There are three main manganese oxides in polymetallic nodules. They are vernadite、todorokite and birnessite. Different manganese oxides have their specific origins and growth environments. It is found that there is a special opal-layer which is full of todorokite veinlets in the polymetallic nodules from the Eastern Pacific Ocean. Microscope、EPMA (Electron Probe MicroAnalysis) and ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer) methods were used to find out the micro-structure and geochemical features of the opal-layer and its surrounding minerals, in order to investigate the origin of the opal-layer. It shows that: (1) The special opal-layer and its surrounding area are mainly composed of todorokite,vernadite and opal. (2) Todorokite has high reflectivity and Mn/Fe ratio. It is mainly composed of MnO and of diagenetic origin. (3) Vernadite has moderate reflectivity and low Mn/Fe ratio. It is mainly composed of MnO and FeO, it is of hydrogenic origin. (4) Opal has low reflectivity. It is mainly composed of SiO2 and of biogenic origin. A model was set up for the formation of the opal-layer and the todorokite veinlets inside. It could be divided into 5 stages as follows:(1)In the oxidizing environment, vernadite was formed on the surface of the half-buried nodule.(2) The dissolving of biogenic opaline silica leaded to a vast amount of colloidal silica in the upper water of the seafloor. The colloidal silica deposited on the surface of the half-buried nodule, forming the opal-layer.(3) In the oxidizing environment, vernadite was formed on the surface of the half-buried nodule. At the same time, the diagenesis of the nodule and dehydration of opal led to the formation of cracks in the opal-layer.(4) The nodule was buried by the silica-rich sediments. In the reducing environment, the todorokite veinlets formed in the previous cracks by the invasion of interstitial water from the surrounding sediments.(5) The nodule re-exposured in the water, forming vernadite on its surface in the oxidizing environment.
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