Effects of environmental Fe concentrations on formation and evolution of allophane in Al-Si-Fe systems: Implications for both Earth and Mars
Effects of environmental Fe concentrations on formation and evolution of allophane in Al-Si-Fe systems: Implications for both Earth and Mars
复制标题
环境铁浓度对 Al-Si-Fe 体系中水铝英石形成和演化的影响：对地球和火星的影响
DOI：
10.1029/2020je006590
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发表时间：
2020
期刊：
影响因子：
--
通讯作者：
Liu Chengshuai
中科院分区：
文献类型：
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作者：
Du Peixin;Yuan Peng;Liu Jiacheng;Yang Yixuan;Bu Hongling;Wang Shun;Zhou Junming;Song Hongzhe;Liu Dong;Michalski Joseph R.;Liu Chengshuai
水铝英石是地球上常见的成分，也是火星上无定形成分的可能成分，它以结构性铁和/或铁（氢氧化物）氧化物涂层的形式与铁密切相关。然而，到目前为止，水铝英石作为环境铁浓度的产物的形成和演化的研究很少。研究了不同Fe/（Fe + Al）摩尔比（n，0 ≤n≤ 1.0）的凝胶中水铝英石的沉淀。产物的X射线衍射和傅里叶变换红外光谱表明，当n ≤ 0.2时，水铝英石几乎是唯一的产物，富铁水铝英石的结晶度随n的增大而降低。结合透射电子显微镜和穆斯堡尔谱的结果，发现Fe主要结合到水铝英石的三水铝石状片中，形成团簇;粗略估计Fe对Al的最高取代含量为20 mol.%。当n进一步增加时，水铝英石的形成越来越受到抑制，并且Fe相开始与Al-Si相分离，导致初始水铝英石和初始赤霞石的混合物，最后是赤霞石。产物的近红外光谱数据（1.2-2.6μm）显示无法识别Al-Si-Fe系统中的无序矿物，而0.4-1.2μm范围内的特征对于研究产物中的铁的存在是有力的。这些发现不仅提供了对地球上铝硅铁系统中水铝英石的形成，演化和地质作用的见解，而且还有助于限制火星上水铝英石和赤霞石共存的地点的古环境。
Allophane, a common component on Earth and a probable constituent of the amorphous component on Mars, is closely associated with Fe in the form of structural Fe and/or iron (oxyhydr)oxide coatings. However, until now, the formation and evolution of allophane as products of environmental Fe concentrations have rarely been studied. We investigated allophane precipitation from gels with different Fe/(Fe + Al) molar ratios (n, 0 ≤n≤ 1.0). X‐ray diffraction patterns and Fourier transform infrared spectra of the products showed that allophane was nearly the only product atn≤ 0.2 and that the crystallinity of Fe‐rich allophane decreased with increasingn. Combined with the results of transmission electron microscopy and Mössbauer spectroscopy, Fe was found to be mainly incorporated into the gibbsite‐like sheet of allophane, forming clusters; the highest Fe‐for‐Al substitution content was roughly estimated to be 20 mol.%. Asnincreased further, the formation of allophane was increasingly suppressed, and the Fe phases began to separate from the Al‐Si phases, resulting in mixtures of incipient allophane and incipient akaganeite and finally akaganeite. The near‐infrared spectroscopic data (1.2–2.6μm) of the products showed incapability in identifying poorly ordered minerals in Al‐Si‐Fe systems, while the features in the range of 0.4–1.2μm were powerful for studying iron occurrence in the products. These findings not only offer insights into the formation, evolution and geological role of allophane in Al‐Si‐Fe systems on Earth but also help constrain the paleoenvironment of locations where allophane and akaganeite co‐occur on Mars.