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.