Manganese oxides/oxyhydroxides (MnOx) are based on Mnn+O6 and Mnn+O4 polyhedra arranged such as to form compact, channel or layered structures. In geology, they are precious archives of past redox conditions for palaeoclimatic reconstructions; in material sciences, they are used for a variety of applications, from pigments to environmental remediation and energy storage. Thus, the fast, remote and non-destructive identification of MnOx is critical in several disciplines. Micro-Raman spectroscopy is often used for this purpose, although a systematic characterization of their stability under the laser beam is still lacking. In this work, we present our results on the behaviour of the most common MnOx having compact and channel structures when a 532-nm laser with intensity between similar to 23 mu W/mu m(2) and similar to 36.8 mW/mu m2 is used. The compact structures of manganosite (NaCl-like) and hausmannite (spinel-like) are stable up to similar to 36.8 mW/mu m(2). The stability of oxides with channel structures depends on channel size, charge of channel cations and valence state of Mn. Hausmannite is the final degradation product of all MnOx with channel structures, irrespective of the starting phase. Pyrolusite, manganite, hollandite and romanechite are relatively stable under the laser beam, and the transition to the spinel structure occurs above 2.5 mW/mu m(2) while the degradation of cryptomelane and todorokite starts similar to 226 mu W/mu m(2). The analysis of MnOx thus needs very accurate experimental conditions to avoid misleading and incorrect phase identifications. Based on our data, we propose an analytical protocol for a proper characterization of these minerals via Raman spectroscopy.

A multi‐analytical approach based on colourimetry, micro‐Raman spectroscopy, scanning electron microscopy, optical microscopy and powder X‐ray diffraction, has been applied to investigate Roman cooking ware samples dating back to imperial age (I‐II century AD). In particular, the highly distinctive production of pots coming from two different archaeological sites, ‘Villa della Piscina’ at Centocelle district and the so called ‘Minerva Medica Temple’ at Esquilino district (Rome, Italy), was studied characterising the colour, the chemical, mineralogical and petrographic distinctive properties of the investigated samples, in order to compare technological and provenance aspects. Classification of ceramic fragment by colorimetry, integrated by compositional studies with a main contribution of micro‐Raman spectroscopy, allows to discriminate between superior and inferior quality cooking ware and evaluate the compatibility of the investigated samples with some pottery realised in known manufacturing sites in Lazio. Compositional and petrographic features allow assignment of samples from ‘Villa della Piscina’ testifying productions in the surroundings of Rome, while an importation from outsider workshops is hypothesised for the ceramic fragments from the so called ‘Minerva Medica Temple’. For these, an importation from outsider workshop is hypothesised.

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 highresolution (- 1 mu m) 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.

In this paper, we review 10 years of research on high-temperature oxidation process on amphiboles with variable composition. One notable feature that has emerged from our experiments is that iron oxidation is reversible over a relatively large thermal range before the kinetic energy of delocalized electrons and H+ ions becomes high enough to allow their ejection from the crystal. Experiments under different oxidation conditions showed that the sodic amphibole riebeckite follows two distinct paths: under oxidizing conditions, it undergoes both Fe oxidation and dehydrogenation and the resulting oxo-riebeckite is stable up to 900 °C; under vacuum conditions, neither Fe oxidation nor dehydrogenations occur, and the amphibole is stable up to 800 °C, via a rearrangement of the octahedral cations. In situ HT Raman measurements on grunerite provided the first atomic-scale proof for the thermal activation of polarons in Fe-amphiboles; further studies showed that this process is a general feature of Fe2+-bearing amphiboles. For riebeckite, it starts at 227 °C and is complete at 377 °C under both reducing and oxidizing conditions. Above 377 °C, external oxygen triggers the expulsion of H+ and e− from the crystal. The temperature range observed for the development of charge carriers accurately fits the temperatures for the development of high-conductivity layers in warm and cold subduction zones and provides the atomic-scale picture for large-scale processes such as the development of anomalous conductivity layers at convergent-plate margins. Our work shows that Raman spectroscopy may provide relatively straightforward access to the properties of rock-forming minerals of geophysical interest.

