Micro-Raman mapping of critical metals (Li, Co, Ni) in a rhythmically laminated deep-ocean ferromanganese deposit

Deep-ocean ferromanganese deposits represent one of the most important strategic reservoirs for rare and critical metals. In particular, Mn-oxyhydroxides, such as asbolane and lithiophorite, concentrate large amounts of Li, Ni, and Co into polymetallic nodules and crusts. However, because of their poor crystallinity and the presence of finely intermixed additional phases, these minerals cannot be unambiguously identified by standard X-ray powder diffraction methods. In addition, Li cannot be routinely detected by standard X-ray spectroscopy techniques.

In this work we show how the spatial distribution of asbolane (the Ni-Co-rich Mn-oxide) and lithiophorite (the Li-rich Mn-oxide) across strongly inhomogeneous ferromanganese mineralizations can be investigated at high-resolution (∼ 1 μm) via fast and easily accessible Raman scattering measurements. Because of the strong selectivity of these minerals to the incorporation of critical metals, the obtained micro-Raman maps provide also an indirect map of the Co and Ni vs. Li distribution in the crusts. The described results thus show that our spectroscopic approach could represent an efficient and valuable in situ tool for mineral chemistry and resource evaluation of these elements in ferromanganese deposits from deep-ocean environments. This research opens a new frontier for the application of Raman spectroscopy in ore prospecting for critical minerals and metals.