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 at n ≤ 0.2 and that the crystallinity of Fe‐rich allophane decreased with increasing n. 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.%. As n increased 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 (1.2–2.6 μm) of the products showed incapability in identifying poorly ordered minerals in Al‐Si‐Fe systems, while the features in the range of 0.4–1.2 μm 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.

中文翻译：

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环境铁浓度对Al-Si-Fe系统中脲烷的形成和演化的影响：对地球和火星的影响

Allophane是地球上的常见成分，也是火星上非晶成分的可能成分，它以结构Fe和/或氧化铁（羟基氧化物）涂层的形式与Fe紧密结合。然而，直到现在，很少研究作为环境铁浓度的产物的脲基甲烷的形成和演化。我们研究了水铝英石沉淀从具有不同的Fe /（铁+ Al）的摩尔比（凝胶Ñ，0≤ Ñ ≤1.0）。X射线衍射图案和傅立叶变换产品的红外光谱表明，水铝英石几乎在唯一的产品Ñ ≤0.2和富Fe英石的结晶度随下降Ñ。结合透射电子显微镜和Mössbauer光谱分析的结果，发现Fe主要掺入到苏铁石状的辉石状片中，形成团簇。最高的铝铁取代含量粗略估计为20摩尔％。作为n进一步增加，脲基铝石的形成受到越来越多的抑制，Fe相开始从Al-Si相中分离出来，形成了初始脲基铝石和初始赤霞石的混合物，最后是赤霞石的混合物。产品的近红外光谱数据（1.2-2.6μm）显示无法识别Al-Si-Fe系统中不良有序的矿物，而0.4-1.2μm范围内的特征对于研究产品中的铁元素具有强大的作用。 。这些发现不仅提供了对地球上Al-Si-Fe系统中脲烷的形成，演化和地质作用的见解，而且还有助于约束在火星上脲烷和赤辉石共生的位置的古环境。