Abstract: A process for determining an end member age of an illite component in an earth sample which contains a diagenetic illite component and a detrital illite component. As a first step, the sample is separated into a plurality of fractions so that the fractions will have different mean ages of illite. Next, the relative percentage of one of the illite components is determined for each fraction. The mean age of total illite in each fraction is determined using potassium-argon age analysis. The determined relative percentages are then correlated with the determined mean ages to generate a linear relationship. The end member age for each illite component can then be determined by using the linear relationship since zero percent of one of the illite components corresponds to the end member age of the other illite component.