By controlled experiments that simulate marine conditions, it is shown that accelerated weathering and mineral authigenesis occur during the combined process of digestion, ingestion and excretion of sediment by worms. Previously characterized synthetic mud, composed of finely ground, low-grade metamorphic slate dominated by highly crystalline chlorite and muscovite, was unchanged in control tanks over a 20 week time scale, but was significantly different in faecal samples from an experimental tank populated with the annelid Arenicola marina. Chlorite was preferentially destroyed during digestion and both chlorite and muscovite underwent peak broadening with a skew developing towards higher lattice spacings. Neoformed minerals with very high d-spacings and a mineral with 0.75 nm basal lattice spacing (possibly berthierine) were detected in the cast samples. It is postulated that the lowered pH microenvironment in the guts of organisms may accelerate mineral dissolution and precipitation processes during ingestion. These results show that macrobiotic activity significantly accelerates weathering and mineral degradation along with mineral authigenesis. Neoformed minerals produced in this way would be likely to form the precursors of diagenetic clay mineral cements and thus have an impact on sandstone reservoir quality.