Mineral dust supplies iron to remote oceans, stimulating phytoplankton growth and carbon dioxide decrease. Iron-bearing clay minerals are the dominant phase in mineral dust. However, their mineralogical properties are largely unknown. We first determined microstructures and chemical compositions of the clay minerals in individual dust particles by transmission electron microscopy. Nanocrystalline illite-smectite series clay minerals and iron-rich chlorite are probably important sources of iron.