—The paper presents a study into the total chemical and mineralogical compositions of fine colloids (fraction <0.08 μm) in the upper horizons of soils of contrasting compositions and properties. Based on the analytical data, the crystallochemical formulas of clay minerals with a 2:1 structure have been calculated, and the chemical potentials of independent components and the activity coefficients of the main end-members have been determined. Using the Selector software, the equilibrium phase-multicomponent composition of soils has been obtained, and the similarity and difference in the crystal chemistry of 2:1 clay minerals produced by two different methods are shown. The calculated Gibbs energy values of these minerals are close to the calorimetric data for smectites. It is demonstrated that the contents of typical elements in 2:1 clay minerals reflect the genetic properties of soils. In soils of alkaline series (Solonetz), such minerals are relatively enriched in sodium and magnesium; in soils of acidic series (Podzoluvisols and Cambisols), in hydrogen and iron; and in soils of the neutral series (Chernozems, Arenosols, and Umbric Gleysols), in calcium.