In coastal muds downdrift from the Amazon River mouth, marine diatom frustules are rapidly converted to various forms of authigenic aluminosilicate phases during burial. The dominant neoformed crystalline phases have a composition similar to K-smectite, yield electron diffraction patterns characteristic of clay minerals, and exhibit a range of crystal sizes and morphologies, including euhedral pseudohexagonal crystals and anhedral flakes replacing biogenic silica. A poorly crystalline or microcrystalline K-rich and Fe-rich aluminosilicate material also replaces the siliceous frustules. The conversion process is not always complete, leaving relics of the original frustule in the resulting authigenic aluminosilicate phases. Laboratory incubation experiments with cultured diatoms demonstrate that the conversion process occurs in 20–23 months. The conversion of biogenic silica to authigenic clays occurs throughout the Amazon deltaic deposits and presumably takes place in other comparable depositional settings. Biogenic silica alteration provides proof for a direct link between the biogeochemical cycle of silica in nearshore environments and the neoformation of cation-rich aluminosilicate phases, and it may prove to be important for oceanic geochemical cycles as a sink for Si, K, and other elements incorporated in the authigenic aluminosilicates. Rapid formation of authigenic K-smectite may also represent a reaction stage leading to eventual formation of illitic clays during later diagenesis.