We have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in-situ K-feldspar grain dissolution or imported into the model reference frame. Illite fiber nucleation and growth are modeled using Arrhenius expressions that consider saturation state in addition to temperature and time. Nucleation occurs on pore walls, and muscovite and detrital illite may be defined as energetically favorable substrates. The model is integrated with other Touchstone (TM) models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations. We evaluated the illite model performance on two data sets: (1) Jurassic quartzose samples from offshore mid-Norway with maximum temperatures ranging from 108 to 173 degrees C (226 to 343 degrees F) and (2) Miocene lithic samples from offshore Southeast Asia that have maximum temperatures ranging from 157 to 182 degrees C (315 to 360 degrees F). The model matches measured abundances of illite, kaolinite, and K-feldspar in both data sets using identical kinetic parameters. Predicted K-Ar ages are consistent with available data given uncertainties associated with detrital contaminants. Although no illite particle-size data are available from the analyzed samples, modeled crystallite thicknesses from the Norway data set are comparable to published measurements of 0.004 to 0.012 mu m from North Sea samples with similar temperature histories.