Todorokite Veinlets in Opal-A Layers of Deep-Sea Manganese Nodules from the Northern Central Pacific

Deep-sea Manganese nodules from the northern Central Pacific are characterized by todorokite veinlets. The veinlets develop as a network in concentric layers around a nodule core. The layers display varying degrees of reflectivities which are classified into four optically different zones (internal reflection, and low, moderate, and high reflectivity zones based on an arbitrary scale) under the ore microscope. The internal reflection zone is significantly enriched in Si and Fe, and is mainly composed of X-ray amorphous opal-A, probably of biogenic origin. The difference in reflectivity is attributed to varying amounts of todorokite crystallites and associated chemical elements across the layer. These differneces develop during nodule diagenesis.