Elucidating the high-temperature behavior of rock-forming minerals such as amphiboles (AB2C5 T8O22W2) is critical for the understanding of large-scale geological processes in the lithosphere and, in particular, the development of high conductivity in the Earth's interior. Recently, we have shown that at elevated temperatures, CFe-bearing amphiboles with a vacant A site develop two types of charge carriers: (1) small polarons and (2) delocalized H+ ions.To elucidate the effect of A-site cations on the formation and stability of charge carriers within the amphibole structure, here we analyzed synthetic potassic-ferro-richterite as a model Fe-rich amphibole with a fully occupied A site via in situ temperature-dependent Raman spectroscopy. We further compare the results from in situ time-dependent Raman-scattering experiments on pre-heated and rapidly quenched potassic-ferro-richterite and riebeckite as a model Fe-rich amphibole with a vacant A site.We show that the presence of A-site cations (1) reduces the activation temperature of mobile polarons and delocalized H+ cations; (2) decreases the magnitude of the polaron dipole moment; (3) slows down the process of re-localization of electrons on cooling; and (4) makes the electrons inert to rapid change in external conditions, supporting the persistence of a metastable state of pre-activated delocalized electrons even at room temperature.Our results have important geological implications demonstrating that the A-site cations may control the depth of development of high conductivity in subducted amphibole-bearing rocks. Moreover, from the viewpoint of mineral-inspired materials science, our results suggest that the amphibole-structure type has great potential for designing functional materials with tunable anisotropic-conductivity properties.

Phosphatized Mn and Fe rich hardgrounds and condensed pelagic deposits in carbonate platform successions are precious archives of abrupt climate and environmental changes (redox conditions and phosphorous availability) in the past shallow-water marine environment. While numerous examples have been documented in the Cretaceous successions of the Northern Tethys, the scarcity of similar descriptions from the southern margins suggests differences in sedimentary processes or preservation conditions. In this work we study three phosphatized Mn and Fe rich hardgrounds and pelagic condensed deposits that mark the repetitive demise of the Panormide carbonate platform developed in the Southern Tethyan margin during the Cretaceous. The integration of SEM-EDS, PXRD, and Micro-Raman spectroscopy data shows that these hardgrounds consist of fine-grained Fe (goethite and hematite) and Mn (birnessite and/or vernadite) oxides dispersed in a calcite and apatite matrix. Micro-Raman spectroscopy shows the presence of oxidized Mn species: Mn3+ and Mn4+. The oxidation of Mn2+ -> Mn3+/4+ and/or Fe2+ -> Fe3+ occurred at the sediment-seawater interface under oxic conditions (where both Mn and Fe oxidize) or suboxic conditions (where only Fe oxidizes). The paleoenvironmental perturbations that triggered the formation of both hardgrounds and condensed pelagic deposits were likely related to pCO(2) cycle, upwelling of P-Mn-Fe-rich water masses, eutrophication and phosphatization related to the Cretaceous climate oscillations during the main Oceanic Anoxic Events. These perturbations were likely enhanced by tectonic activity. Moreover, we show that the formation of the phosphatized metals-rich hardgrounds and the recovery of shallow-water sedimentation occurred after long-term periods (6-12 Ma). Thus, the Panormide serves as a remarkable example of resilience amidst significant climatic changes.

In this paper, we address two key features of the behaviour of Fe-rich amphibole at high temperatures: (1) the Fe2+. Fe3+ + e exchange within the crystal bulk, and (2) the consequent rise in electrical conductivity. Cycling heating-cooling experiments were done in situ up to 542. C (815 K) at beamline B11 of the Diamond Synchrotron Laboratory (UK). X-ray absorption spectra at the Fe K-edge and electrical resistivity were measured simultaneously on a single crystal of riebeckite with a composition very close to the ideal formula A.BNa2C(Fe23+Fe23+)TSi8O2W2(OH)2. The Fe3+/Fetot ratio was monitored via analysis of the pre-edge feature in the XANES spectra. Our data show slight oscillations of the oxidation state of Fe with temperature cycling up to around 400.C (673 K), followed by a substantial gradual increase in Fe2+. Fe3+ oxidation that starts at 450. C (723 K) and is completed at 525. C (798 K). The conductivity (s) measured along the crystallographic c-axis oscillates strongly with cycling temperature allowing us to conclude that it is intrinsically related to the electron hopping induced by thermal treatment. The activation-energy derived from the s(T) trend is Ea = 74.4 +/- 0.6 kJ/mol (0.77 +/- 0.01 eV), in agreement with small-polaron conduction. This study provides direct and robust support of the conduction mechanisms in Fe-amphibole previously inferred from indirect methods. Given that riebeckite is a significant component in the glaucophanitic amphiboles common in blueschists associated with subducted oceanic crust, our data provide a link between atomic-scale processes and Earth-scale anomalous conductivity observed via geophysical measurements.

