The chemical stability of coffinite, USiO 4 ·nH 2 O; 0 b n b 2, associated with organic matter: A case study from Grants uranium region, New Mexico, USA Artur P. Deditius ⁎, Satoshi Utsunomiya 1 , Rodney C. Ewing Department of Geological Sciences, University of Michigan,1100 N. University Avenue, Ann Arbor, MI 48109-1005, United States ABSTRACT ARTICLE INFO Article history: Received 23 March 2007 Received in revised form 6 February 2008 Accepted 11 February 2008 Edited by: D. Rickard Keywords: Coffinite Organic matter Alteration Reducing conditions Oxidizing conditions Coarse-grained coffinite [USiO 4 ·nH 2 O; n =0–2] from the Upper Jurassic Morrison Formation in the Grants uranium region, New Mexico, USA, has been investigated in order to understand the processes of coffinite formation and alteration under reducing and oxidizing conditions. Elongated (up to 35 μm) prismatic crystals of coffinite precipitated contemporaneously with layers of organic matter (OM) and a vanadium-rich mica in this sandstone. Three different populations of coffinite were determined based on paragenesis and chemical composition; i) primary coffinite, (U 0.924 Ca 0.218 Y,REE 0.021 ) 1.163 (Si 0.835 P 0.033 As 0.010 ) 0.878 O 4 ·nH 2 O, that was partially dissolved by migrating organic acids under reducing conditions; ii) recrystallized coffinite, (U 0.805 Ca 0.156 Y,REE 0.022 Zr 0.001 ) 0.984 (Si 0.954 P 0.036 As 0.005 ) 0.995 O 4 ·nH 2 O, for which there was a 13% loss of U [apfu] and 28% loss of Ca [apfu], while Y + La + Ce + Nd [apfu] is enriched 5%. Uranium and REEs were incorporated into nano-scale crystals of secondary coffinite within the layers of organic matter; iii) coffinite formed during extensive alteration under oxidizing conditions. This coffinite, (U 0.619 Ca 0.117 Y,REE 0.018 ) 0.754 (Si 1.127 P 0.040- As 0.004 ) 1.171 O 4 ·nH 2 O, is depleted ~23% of U [apfu], 25% of Ca [apfu] and 18% of Y +REE [apfu], as compared with coffinite-(ii). (Na,K)-boltwoodite, [(Na,K)(UO 2 SiO 3 OH)(H 2 O)], and a mixture of various uranyl sulfates, precipitated at the expense of coarse-grained coffinite-(i) as the final products of the alteration under oxidizing conditions. Based on charge balance calculations of coffinite groups (i) and (ii), the amount of U 6+ was estimated to be in the range of 0.1–0.19 [apfu]. An upper limit of 0.2 [apfu] U 6+ is postulated for the coffinite structure. Thus, the ideal chemical formula of coffinite is: (U 4+ 1 - x U 6+ x )Si 1 - x O 4 ·nH 2 O, where 0 b x b 0.2; 0 b n b 2. Compositional variations of coffinite are governed by the substitutions: U 4+ +Si 4+ f U 6+ + [2(OH) - , 0.5 □]; REE 3+ + As 5+ f U 4+ +Si 4+ ; Ca 2+ + (As, P) 5+ f REE 3+ +Si 4+ . The variable, but high, totals of the chemical analyses (92.8–99.7 wt.%) suggest that coffinite contains molecular H 2 O, which is not an essential component of coffinite structure. The amount of H 2 O varies between 0 and 2 molecules per formula unit, and a minor amount of (OH) - may be accommodated into the structure. The U–Pb chemical ages limit the age of coffinite precipitation to between 36.6 and 0.7 Ma, which indicates continuous precipitation of coffinite since the mid-Tertiary. These results suggest that organic matter, which has preserved reducing conditions even in the presence of oxidizing fluids, over this long period of approximately 30 million years, plays an important role in the sustained presence of coffinite in these deposits. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Coffinite, nominally USiO 4 ·nH 2 O, I4 1 /amd and Z =4, is one of the major U 4+ -minerals in economically exploitable U-ores, and coffinite is the second most abundant source of U in the world (Plant et al., 1999). Coffinite is an orthosilicate with the general formula of ABO 4 . There are a large number of possible substitution schemes: A-site: U 6+ , Zr 4+ , Chemical Geology 251 (2008) 33–49 ⁎ Corresponding author. Tel.: +1 734 763 5344; fax: +1 734 647 5706. E-mail address: deditius@umich.edu (A.P. Deditius). 1 Current address. Kyushu University, Department of Chemistry, Ropponmatsu, 4-2-1, Chuou-ku, Fukuoka-shi, 810-8560, Japan. Th 4+ , Hf 4+ , Ca 2+ ,Y 3+ , REE 3+ ; while for B-site: P 5+ , As 5+ ,V 5+ ,S 6+ ,F - and OH - groups, as well as the possibility of vacancies at the tetrahedral site (e.g., Stieff et al., 1956; Janeczek and Ewing, 1992a; Burns, 1999; Finch and Hanchar, 2003; Förster, 2006). The presence of multivalent elements in coffinite implies the possibility of limited or complete solid solutions between: coffinite–zircon (ZrSiO 4 )–thorite (ThSiO 4 )– xenotime (YPO 4 )–ningyoite UCa(PO 4 ) 2 ·1–2H 2 O (e.g., Smits, 1989; Janeczek and Ewing, 1992a; Finch and Hanchar, 2003; Förster, 2006). However, compositional data for coffinite are limited because: 1) it usually occurs as very fine-grained crystals, b10 μm; 2) it occurs in intimate intergrowths with large amounts of associated minerals, such as illite, uraninite and REE-phosphates, which is often the 0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2008.02.009 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo