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.
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XRD patterns were collected with a Bruker D8 Advance diffractometer (Germany), operating at 40 kV and 40 mA using CuKα radiation. The synthesized products were investigated from 3 to 70° (2θ) with a scanning speed of 3°/min. The humidity of testing evironment is controlled at ca. 30% by an automatic air dehumidifier. Samples, which were randomly oriented, were prepared by pressing powder into a cavity up to the reference level of the sample holders. Near Infrared Spectroscopy measurements were carried out using an ASD Fieldspec Pro spectrometer in the range from 350 to 2500 nm. The spectral resolutions are 3 and 10 nm for the range of 350−1000 and 1000−2500 nm, respectively. An accessory light source was employed to measure spectrum without sunlight. During the measurements, the detector was attached to a relatively flat surface of the sample to avoid possible influence from fluorescence lighting in the environment. A white Teflon reference was introduced during measurement to optimize and calibrate the instrument. To facilitate comparison of the combination bands (1.80−2.50 μm), the spectral continuum was removed from 0.35 to 2.50 μm for all spectra using a straight−line continuum across the extreme convex points [Bishop et al., 2008]. Spectral band depths of smectites were calculated according to the formulas by Viviano−Beck et al. . The gently mixed/homogenized synthetic samples (< 200 mesh) were dried at 80℃ for 2 h, and then used for the NIR test. Each sample was measured in different directions at an interval of ca. 30° for 5−7 times. After averaging, the spectral data were processed to eliminate the random error caused by the nonuniformity of the sample. The X-ray diffraction (XRD) data were analyzed and processed through Eva software. The CRISM datasets were download from the Planetary Data System (PDS) and underwent processing through the CRISM Analysis Toolkit v7.4 using standard procedures for geometry calibration and atmospheric correction (Ehlmann, 2012). All XRD patterns and NIR spectra were further processed using SciDAVis software.