Reflectance and emission spectroscopy study of four groups of phyllosilicates: smectites, kaolinite-serpentines, chlorites and micas

Abstract

Coordinated visible/near-infrared reflectance/mid-infrared reflectance and emissivity spectra of four groups of phyllosilicates were undertaken to provide insights into the differences within and among groups of smectites, kaolinite-serpentines, chlorites and micas. Identification and characterization of phyllosilicates via remote sensing on Earth and Mars can be achieved using the OH combination bands in the 2.2–2.5 μm region and the tetrahedral SiO₄ vibrations from ~8.8–12 μm (~1140–830 cm^–1) and ~20–25 μm (500–400 cm^–1). The sharp and well resolved OH combination bands in the 2.2–2.5 μm region provide unique fingerprints for specific minerals. Al-rich phyllosilicates exhibit OH combination bands near 2.2 μm, while these bands are observed near 2.29–2.31, 2.33–2.34 μm and near 2.35–2.37 μm for Fe³⁺-rich, Mg-rich and Fe²⁺-rich phyllosilicates, respectively. When a tetrahedral substitution of Al or Fe³⁺ for Si occurs, the position of the Si(Al,Fe)O₄ stretching mode absorption shifts. Depending on the size of the cation, the Si(Al,Fe)O₄ bending mode near 500 cm^–1 is split into multiple bands that may be distinguished via hyperspectral remote sensing techniques. The tetrahedral SiO₄ vibrations are also influenced by the octahedral cations, such that Al-rich, Fe-rich and Mg-rich phyllosilicates can be discriminated in reflectance and emissivity spectra based on diagnostic positions of the stretching and bending bands. Differences among formation conditions for these four groups of phyllosilicates are also discussed. Hyperspectral remote sensing can be used to identify specific phyllosilicates using electronic and vibrational features and thus provide constraints on the chemistry and formation conditions of soils.