Coastal areas are extremely exposed to litter pollution. In this work, we assessed the abundance and distribution of microlitter in the water column and surface waters, as well as its summertime transport trajectories in the northern Latium coastal area (Tyrrhenian Sea, Italy). Microlitter items were classified according to their different shapes (fragments, films, and filaments), sizes, colours, and types by visual sorting and by optical microscopy; their composition was addressed via spectroscopic (Raman and FTIR) microanalysis. Microplastics (MPs) were abundant in all water samples; polypropylene, polyethylene, and poly-methyl methacrylate were identified. The results show a higher concentration of microlitter items in the water column (average concentration of 5.9 ± 1.38 items/m³) compared to surface waters (average concentration of 0.16 ± 0.03 items/m³). These abundances are similar to those found in the Mediterranean Sea and Oceanic waters. The sampling sites closer to the coast showed a higher abundance of particles, especially filaments, in the water column compared to the offshore areas. This feature can be explained based on the presence of land-based pollution sources, bathing areas, and the resuspension of microlitter items from the bottom. Analysis of the surficial trajectories pointed to a general northward transport direction of microlitter and the presence of possible enrichment spots along the coast.

Phlogopite solid-solutions have a wide stability field and are ubiquitous in a wide variety of geological environments; their composition may thus provide important information regarding the host-rock crystallization conditions. In this paper we examine micas from a lamprophyre minette from St. Helier (Island of Jersey, UK). The host rock consists of dominant phlogopite with subordinate clinopyroxenes, rare amphiboles and remnants of olivine; the microcrystalline groundmass consists of potassium feldspar and (Fe,Mg)-mica. Phlogopite phenocrystals from lamprophyres typically show normal and continuous compositional zoning, however those from the studied minette show dark brown cores enveloped by euhedral oscillatory rims. From electron microprobe (EMP) data, the compositional zoning is mainly related to the relative (Mg,Fe) and Ti contents, the latter being extremely high (TiO2 ≈ 8 wt%) at the crystal core. Thermometric modelling based on phlogopite/rock Ti-partitioning yields comparable T values for both the inner and outer rims with a mean value of T = 1030 ± 50°C. Combination of EMP and single-crystal FTIR in the OH-stretching region shows that the substitution mechanism responsible for the exceptional and oscillatory Ti contents is the “Ti-vacancy” mechanism that is typical of HP/HT crystallization environments. Raman imaging provides additional insight in the crystal chemistry of the studied micas, enabling characterization of the trioctahedral vs. dioctahedral character, and of the Ti or Mg/Fe2+ distribution during crystal growth. Our study shows how combination of these vibrational spectroscopies may provide access to petrological processes at a very fine scale.

This work relates to the occurrence and crystal-chemical characterization of fibrous erionite, a carcinogenic zeolite, discovered for the first time in the volcanic rocks of Latium, Italy. The erionite samples were investigated using SEM, TGA, PXRD, FTIR and Raman spectroscopies. Cell parameters, fractional coordinates and site scattering were refined using the Rietveld method. Two different types of erionite were found, having different crystal morphologies, chemical composition, and structure. The first type is an extremely fibrous erionite-K, with Si/(Si+Al) ratio of 0.77, cell parameters a=13.255 ;k, c=15.053 ;k and cell volume V=2290.49 ;k3. The second type is acicular to highly fibrous erionite-Na, with a lower Si/(Si+Al) ratio (0.72-0.73) and larger cell parameters (a=13.291 ;k, c=15.146 ;k, cell volume V=2317.35 ;k3). Both erionite types occur as fibers of inhalable size. The structure of both samples, refined by the Rietveld method on powder X-ray diffraction data, collected in transmission mode on capillaries, is consistent with that of samples with similar extra-framework cations content. Raman and FTIR data are presented and discussed. The described finding of potentially carcinogenic erionite in the volcanic rocks of Latium that are typically rich in zeolites of various nature, both as cavity filling and devitrification products of glass, suggests that this hazardous mineral might be more common than previously believed in the area and points to the need for detailed and more systematic studies.

