Mineralogical and spectroscopic investigation of enstatite chondrites by X-ray diffraction and infrared reflectance spectroscopy

摘要

The mineralogy and infrared reflectance spectra of 13 Enstatite (E) chondrite meteorite finds spanning the full range of textural alteration grades in both EL and EH classes have been investigated. Rietveld refinement of high-resolution powder X-ray diffraction (XRD) data was used to determine quantitative major mineral abundances. Sample-correlated mid-infrared (2.0 to 25.0 μm; 4500 cm~(-1) to 400 cm~(-1)) reflectance infrared spectra were collected for each meteorite. Spectral features due to the fundamental lattice vibrations of the silicates, primarily enstatite, dominate the spectra of these meteorites over most of the spectral range investigated. The spectral features related to primary (i.e., pre-terrestrial) mineralogy include fundamental stretching and bending lattice modes (～8.3-25.0 μm; 1200-400 cm~(-1)), overtones and combinations of the fundamental modes (～4.5-6.1 μm; 2200-1650 cm~(-1)), and the principle Christensen feature (～8.3 μm; 1200 cm~(-1)). Terrestrial weathering products including Fe-oxyhydroxides, gypsum, and carbonates occur in most of these meteorites and contribute to some spectral features: particularly an asymmetric feature near ～2.6 to 3.8 μm (3800 to 2600 cm~(-1)) attributed to adsorbed, hydrogen-bonded, and/or structural OH and H_2O, and a feature near ～6.2 μm (1625 cm~(-1)) attributed to adsorbed, hydrogen-bonded, and/or structural H_2O. Modal mineral abundances determined by Rietveld refinement have been used to calculate model grain densities for each meteorite. Bulk magnetic susceptibility measurements combined with modal mineralogy and grain densities reveal a trend toward lower grain density and lower bulk susceptibility with increased terrestrial weathering.