Experimental study of chromium (III) coprecipitation 3 with calcium carbonate
Experimental study of chromium (III) coprecipitation 3 with calcium carbonate
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铬(III)与碳酸钙共沉淀3的实验研究
DOI：
10.1016/j.gca.2022.01.019
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发表时间：
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影响因子：
5
通讯作者：
Liping Qin
中科院分区：
文献类型：
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作者：
Ziyao Fang;Wei Liu;Tao Yao;Gentao Zhou;Shiqiang Wei;Liping Qin
海洋碳酸盐岩的铬同位素组成已被广泛用于追踪地质历史时期大气和海洋中氧含量的演化。最近的研究表明，在天然碳酸盐中发现的Cr主要是三价Cr，也称为Cr（III）。然而，Cr（III）在碳酸盐中的掺入行为仍然不确定。在这里，我们进行了铬（III）与碳酸钙的共沉淀实验，以限制铬（III）在碳酸盐沉积环境中的行为。扩展X射线吸收精细结构（EXAFS）的结果表明，Cr（III）是不相容的方解石晶格。在共沉淀样品中可能存在多种Cr物种。大部分Cr吸附在方解石的晶体表面或以无定形Cr氢氧化物[Cr（OH）3]的形式存在。一小部分Cr可以存在于方解石晶体中，但Cr 3+不能直接取代Ca 2+。它可能存在于晶体中的间隙空隙中，或以二价Cr氢氧化物阳离子[Cr（OH）2+]的形式占据Ca 2+的位置，导致扭曲的结构。此外，我们发现，Cr（III）可以刺激多晶型选择的球文石[碳酸钙（CaCO 3）的亚稳多晶型]在碳酸钙沉淀过程中，可能是由于铬氢氧化物的球文石晶体的表面上形成，阻碍其转化为方解石。由于铬（III）是不相容的碳酸盐晶格，我们认为，大多数天然碳酸盐中的铬吸附在晶体表面上，并可能获得在水-沉积物界面碳酸盐沉淀后。因此，碳酸盐岩的铬同位素组成很容易受到沉积后成岩作用的影响，沉积碳酸盐岩中铬同位素的变化不应完全对应于大气和海洋中氧含量的变化。利用碳酸盐岩铬同位素组成重建古环境时，应考虑沉积后的作用。
Chromium (Cr) isotope compositions of marine carbonates have widely been used to trace the evolution of oxygen levels in the atmosphere and ocean during geological history. Recent studies have indicated that Cr found in natural carbonates is dominated by trivalent Cr, also known as Cr(III). However, the incorporation behavior of Cr(III) into carbonates remains undefined. Here, we conducted coprecipitation experiments for Cr(III) with calcium carbonate to constrain the behavior of Cr(III) in carbonate deposition environments. Extended X-ray absorption fine structure (EXAFS) results suggest that Cr(III) is incompatible with the calcite crystal lattice. There are likely to be several Cr species in the coprecipitation samples. Most Cr is adsorbed on the crystal surface of calcite or exists as amorphous Cr hydroxide [Cr(OH)3]. A small fraction of Cr can reside in the calcite crystal, but Cr3+cannot directly substitute for Ca2+. It may exist in interstitial voids in crystals or occupy the position of Ca2+in the form of divalent Cr hydroxide cation [Cr(OH)2+], leading to a distorted structure. Moreover, we find that Cr(III) can stimulate polymorph selection of vaterite [a metastable polymorph of calcium carbonate (CaCO3)] during CaCO3precipitation, likely due to the formation of Cr hydroxide on the surface of vaterite crystals, hindering its transformation to calcite. Because Cr(III) is incompatible with the carbonate crystal lattice, we suggest that most Cr in natural carbonates is adsorbed on the crystal surface and may be acquired at water–sediment interfaces after carbonate precipitation. Therefore, Cr isotope compositions of carbonates may easily be affected by post-depositional diagenetic processes, and Cr isotope variations in sedimentary carbonates should not exclusively correspond to changes in oxygen levels in the atmosphere and ocean. Paleoenvironment reconstruction based on Cr isotope compositions of carbonates should take post-depositional processes into consideration.