Anomalous high-conductivity layers are typical of subduction zones, the largest recycling systems of the Earth. Understanding the mineral physics underlying the high conductivity of rocks has paramount implications for several planetary-scale processes, including global water cycling, earthquake activity, and arc magmatism. Here, by using in situ polarized Raman spectroscopy, we provide a direct proof for the development of anisotropic electron-phonon excitations (polarons) and delocalized H+ in riebeckite, a Fe-bearing sodic amphi-bole typical of blue-schist metamorphic facies. The activation of polarons starts at 500 K (227 degrees C) and is complete at 650 K (377 degrees C) under both reducing and oxidizing conditions. At higher temperatures external oxygen triggers the expulsion of H+ and e(-) from the crystal bulk. The temperature range observed for the development of charge carriers is in excellent agreement with the conductivity trends measured for riebeckite in previous studies, and nicely fits the temperatures for the development of high-conductivity layers in warm and cold subduction zones. The study directly demonstrates the activation of polarons at temperatures characteristic of convergent plate margins provides the atomic-scale picture whose macroscopic-scale expression is the anomalous conductivity measured in subduction zones.

The existence of thermally-activated quasiparticles in amphiboles is an important issue, as amphiboles are among the main hydrous complex silicate minerals in the Earth’s lithosphere. The amphibole structure consists of stripes of 6-membered TO4-rings sandwiching MO6 octahedral slabs. To elucidate the atomistic origin of the anomalous rock conductivity in subduction-wedge regions, we studied several Fe-containing amphiboles with diverse chemistry by using in situ, temperature-dependent, polarised Raman spectroscopy. The occurrence of resonance Raman scattering at high temperatures unambiguously reveal temperature-activated small polarons arising from the coupling between polar optical phonons and electron transitions within Fe2+O6 octahedra, independently of the amphibole chemical composition. The FeO6-related polarons coexist with delocalised H+; that is, at elevated temperatures Fe-bearing amphiboles are conductive and exhibit two types of charge carriers: electronic polarons with highly anisotropic mobility and H+ cations. The results from density-functional-theory calculations on the electron band structure for a selected amphibole compound with a relatively simple composition are in full agreement with experimental data. The polaron activation temperature, mobility, and polaron-dipole magnitude and alignment can be controlled by varying the mineral composition, which makes amphiboles attractive “geo-stripes” that can serve as mineral-inspired technology to design thermally-stable smart materials with anisotropic properties.

Abstract Anomalous high-conductivity layers (HCL) are typical of subduction zones, the largest recycling systems of our planet. Understanding the underlying physics of rock high conductivity has paramount implications for several planetary-scale processes, including the global water cycling, earthquake activity, and arc volcanism. Here we provide a direct proof for the activation of anisotropic electron-phonon excitations (small polarons) and delocalized H+ in riebeckite, a Fe-rich sodic amphibole typical of blue-schist metamorphic facies. This phenomenon occurs above 500 K, regardless of the oxygen fugacity, whereas the expulsion of H+ from the crystal surface occurs above 650 K only in the presence of exO2. Notably, the expected depth of activation of charge carriers, for modelled warm and cold subduction zones, fits the identified HCL providing the atomic-scale evidence for the observed anomalous electrical conductivity. This work thus gives new insights into the link between atomic-scale phenomena and Earth-scale processes interacting at convergent plate margins.

The growth of ferromanganese (FeMn) crusts on soft substrates is uncommon. FeMn crusts generally accrete on hard-rock surfaces, where sedimentation rates are low and the rocks free of sediment cover. Here we use X-ray Powder Diffraction, Fourier Transform-Infra Red spectroscopy, Raman spectroscopy, X-ray Computed Micro Tomography, Scanning Electron Microscopy and Particle Size Distribution analysis to investigate FeMn crusts associated with a porous, weakly consolidated, and bioturbated siltstone consisting mainly of authigenic palygorskite. FeMn crusts occur both on the surface of the siltstone and as FeMn lining and/or infilling of bioturbation burrows. Our results show that variations in the water redox conditions lead to a micrometric alternation of hydro genetic vernadite and diagenetic todorokite and asbolane. These variations affected the oxidation state of Mn, which increased during the diagenetic formation of todorokite. The mineralogy of the FeMn oxides lining bioturbation traces is similar to that of the crusts growing on the siltstone, suggesting that they are genetically related and probably contemporaneous. Bioturbation burrows lined by FeMn oxides are unfilled (the siltstone porosity is 5-10%) or filled by bioclastic carbonate sediment. The soft siltstone possibly resulted from the alteration of volcaniclastic-pyroclastic sediments, forming authigenic palygorskite. Bioturbation of the sediment by burrowing organisms lead to seawater percolation, followed by hydrogenetic or diagenetic precipitation of FeMn linings under oxic/suboxic conditions within the burrows. These findings emphasize the importance of weakly-consolidated substrates with bioturbation cavities for the formation of marine polymetallic deposits.

