Microanalysis and mineralogy of Asian and Saharan dust

Abstract

Mineral dust is transported over long distances from desert sources, interacting with Earth environments. The mineralogy and microstructures of individual dust particles are required to understand the interactions. Here, I summarize recent findings from electron microscopy of dust particles, focusing on Asian and Saharan dust. Dust particles are heterogeneous mixtures of clay and nonclay minerals. Clay minerals account for more than half of the mass of mineral dust. Fine grains of clay minerals form their own aggregates, coat coarse nonclay minerals, or become a matrix of composite particles. The most abundant clay minerals are illite‒smectite series clay minerals (ISCMs) dominated by illite and interstratified illite‒smectite. Saharan dust is distinct from Asian dust by the high contents of palygorskite and hexagonal kaolinite. Common nonclay silicates are quartz, K-feldspar, and Na-rich plagioclase. Amorphous silica is associated with clays in Saharan dust. Calcite occurs as nanofibers as well as coarse grains, reacting with atmospheric acids to precipitate gypsum. The submicron grains of iron oxides and titanium oxides are scattered through the fine matrix of dust particles. ISCMs, chlorite, biotite, and iron oxides are iron carriers to remote ecosystems. The shapes of dust particles approximate ellipsoids whose aspect ratios increase with clay contents. The mineralogical classification of dust particles has led to the determination of the bulk mineral composition of a very small quantity of samples. The constituent mineralogy of dust particles is discussed in an environmental context with a brief introduction of the geological backgrounds of the minerals in their source areas.