Saprolites are residual soils which preserve the textures of their parent rocks and thus have evolved by an isovolumetric process of weathering ( MILLOT, 1970, The Geology of Clays, Springer). Using bulk density, saprolite elemental analyses can be converted to units of g cm -3. Furthermore, an empirical reaction progress diagram can be constructed for a suite of saprolite samples by plotting element concentrations (in g cm -3) against bulk density (B.D.). Our data for a granite saprolite show that Al 2O 3 and SiO 2 decrease in a linear fashion from B.D. 2.1g cm -3 to 1.5g cm -3 but that K 2O follows a curvilinear trend such that it decreases from 75% of its fresh rock value at B.D. 1.6 g cm -3 to nearly zero at B.D. 1.5 g cm -3. The only hypothetical reaction paths that are compatible with these B.D. vs A1 2O 3, SiO 2 and K 2O constraints are those in which orthoclase alters to kaolinite through an intermediate potassium phase similar to KAl 3Si 3O 10(OH) 2 or KAl 2Si 2O 6(OH) 3 (hypothetical K-kaolinite). Normative mineral calculations, X-ray diffraction data and structural H 2O data are employed to test this conclusion.