Petrographic and mid-infrared spectroscopy study of shocked feldspar in the Asuka-881757 Lunar gabbro meteorite sample

摘要

We investigated three types of shocked feldspar in the Asuka-881757,531-2 sample with mid-infrared spectroscopy (reflectance mode). Under the petrographic microscope three types of site were characterized by (1) undulatory extinction, (2) modulatory extinction with isotropic patches and decreased interference color, and (3) isotropic, lath-shaped feldspars, which is indicative of maskelynite. The IR emissivity maximum (Christiansen feature=CF) changes with the chemical composition of feldspar. One of the Christiansen composition features exhibits a wave length peak of 1234 cm~(-1) for anorthite; another feature appears at 1245 cm~(-1) for maskelynite (Palomba et al. 2006). With the help of IR spectroscopy we observed three vibrational types in our spectra: (1) peaks of depolimerization of SiO_4 tetrahedra (500-650 cm~(-1), 950-1150 cm~(-1)), (2) peaks of stretching and bending vibrational modes of SiO_6 octahedra (750-850 cm~(-1)), and (3) Si-O stretching vibration of SiO_4 units (Johnson and Horz 2003; Johnson et al. 2003, 2007). All these vibration types were observed at the less shocked sites. In the spectrum of highly shocked maskelynite only a broader band close to 1000 cm"1 was observed, which is the main vibrational band of maskelynite (Palomba et al. 2006). The calculated FWHM showed the disordering rate of shocked feldspars. On the basis of the measurements it could be concluded that the estimated shock pressure range gradually increases from 17-35 GPa for different degrees of undulatory sites, to 35-45 GPa for maskelynite sites.