Textural variation in rocks bears promise of yielding large amounts of petrogenetically valuable information. This information, concerning reaction pathways and kinetics, can be obtained through evaluation of carefully selected quantitative textural variables such as grain shape, as described by Fourier-series approximation, and surface area of phase contacts per unit volume. As an example, it is shown that the response of these two variables to a metamorphic gradient is more sensitive than standard compositional variables. In addition, these textural variables record processes to which composition cannot respond. Conclusions from the empirical data indicate that surface free energy, which is a function of surface area and grain neighborhoods, may in some instances be a dominant factor in petrogenesis.