The Martian dust spectra obtained by the infrared interferometer spectrometer (Iris) experiment on Mariner 9 have been compared with modeling computations for various minerals and rocks. The model used was based upon a recently developed theory of absorption and scattering by particulate materials. The results indicate that the composition of the Martian dust is biased toward silicate structures which involve a high degree of silicate oxygen sharing. A feldspar-rich dust of approximately 1-μm particle size appears to be the best candidate, although sheet structure silicates cannot be ruled out.