Abstract Mineral dust exerts a major but uncertain radiative influence on Earth’s climate. Using near‐global mineralogical constraints from NASA’s Earth Surface Mineral Dust Source Investigation (EMIT) mission in an Earth system model, we quantify the relative importance of iron‐oxide content and particle size for estimating the dust direct radiative effect (DRE). We show that the shortwave DRE is governed by a nonlinear interaction between the two factors, which jointly determine dust absorption. In contrast, the longwave DRE depends primarily on particle size and is largely insensitive to mineral composition under the assumptions of mixing state and aggregate structure. In the Earth system model, constraining the soil iron‐oxide distribution using EMIT shifts the dominant source of dust DRE uncertainty toward processes controlling atmospheric dust particle size distribution and loading. These findings highlight the importance of improved observational and process‐based constraints on dust size evolution and loading for better quantifying its climate impact.