Modelling the interaction of dust with longwave (LW) radiation is still a challenge due to the scarcity of information on the complex refractive index of dust from different source regions. In particular, little is known on the variability of the refractive index as a function of the dust mineralogical composition, depending on the source region of emission, and the dust size distribution, which is modified during transport. As a consequence, to date, climate models and remote sensing retrievals generally use a spatially-invariant and time-constant value for the dust LW refractive index. In this paper the variability of the mineral dust LW refractive index as a function of its mineralogical composition and size distribution is explored by in situ measurements in a large smog chamber. Mineral dust aerosols were generated from nineteen natural soils from Northern Africa, Sahel, Middle East, Eastern Asia, North and South America, Southern Africa, and Australia. Soil samples were selected from a total of 137 samples available in order to represent the diversity of sources from arid and semi-arid areas worldwide and to account for the heterogeneity of the soil composition at the global scale. Aerosol samples generated from soils were re-suspended in the chamber, where their LW extinction spectra (2–16 µm), size distribution, and mineralogical composition were measured. The generated aerosol exhibits a realistic size distribution and mineralogy, including both the sub- and super-micron fractions, and represents in typical atmospheric proportions the main LW-active minerals, such as clays, quartz, and calcite. The complex refractive index of the aerosol is obtained by an optical inversion based upon the measured extinction spectrum and size distribution. Results from the present study show that the LW refractive index of dust varies greatly both in magnitude and spectral shape from sample to sample, following the changes in the measured particle composition. The real part ( n ) of the refractive index is between 0.84 and 1.94, while the imaginary part ( k ) is ~ 0.001 and 0.92. For instance, the strength of the absorption at ~ 7 and 11.4 µm depends on the amount of calcite within the samples, while the absorption between 8 and 14 µm is determined by the relative abundance of quartz and clays. A linear relationship between the magnitude of the refractive index at 7.0, 9.2, and 11.4 µm and the mass concentration of calcite and quartz absorbing at these wavelengths was found. We suggest that this may lead to predictive rules to estimate the LW refractive index of dust in specific bands based on an assumed or predicted mineralogical composition, or conversely, to estimate the dust composition from measurements of the LW extinction at specific wavebands. Based on the results of the present study, we recommend using refractive indices specific for the different source regions, rather than generic values, in climate models and remote sensing applications. Our observations also suggest that the refractive index of dust in the LW does not change due to the loss of coarse particles by gravitational settling, so that a constant value could be assumed close to sources and during transport. The results of the present study also clearly suggest that the LW refractive index of dust varies at the regional scale. This regional variability has to be characterized further in order to better assess the influence of dust on regional climate, as well as to increase the accuracy of satellite retrievals over regions affected by dust. We make the whole dataset of the dust complex refractive indices obtained here available to the scientific community by publishing it in the supplementary material to this paper.