Using controlled experiments that simulate interlayered sand and mud in marine conditions, it is shown that clay rims develop on sand grains during the digestive processes of the polychaete worm Arenicola marina. The clay-size minerals also undergo accelerated weathering and mineral authigenesis. Synthetic mud (crushed slate) and clean, eolian sand of known composition and texture were horizontally layered in experimental and control tanks. Natural seawater was added to these tanks along with several A. marina. Fecal casts were collected at regular time intervals, and these were analyzed texturally using electron microscopy and mineralogically using X-ray diffraction (XRD). Total bioturbation of the sand and mud layering by A. marina was achieved after only eight weeks of feeding activity. Sand grains in the fecal casts developed clay-mineral rims, a feature absent in the original material and not found in the control tanks. Thin, composite, curvilinear structures, assumed to be detached clay rims, were also produced. Over a period of many months the synthetic mud proved to be unchanged in the control tank, but it was significantly different in the fecal casts from the experimental tank that contained the worm A. marina. The original minerals were degraded and new authigenic clay minerals formed. It is postulated that a combination of low pH and the biologically active microenvironment in the guts of annelid worms may radically accelerate mineral dissolution and clay-mineral precipitation processes during digestion. It is likely that mucous produced by the worms not only led to the aggregation of the fine-grained material into fecal casts but also adhered the fine sediment onto sand grains, thus creating clay-mineral rims. In this case the solid part of the clay rim will be a combination of the primary material (crushed slate) and the neoformed clay minerals.