The chemical stability of coffinite, USiO4•nH2O;: 0<n<2, associated with organic matter:: A case study from Grants uranium region, New Mexico, USA
The chemical stability of coffinite, USiO4•nH2O;: 0<n<2, associated with organic matter:: A case study from Grants uranium region, New Mexico, USA
复制标题
DOI：
10.1016/j.chemgeo.2008.02.009
复制
发表时间：
2008-06-01
期刊：
影响因子：
3.9
通讯作者：
Ewing, Rodney C.
中科院分区：
文献类型：
--
作者：
Deditius, Artur P.;Utsunomiya, Satoshi;Ewing, Rodney C.
对美国新墨西哥州赠款铀矿区上侏罗统莫里森组中的粗粒钙霞石[USiO 4 center dot nH（2）O; n=0-2]进行了研究，以了解在还原和氧化条件下钙霞石的形成和蚀变过程。拉长（高达35 pm）的棱柱形晶体的coffinite沉淀同时层的有机质（OM）和富钒云母在这个砂岩。基于共生和化学成分确定了三种不同的coffinite种群：i）原生coffinite，（U0.924Ca0.218Y，REE0.021）（1.163）（Si0.835P0.033As0.010）（0.878）O-4中心点nH（2）O，在还原条件下被迁移的有机酸部分溶解; ii）重结晶的钙钛矿，（U0.805Ca0.156Y，REE0.022Zr0.001）（0.984）（Si0.954P0.036As0.005）（0.995）O-4中心点nH（2）O，U [apfu]损失13%，Ca [apfu]损失28%，Y+La+Ce+Nd [apfu]富集5%。铀和稀土元素被纳入纳米级晶体的次生coffinite层内的有机质; iii）coffinite在氧化条件下广泛的蚀变过程中形成。该钙钛矿（U0.619Ca0.117Y，REE0.018）（0.754）（Si1.127P0.040-AS（0.004））（1.171）O-4中心点nH（2）O，与钙钛矿-（ii）相比，其U [apfu]、Ca [apfu]和Y+REE [apfu]分别亏损23%、25%和18%。(Na（Na，K）（UO_2SiO_3 OH）（H_2O）]和各种铀酰硫酸盐的混合物，在氧化条件下以粗粒钙铀石为代价沉淀出来，作为蚀变的最终产物。上限为0.2 [apfu] U6+被假定为coffinite结构。因此，理想的钙钛矿化学式为：（U1-x4+ Ux 6+）Si 1-xO 4中心点nH（2）O，其中0
Coarse-grained coffinite [USiO4 center dot 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 pm) 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, (U0.924Ca0.218Y,REE0.021)(1.163)(Si0.835P0.033As0.010)(0.878)O-4 center dot nH(2)O, that was partially dissolved by migrating organic acids under reducing conditions; ii) recrystallized coffinite, (U0.805Ca0.156Y,REE0.022Zr0.001)(0.984)(Si0.954P0.036As0.005)(0.995)O-4 center dot 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, (U0.619Ca0.117Y,REE0.018)(0.754)(Si1.127P0.040-AS(0.004))(1.171)O-4 center dot nH(2)O, is depleted similar to 23% of U [apfu], 25% of Ca [apfu] and 18% of Y+REE [apfu], as compared with coffinite-(ii). (Na,K)-boltwoodite, [(Na,K)(UO2SiO3OH)(H2O)], 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 U6+ was estimated to be in the range of 0.1-0.19 [apfu]. An upper limit of 0.2 [apfu] U6+ is postulated for the coffinite structure. Thus, the ideal chemical formula of coffinite is: (U1-x4+Ux6+)Si1-xO4 center dot nH(2)O, where 0