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The a-, a-and n-spectra of quartz in the IR regions were measured under FTIR spectroscopy, and overtone and combination modes perpendicular to the c-axis of quartz were confirmed to be electric vector active by the coincidence of the a-spectrum with the a-spectrum among the three kinds of spectra. Absorption coefficients of overtone and combination modes intrinsic to the a-, a-, and n-spectra of quartz were measured. The thickness of quartz thin sections can be determined by applying the absorption coefficients obtained from the a-and a-spectra to the polarized IR absorption spectra measured under E L the c-axis.