Thermal infrared spectra of experimentally shocked andesine anorthosite

Abstract

Andesine-rich (An36–46) anorthosite rocks experimentally shocked to high pressures (16–57GPa) exhibit changes in spectral features with increasing pressure in laboratory thermal infrared emission spectra (250–1400cm−1). These results are similar to previous studies of shocked bytownite- and albite-rich rocks, albeit with differences in absorption band centers characteristic of mineralogy and composition. Typical spectral absorptions result from Si–O antisymmetric stretch motions of the silica tetrahedral (1000–1250cm−1) and weaker absorptions due to Si–O–Si octahedral bending vibrations (350–700cm−1). Many of these features persist to higher pressures in the andesine spectra compared to similar features in measurements of shocked bytownite. This is consistent with previous thermal infrared absorption studies of shocked feldspars and likely is related to differences in density, hardness, and Al content. A transparency feature at ∼832cm−1 observed in powdered andesine spectra also degrades with increasing pressure, intermediate between the ∼828cm−1 and ∼855cm−1 transparency features in spectra of powders of shocked bytownite and albitite, respectively. These data can be incorporated into thermal infrared spectral analyses of cratered planetary surfaces (or laboratory spectra of shocked samples) to help constrain the occurrence and degree of shock in plagioclase feldspars.