Simultaneous size distributions and Fourier transform infrared extinction spectra have been measured for several clay components of mineral dust aerosol (illite, kaolinite, and montmorillonite) in the fine particle–sized mode (D = 0.1–1 μm). Published optical constants have been used in combination with the measured size distributions in spectral simulations for comparison to the measured extinction spectrum. In general, the Mie theory simulation does not accurately reproduce the peak position or band shape for the prominent Si—O stretch resonance near 9.5 μm (ca. 1050 cm−1) for any of the clays. The resonance peak in the Mie simulation is consistently blue shifted (27–44 cm−1) relative to the experimental spectrum. Additionally, the integrated absorbance in the resonance band is underpredicted for all three clay compounds. Spectral simulations based on various distributions of ellipsoid-shaped particles better reproduce the experimental spectrum including the peak position, band shape, and integrated absorbance for these fine clay aerosols.