Conversion of diatoms to clays during early diagenesis in tropical, continental shelf muds

In coastal muds downdrift from the Amazon River mouth, marine diatom frustu les are rapidly converted to various forms of authigenic aluminosilicate ph ases during burial. The dominant neoformed crystalline phases have a compos ition similar to K-smectite, yield electron diffraction patterns characteri stic of clay minerals, and exhibit a range of crystal sizes and morphologie s, including euhedral pseudohexagonal crystals and anhedral flakes replacin g biogenic silica. A poorly crystalline or microcrystalline K-rich and Fe-r ich aluminosilicate material also replaces the siliceous frustules. The con version process is not always complete, leaving relies of the original frus tule in the resulting authigenic aluminosilicate phases. Laboratory incubat ion experiments with cultured diatoms demonstrate that the conversion proce ss occurs in 20-23 months. The conversion of biogenic silica to authigenic clays occurs throughout the Amazon deltaic deposits and presumably takes pl ace in other comparable depositional settings. Biogenic silica alteration p rovides proof for a direct link between the biogeochemical cycle of silica in nearshore environments and the neoformation of cation-rich aluminosilica te 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 alum inosilicates. Rapid formation of authigenic K-smectite may also represent a reaction stage leading to eventual formation of illitic clays during later diagenesis.