In this study, Raman and infrared spectroscopy is applied to investigate two manganese oxide phases: lithiophorite [(Al,Li)Mn4+O2(OH)2] and asbolane [(Ni,Co)xMn4+(O,OH)4.nH2O], along with their intermediates (“Asbolane-Lithiophorite Intermediates”: ALI). These oxides typically incorporate variable concentrations of Co, Ni, Cu and Li. They represent a group of economically interesting phases that are difficult to identify and characterize with classical X-ray diffraction techniques. They were described in many places around the world, including the oxidized horizon of large ore deposits in New-Caledonia, Australia, the Democratic Republic of the Congo (DRC) amongst others. They also represents phases encountered as Ni-Co enriched manganese nodules of the deep ocean floors. Our results show that Lithium-bearing manganese oxides with typical X-ray diffraction lines of lithiophorite can exhibit two different Raman responses, namely the one of a typical lithiophorite and the one of ALI. This difference of reaction between X-ray and Raman methods strengthen the model developed in literature [2] that the X-ray diffraction lines of these oxides result primarily from one component of the structure, the MnO6 octahedra layers. In the same way, the reflectors associated with the unstructured Ni-Co oxide layers in asbolane are too weak to be visible on X-ray diffraction patterns. By contrast, the Raman responses are also driven by the chemical composition of the samples, allowing a more precise characterization. We propose reference Raman spectroscopic signatures for lithiophorite, asbolane and ALI phases. These spectra are mainly composed of two spectral domains, the first one is located between 370-630 cm-1 and the second one between 900-1300 cm-1. We then assess the impact of their highly variable chemistry on their Raman peak positions, intensities and FWHM using a semi-systematic curve-fitting method profiled for these phases. The strong affinities observed between the Raman spectral content of asbolane, lithiophorite and their intermediates, combined with the progressive trend observed for some peak parameters indicate that the studied phases represent probably a solid solution.
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RBINS Staff Publications 2016
