Evaluation of the coprecipitation of incompatible trace elements with fluoride during silicate rock dissolution by acid digestion
Evaluation of the coprecipitation of incompatible trace elements with fluoride during silicate rock dissolution by acid digestion
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DOI：
10.1016/s0009-2541(98)00206-x
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发表时间：
1999-05-24
期刊：
影响因子：
3.9
通讯作者：
Nakamura, E
中科院分区：
文献类型：
--
作者：
Yokoyama, T;Makishima, A;Nakamura, E
镁基硅酸盐岩石在HF消解过程中形成的不溶性氟析出物在其结构中共沉淀出Rb、Sr、Y、Ca、Ba、REE、Pb、Th、U等微量元素，妨碍了其准确测定。我们定量地估计了微量元素在这些氟化物中的共沉淀，并提出了一种有效的消化方法，可以完全抑制氟化物的沉淀。对3种镁铁质和超镁铁质硅酸盐岩石样品进行常规酸溶，会产生粘稠物质的沉淀，产物中某些微量元素的产率很低。XRD分析表明，析出相由CaAlF5、CaMg2Al2F12、Na0.88Mg0.88Al1.12(F，OH)(6)等氟化物组成。H2O和MgF2，它们的形成取决于岩石样品的主要元素组成。微量元素的共沉淀似乎受到元素离子半径和价态以及宿主氟化物沉淀种类的强烈控制，导致元素选择性地损失到这些氟化物中。另一方面，几乎100%的微量元素被回收使用更大的HClO4比传统使用和蒸发样品干燥在一个循序渐进的方式。采用该方法，样品分解后形成白色析出物，为高场强元素的氧化物。除Th外，本研究中感兴趣的微量元素与氧化物的共沉淀可以忽略不计，其0.5-3.2%（按重量计）可能作为不溶性氧化物共沉淀。由于该方法的空白值也可以忽略不计，因此既可以用于ICP-MS精确测定痕量元素，也可以用于TIMS同位素分析。(C) 1999 Elsevier Science B.V.版权所有
Insoluble fluoride precipitates which form during HF digestion of mafic silicate rocks coprecipitate in their structures the trace elements such as Rb, Sr, Y, Ca, Ba, REE, Pb, Th and U, thus hindering their accurate determination. We have estimated quantitatively the coprecipitation of trace elements into such fluorides, and suggest an effective method of digestion that can suppress completely fluoride precipitation. Conventional acid digestion of three samples of mafic and ultramafic silicate rocks resulted in the precipitation of sticky material and very poor yields of certain trace element in the resultant solution. XRD analysis indicated that the precipitates were composed of fluorides such as CaAlF5, CaMg2Al2F12, Na0.88Mg0.88Al1.12(F,OH)(6). H2O and MgF2, the formation of which depended on the major element composition of the rock sample. Coprecipitation of trace elements appeared to be strongly controlled by both ionic radius and valency of the elements as well as the species of the host fluoride precipitate, resulting in selective losses of the elements into these fluorides. On the other hand, almost 100% of the trace elements were recovered using larger amounts of HClO4 than is conventionally used and evaporating the sample to dryness in a step-wise fashion. Using this method, white precipitates were formed as oxides of high field strength elements after decomposition of the sample. Coprecipitation of trace elements of interest in this study with the oxides is negligible except for Th for which 0.5-3.2% by weight was coprecipitated probably as the insoluble oxide. As our method also results in negligible blank values, it can be used for both the accurate determination of trace element using ICP-MS as well as isotope analysis using TIMS. (C) 1999 Elsevier Science B.V. All rights reserved.