47-1 Raman Spectroscopy Study of Ocean Nodule Manganese Oxide Mineralogy

Session: Minerals in Motion: Tracking Mineral Reactions Using In Situ and Synchrotron Techniques, A Celebration of the Career of Peter Heaney

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Abstract:

Increased demand for critical metals and advancements in recovery methods have reinvigorated commercial and scientific interest in ferromanganese ocean nodules. Ocean nodules are primarily composed of Mn and Fe oxides and commonly contain minor amounts of Ni, Co and Cu. Despite a voluminous literature, considerable discussion continues about nodules regarding the most basic concepts, including age and rate of growth, formation mechanism and role of microorganisms, mineral phase composition, source of metals, etc.  Investigations of the nodules are complicated by their microscale chemical heterogeneity and textural complexity.

Critical to resolving many of the questions about ocean nodules is a detailed understanding of their mineral components.  An historic emphasis on diffraction methods strongly influenced the current Mn oxide mineralogy descriptions and nomenclature. Most X-ray diffraction (XRD) patterns for nodule Mn oxides show only broad features, giving rise to terms such as “vernadite” for disordered layer Mn oxides, and provide limited details about the structures, or even phase identity, or if the sample is a mixture of phases.

Micro-Raman spectroscopy is an effective tool for identifying Mn oxide/hydroxide phases and provides an alternate approach for characterizing ocean nodule mineralogy.  Raman spectra can provide information about structural details, even for disordered and amorphous materials, and in situ measurements are possible from ~1-2 μm diameter spots. We used Raman spectroscopy in coordination with back-scattered electron imaging and energy-dispersive X-ray spectroscopy (EDS) mapping to characterize in detail the Mn oxide mineralogy of four Central-Pacific Ocean nodules.  EDS maps were used to coordinate Raman spectra and chemical analyses from the same spot areas on the nodules. Most of the several hundred Raman spectra represented multiple phases and were modeled using a classic least-squares fitting procedure and component spectra from well-characterized standards, including a Mg/Ni asbolane, a hexagonal birnessite/rancieite, triclinic birnessite, and todorokite, and an Fe oxyhydroxide. Raman and compositional data show that Mg/Ni-rich asbolane is a major Mn oxide phase in ocean nodules, and todorokite is an important but rarely predominant phase. Presumably, an XRD pattern of some combination of these disordered structures would be recognized as “vernadite”, a term that is essentially irrelevant for Raman spectroscopy. Raman spectroscopy provides new insights into Mn oxide mineralogy in ocean nodules and a potential for better understanding their formation. 

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7349

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