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首页> 外文期刊>The American mineralogist >Synchrotron-based speciation of chromium in an Oxisol from New Caledonia: Importance of secondary Fe-oxyhydroxides
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Synchrotron-based speciation of chromium in an Oxisol from New Caledonia: Importance of secondary Fe-oxyhydroxides

机译：来自新喀里多尼亚的Oxisol中基于同步加速器的铬形态学：仲羟基氧化铁的重要性

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In New Caledonia, the weathering of ultamafic rocks under a tropical climate has led to the residual accumulation of trace elements in lateritic soils widely dominated by Fe-oxyhydroxides. The speciation of trace elements, such as Cr, Ni, and Co, in these Oxisols remains a major subject of interest regarding mining and environmental issues. We have assessed the speciation of chromium in the upper part of an Oxisol, by combining bulk and spatially resolved chemical analyses (EPMA and SEM-EDS) with synchrotron-based spectroscopic data (EXAFS and XANES). EPMA indicates that the main hosts for chromium in the bedrock sample are the silicates forsterite, enstatite, and lizardite. Hosting of chromium in these easily weatherable mineral species could lead to a significant loss of this element upon weathering. However, total analyses of major elements indicate only a slight depletion of Cr, together with an immobility of Fe and Al and drastic losses of Si and Mg, after the weathering of the bedrock. Such a low mobility of chromium is likely related to its significant incorporation in goethite and hematite formed after the weathering of Fe2+-bearing primary silicates. This efficiency of secondary Fe-oxyhydroxides at immobilizing chromium is demonstrated by quantitative analysis of EXAFS data that indicates that these mineral species host between 67 and 75 wt% of total Cr (compared to the 18 to 22 wt% of total Cr hosted by chromite). In addition, SEM observation and SEM-EDS analyses performed on the Oxisol samples also show some evidence for chemical weathering of chromite. Chromite could then represent a past and/or present source of chromium upon extended tropical weathering of the studied Oxisol, rather than a stable host. These results emphasize the importance of secondary Fe-oxyhydroxides, compared to Cr-spinels, on chromium hosting in Oxisols developed upon tropical weathering of ultramafic rocks. Although the trapping mechanism of chromium mainly corresponds to incorporation within the structural network of goethite and hematite, sorption reactions at the surface of these mineral species could also be involved in such a process. In addition, considering their potential oxidative reactivity that can generate Cr6+ or enhance the chemical weathering of chromite, the occurrence of Mn oxides could significantly modify the behavior of chromium upon weathering. These considerations indicate that further studies are needed to assess the actual potential of chromium release from Oxisols developed upon weathering of ultramafic rocks under a tropical climate.
机译：在新喀里多尼亚，热带气候下的超铀岩石风化导致痕量元素在富氧氧化铁所占主导的红土土壤中残留。这些矿质中痕量元素（例如Cr，Ni和Co）的形成仍然是有关采矿和环境问题的主要关注主题。我们通过结合本体和空间分辨化学分析（EPMA和SEM-EDS）与基于同步加速器的光谱数据（EXAFS和XANES），评估了Oxisol上部铬的形态。 EPMA表明，基岩样品中铬的主要基质是硅酸盐镁橄榄石，顽辉石和蜥蜴石。这些易风化矿物中的铬元素可能在风化时导致该元素的大量损失。然而，对主要元素的总体分析表明，在基岩风化之后，Cr仅有少量消耗，同时Fe和Al的固定性以及Si和Mg的大量损失。铬的这种低迁移率可能与其大量掺入含Fe2 +的原始硅酸盐风化后形成的针铁矿和赤铁矿有关。通过EXAFS数据的定量分析证明了仲Fe-羟基氢氧化物固定铬的效率，该数据表明这些矿物质占总Cr的67至75 wt％（相比之下，由铬铁矿占总Cr的18至22 wt％） 。此外，对Oxisol样品进行的SEM观察和SEM-EDS分析也显示了铬铁矿化学风化的一些证据。然后，在研究的Oxisol长时间热带风化时，铬铁矿可能代表过去和/或现在的铬源，而不是稳定的主体。这些结果强调了与Cr-尖晶石相比，次要的羟基氧化铁对超镁铁质岩石的热带风化所形成的Oxisol中铬的寄主的重要性。尽管铬的捕集机理主要对应于针铁矿和赤铁矿结构网络中的结合，但在这些过程中也可能涉及这些矿物种类表面的吸附反应。此外，考虑到其潜在的氧化反应性可产生Cr6 +或增强亚铬酸盐的化学风化，Mn氧化物的出现可显着改变风化时铬的行为。这些考虑表明，需要进一步的研究来评估在热带气候下，超镁铁质岩石风化后，从氧化溶胶中释放出铬的实际潜力。

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