Nano-mineralogy and growth environment of Fe-Mn polymetallic crusts and nodules from the South China Sea

Abstract

Fe-Mn polymetallic crusts and nodules from the South China Sea (SCS) consist of submarine ferromanganese (Fe-Mn) oxide precipitates, and represent important marine mineral resource with substantial economic and scientific research value. Previous studies on the SCS polymetallic crusts and nodules were mainly focused on their bulk mineralogy and geochemistry, whilst research on their nanomineralogy is still lacking. In this study, transmission electron microscopy (TEM), Raman spectroscopic mapping, and in-situ micro X-ray diffraction (XRD) analysis were conducted on the nano-mineralogy of the SCS polymetallic crusts and nodules. It is found that the SCS polymetallic crusts and nodules consist mainly of layered/columnar/mottled nano-phase Fe-Mn minerals and detritus such as quartz, feldspar, and clays. Also, an independent Ti mineral phase has been documented, and the mineralogical analysis reveals the transformation from vernadite to birnessite and todorokite. Titanium forms colloidal minerals in seawater and precipitates into the crusts and nodules with other colloids, such as FeOOH and Si-Al. Vernadite and birnessite can be transformed to todorokite with stable structure under sub-oxic conditions. Therefore, the SCS polymetallic crusts and nodules were formed in a short period of sub-oxic environment and diagenetic process, and the transformation can influence the enrichment of Ni and other metals during the crust/nodule growth.

Enrichment Characteristics and Mechanisms of Critical Metals in Marine Fe-Mn Crusts and Nodules: A Review

“…The highly positive Ti-Fe correlation was interpreted as hydrated TiO 2 being intergrown in the amorphous FeOOH structures, and previous observation has found in situ Ti precipitates associated with Fe-rich areas [32,128,207]. The high titanium contents and the associative distribution with Fe phase are considered indicators for pure hydrogenetic conditions as revealed by TEM and the phase distribution [25,207]. Diagenetic inputs and phosphatization markedly affect the Ti phase distribution and contents, and highly phosphatized crusts demonstrate evident Ti content release and Ti host-phase transfer.…”

“…The high titanium concentration is well known for its almost exclusive positive association with Fe phases under hydrogenetic conditions [32,33]. The highly positive Ti-Fe correlation was interpreted as hydrated TiO 2 being intergrown in the amorphous FeOOH structures, and previous observation has found in situ Ti precipitates associated with Fe-rich areas [32,128,207]. The high titanium contents and the associative distribution with Fe phase are considered indicators for pure hydrogenetic conditions as revealed by TEM and the phase distribution [25,207].…”

Morphology‐Dependent Magnetic Properties in Shallow‐Water Ferromanganese Concretions

“…Early growth was by Fe‐Mn oxide cementation of the substrate, followed by development of chaotic columnar and dendritic growth patterns, sometimes as larger lobes or botryoids, especially in the spheroidal morphotype. Previous research suggests that columnar structures are typical in environments with reduced hydrodynamic activity and rapid growth rates, whereas dendritic growth patterns may result from inconsistent growth on varied locations on the same surface (Ren et al., 2023). Inconsistent growth may be caused by several factors such as physical barriers formed by trapped detrital mineral grains, and bioturbation or other subtle movements and rotations during concretion formation.…”