Optical reflection and transmission measurements have been made between 6 eV and 13 eV on specimens of natural muscovite, phlogopite, biotite and a synthetic fluorphlogopite. Reflectivity peaks are observed near 9.9 eV and 11.8 eV and are thought to be due to the silicate structure of mica. An absorption edge occurs near 7.85 eV in synthetic mica. Additional structure is observed above 7.8 eV in the magnesium micas but not in the aluminium micas. The absorption of phlogopite mica is unusually low in the high energy region. Below 7.8 eV absorption is believed to depend on transition metal impurities in the mica.
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Davidson et al. (1972) studied this question.