Crystal chemistry of ferric smectites and implications for Martian clay detection

We synthesized several series of Fe/Mg smectites, each with a graded composition and crystallinity. We studied the relationship between the layer ordering of these smectites and near-infrared (NIR) wavelength and absorption depth by analyzing the crystallinity index (V/P) from X-ray diffraction and the M-OH absorption and its second-order derivative spectra from NIR spectra at around 2390 nm. Using this analysis, we proposed a semi-quantitative method for evaluating the crystallinity orders of Martian smectites based on CRISM NIR spectra.
Table 1 outlines the experimental conditions for synthesizing Fe/Mg smectites. Figure 1 displays X-ray diffraction (XRD) patterns of the synthesized smectites with varying reaction times and Fe/Mg ratios. Meanwhile, Figure 2 illustrates the near-infrared (NIR) spectra of the synthesized nontronites with differing Fe/Mg ratios and reaction times, and Figure 3 shows the Deriv2 NIR spectra of those depicted in Figure 2. For statistical purposes, Figure 4 shows the position of the M−OH combination bands of smectites with different Fe/Mg ratios. Figure 5 displays a visualization of the smectite crystallinity index (V/P), and Table 2 presents the corresponding crystallinity indices of the smectites based on the XRD results. The correlation between the Deriv2 ratio of the NIR spectra and the crystallinity indices (V/P) of the smectites can be found in Figure 6. Moreover, Figure 7 compares the XRD patterns of a natural nontronite with that of a synthesized saponite. Figures 8a and 8b illustrate the NIR spectra and the Deriv2 NIR spectra of the natural nontronite and the synthesized saponite respectively. Lastly, Figure 9 depicts the NIR spectra of the smectites found on Mars (CRISM) and their analogs on Earth, with various V/P indices.

本研究合成了多系列铁镁蒙脱石（Fe/Mg smectites），各系列的组成与结晶度均呈梯度变化。本研究通过分析X射线衍射（XRD）测得的结晶度指数（V/P），以及近红外（NIR）光谱中约2390 nm处的M-OH吸收峰及其二阶导数光谱，探究了此类蒙脱石的层序结构与近红外波长、吸收深度之间的关联。基于上述分析，本研究提出了一种基于火星紧凑型侦察成像光谱仪（Compact Reconnaissance Imaging Spectrometer for Mars，CRISM）近红外光谱的半定量方法，用于评估火星蒙脱石的结晶度等级。

表1概述了铁镁蒙脱石合成实验的各项条件。图1展示了不同反应时间与铁镁比例下合成蒙脱石的X射线衍射（XRD）谱图。与此同时，图2呈现了不同铁镁比例与反应时间下合成的绿脱石（nontronites）的近红外（NIR）光谱，图3则对应展示了图2中光谱的二阶导数（Deriv2）近红外光谱。为开展统计分析，图4展示了不同铁镁比例下蒙脱石的M-OH组合谱带位置。图5展示了蒙脱石结晶度指数（V/P）的可视化结果，表2则基于X射线衍射结果给出了对应蒙脱石的结晶度指数。图6展示了近红外光谱的二阶导数比值与蒙脱石结晶度指数（V/P）之间的相关性。此外，图7对比了天然绿脱石与合成皂石（saponite）的X射线衍射（XRD）谱图。图8a与图8b分别展示了天然绿脱石与合成皂石的近红外（NIR）光谱及其二阶导数近红外光谱。最后，图9展示了CRISM探测获取的火星蒙脱石光谱及其地球类似物的近红外（NIR）光谱，各光谱对应不同的V/P指数。