We explore the importance of the composition of airborne mineral aerosols for assessments of their direct radiative forcing at infrared wavelengths. Our calculations employing Mie theory and data on spectral refractive indices show that the existing variations in refractive indices can cause large changes in the major aerosol optical characteristics. Calculations of IR radiative forcings at the top of the atmosphere and IR downward and upward fluxes, based on an one-dimensional radiation transfer code, give a wide range of results for varying optical models of the mineral aerosols. We estimate that for a low dust loading scenario the changes in IR downward flux at the surface relative to dust free conditions are in the range from 7 to 14 W/m2 depending upon the mineral aerosol selected. Under dry tropics atmospheric conditions the IR forcing at the top of the atmosphere is in the range from 2 to 7 W/m2. In turn, for a high dust loading scenario the calculated changes, relative to dust free conditions, in IR downward flux at the surface vary from 50 to 80 W/m2, and the IR forcing at the top of the atmosphere varies from 15 to 25 W/m2. Therefore, we conclude that incorporation of regionally and temporally varying dust mineralogical composition into general circulation models could be beneficial for decreasing the currently large uncertainties in the assessment of radiative forcing by the natural and anthropogenic components of the airborne mineral aerosols. Also the use of appropriate mineralogical data is required for remote sensing of the atmospheric aerosols using satellite infrared observations. ¿ 1998 American Geophysical Union