Illite, as it was defined, may vary in composition over as wide a spectrum as the range of compositions of diverse detrital and authigenic micaceous minerals. From a contrasting viewpoint, however, "reference" minerals of illite have been proposed whose compositions are defined sharply, almost uniquely. This paper reports data and an interpretation that reconciles the two differing viewpoints of illite composition. Free energies of formation of six separate illites were calculated from aqueous dissolution data on six different illites. Illitic parent materials, whether detrital, from solution, authigenic, or diagenetic, apparently have reacted with their aqueous environments to yield uniformly lowest-energy final products consistent with parent material and environments. Because the marine, or alternatively, fresh-water non-marine, environments have been relatively uniform during geologic time, the final compositions of illite parent materials will tend to move also toward uniformity in energy, and in consistency with the parent materials present. This offers a thermodynamic basis for proposing "reference" illite minerals whose compositions are much narrower in range than those of original detrital micas, although the latter fulfill the definition of illite.