The variability of the elements implies numerous couple substitutions and possible solid solutions in the system coffinite-zircon (ZrSiO4)-thorite (ThSiO4)-xenotime (YPO4)-ningyoite(U,Ca,Ce)2(PO4)2•1-2 H2O [7, 8]. Coffinite in nature is often associated with organic matter or/and sulfide minerals such as galena (PbS), pyrite (FeS), and chalcopyrite (CuFeS2) indicating the reducing conditions in the system. However, crystallization and alteration processes of coffinite as well as its thermodynamic property have not yet been fully understood. The lack of information is due to the fine-grained nature (20µm) precipitat
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