Porosity evolution and crystallization-driven fragmentation during weathering of andesite

A 10 meter thick andesitic sill intrusion from the Neuquen Basin, Argentina, shows spectacular examples of spheroidal weathering and Liesegang banding (see Figure). The Liesegang patterns demonstrate how andesite blocks, initially cut out by a pre-weathering joint set, are subdivided by fractures forming during the spheroidal weathering process. The stresses causing fracturing originate from the growth of ferrihydrite and calcite in the pore space of the andesite, partly at the expense of original ilmenite, amphibole, and plagioclase. The porosity evolution and fracture formation during progressive weathering has been characterized by SEM studies, X-ray computed tomography (CT), He- and Hg-porosimetry. Fresh andesite has a porosity of ca. 8%, and a major fraction (>80%) of the pore volume is comprised of pores < 10 μm in diameter. The extent of pore filling during weathering increases with pore-size. Pores > 100 μm are almost completely filled by an intimate intergrowth of calcite and ferrihydrite, whereas pores < 10 μm stay open. The fracturing associated with spheroidal weathering is caused by growth in pores comprising the largest 10-20% of the total porosity. Periodic precipitation of weathering product to form Liesegang bands indicates a significant superaturation treshold before nucleation commences. Preferential growth of weathering products in large pores is most likely due to a higher probability of nucleation in pores with a large surface area. A simple model that couple the mechanical and chemical processes involved will be presented.