Infrared, Raman, and cathodoluminescence studies of impact glasses

Abstract

We studied the infrared reflectance (IR), Raman, and cathodoluminescence (CL) spectroscopic signatures and scanning electron microscope-cathodoluminescence (SEM-CL) images of three different types of impact glasses: Aouelloul impact glass, a Muong Nong-type tektite, and Libyan desert glass. Both backscattered electron (BSE) and CL images of the Muong Nong-type tektite are featureless; the BSE image of the Libyan desert glass shows only weak brightness contrasts. For the Aouelloul glass, both BSE and CL images show distinct brightness contrast, and the CL images for the Libyan desert glass show spectacular flow textures that are not visible in any other microscopic method. Compositional data show that the SiO2 composition is relatively higher and the Al2O3 content is lower in the CL-bright areas than in the CL-dark regions. The different appearance of the three glass types in the CL images indicates different peak temperatures during glass formation: the tektite was subjected to the highest temperature, and the Aouelloul impact glass experienced a relatively low formation temperature, while the Libyan desert glass preserves a flow texture that is only visible in the CL images, indicating a medium temperature. All IR reflectance spectra show a major band at around 1040 to 1110 cm^(-1) (antisymmetric stretching of SiO4 tetrahedra), with minor peaks between 745 and 769 cm^(-1) (Si-O-Si angle deformation). Broad bands at 491 and 821 cm^(-1) in the Raman spectra in all samples are most likely related to diaplectic glass remnants, indicating early shock amorphization followed by thermal amorphization. The combination of these spectroscopic methods allows us to deduce information about the peak formation temperature of the glass, and the CL images, in particular, show glass flow textures that are not preserved in other more conventional petrographic images.