It has been known for some time that the part of the lunar regolith smaller than 10 micrometers (<10 μm) is chemically distinct from the bulk soil. Specifically, it is enriched in Al, Ca, Na, K, light rare-earth elements, and Th, and depleted in Mg, Fe, Mn, and Sc. Three models have been proposed to explain those systematics. The Exotic Component model holds that a fine-grained, KREEPy exotic component was distributed on a global scale, perhaps by the Imbrian impact. In the Simple Comminution model, the <10 μm chemical systematics are due to differential comminution processes with fine-grained melt-rock mesostasis and feldspar concentration in this fraction. The Soil Mixing model states that the chemical systematics are due to mixing of compositionally distinct soils that have different grain-size distributions. To determine which model is correct. Modal petrographic data were obtained by scanning transmission electron microscopy with energy dispersive X ray analysis for the <10 μm fractions of four Apollo soils: 10084, 14163, 15271, and 64501. Modal results are consistent with chemical data regarding feldspathic and KREEP enrichments in the finest fraction. Soil glass contents, crystalline silica, and plagioclase/mafic mineral ratios are greater in the <10 μm fraction than in the coarser fractions for all soils. Grain-size distributions are inconsistent with a fine-grained exotic component derived from distant sources. Comminution of local lithologies, vertical mixing, and local lateral transport control the <10 μm soil composition.