Crystallinity effects on the vibrational spectral features of saponite: Implications for characterizing variable crystalline phyllosilicates on Mars

Abstract

X-ray diffraction patterns of Martian mudstones acquired by Chemistry & Mineralogy X-Ray Diffraction (CheMin) aboard Mars Science Laboratory (MSL) suggest that the smectites detected in Gale crater have poor crystallinity. This finding poses an urgent question about the structural ordering of phyllosilicates found globally by Visible/Near-Infrared (VIS-NIR) spectroscopic orbital remote sensing on Mars, linked to their formation conditions. In this study, we synthesized saponite (Nz+x/z[M6][Si8-xAlx]O20(OH)4·nH2O, where M and N correspond to the divalent octahedral cations and the interlayer cations, respectively) with variable crystallinity, which bear structural similarities to the smectite discovered at Gale crater. Synthetic saponite was characterized using Field Emission Scanning Electron Microscopy (FE-SEM), powder X-ray diffraction (XRD), then studied the spectral features of these samples using nuclear magnetic resonance spectroscopy (NMR), Raman spectroscopy, and VIS-NIR reflectance spectroscopy. Our study revealed that the crystallinity of these saponite samples increased (as shown in the FWHM of XRD (060) peak), and is accompanied by (1) improved T-O-T layer stacking along the c axis (specified by the intensity and width of the XRD (001) peak); (2) increased uniformity of the SiO4 unit in tetrahedral sheets (based on the peak widths of Raman spectra and the well-resolved 29Si NMR peaks); (3) improved regularity in the distribution of Mg2+ in octahedral sites and thus the regularity of metal-OH bonds in octahedral sheet (based on the resolution of the NIR 2.2–2.4 μm band and the peak width of the XRD (060,330) peak); and (4) the increased Al in tetrahedral sites and decrease of Al in octahedral sites (the width of 27Al NMR peak), the crystallinity of saponite raised. Based on experimental observations, the first derivative spectra of metal-OH absorptions were proposed as a crystallinity index for smectite on Mars. Our results indicate that different vibration spectroscopy techniques can constrain the structural ordering of smectite on Mars and provide insight into their formation conditions.