Caves are dark subsurface environments with relatively constant temperatures that allow studying bio-mineralization processes and paleoenvironmental or climate changes in optimal conditions. In the extreme and oligotrophic cave environment, manganese patinas having stromatolite-like features are uncommon. Here we provide the first detailed mineralogical, geochemical, and microbiological investigation of fine-grained and poorly crystalline MnFe stromatolite-like wall patinas formed in a deep-cave environment in Italy. These mineralizations, about 3 mm thick, consist of an alternation of Mn-layers and Fe-lenses. We show that the microbial communities' composition is dominated by Mn-oxidizing bacteria, such as Bacillus, Flavobacterium, and Pseudomonas. Our multidisciplinary investigation, integrating data from different analytical techniques (i.e., optical microscopy, SEM-EDS, μXRF, XRPD, FT-IR, Raman spectroscopy, and DNA sequencing), revealed peculiar chemical, mineralogical, and biological features: 1) A cyclical oscillation of Mn and Fe along the growth of the patinas. We propose that this oscillation represents the shift between oxic and suboxic conditions related to different phases occurring during paleo-flood events; 2) A typical spatial distribution of mineralogy and oxidation state of Mn, bacterial imprints, detrital content, and stromatolite-like morphologies along the Mn-layers. We propose that this distribution is controlled by the local hydraulic regime of the paleo-floods, which, in turn, is directly related to the morphology of the wall surface. Under less turbulent conditions, the combination of clay mineral catalysis and biological oxidation produced vernadite, a poor-crystalline phyllomanganate with a low average oxidation state of Mn, and branched columnar stromatolite-like morphologies. On the other hand, under more turbulent conditions, the sedimentation of clay minerals and microbial communities' development are both inhibited. In this local environment, a lower oxidation rate of Mn2+ favored the formation of todorokite and/or ranciéite, two compounds with a high average oxidation state of Mn, and flat-laminated or columnar stromatolite-like morphologies.

Manganese oxides are important geomaterials, used in a large number of applications. For instance, as pigments in art works or in the treatment and removal of heavy metals from drinking water. Particularly, ramsdellite [Mn4+O2] and groutellite [(Mn0.54+,Mn0.53+)O1.5(OH)0.5], because of their 2 × 1 frameworks that enable proton diffusion, are very important cathode materials. Manganese oxides commonly occur as crypto-crystalline and very fine mixtures of different Mn-phases, iron oxides, silicates and carbonates. Thus, proper characterization can be a difficult task using XRPD. The lack of Raman data on groutellite and the little and conflicting data on ramsdellite do not allow their proper identification by Raman spectroscopy. In this work we characterize natural mixtures of ramsdellite and groutellite by combining SEM-EDS, XRPD, FT-IR and Raman spectroscopy, to provide reference Raman spectra. Our data show that they have a typical and unmistakable spectra, allowing clear recognition. Moreover, we have investigated their laser-induced degradation. Our data show that groutellite transforms into ramsdellite, by the loss of H+ and the oxidation of Mn3+ to Mn4+, already at a very low laser power. Further increasing the laser power the formation of hausmannite [Mn2+Mn23+O4] occurs via the reduction of Mn cations. Our data can be used to study the discharge mechanism in cathodic battery materials, by monitoring the Mn reduction from ramsdellite to groutellite, and finally to groutite [α-Mn3+OOH]. Moreover, Raman mapping allows the study of their distribution in all the investigated samples and, indirectly, those of H+ and Mn3+, which plays a key-role in electrochemical activity of these compounds.

The Middle Eocene Climatic Optimum (MECO) is a global warming event that occurred at around 40 Ma and lasted about 500 kyr. We study this event in an abyssal setting of the Tasman Sea, using the IODP Core U1511B-16R, collected during the expedition 371. We analyse magnetic, mineralogical, and chemical parameters to investigate the evolution of the sea bottom conditions at this site during the middle Eocene. We observe significant changes indicating the response to the MECO perturbation. Mn oxides, in which Mn occurs under an oxidation state around +4, indicate a high Eh water environment. A prominent Mn anomaly, occurring just above the MECO interval, indicates a shift toward higher pH conditions shortly after the end of this event. Our results suggest more acid bottom water over the Tasman abyssal plain during the MECO, and an abrupt end of these conditions. This work provides the first evidence of MECO at abyssal depths and shows that acidification affected the entire oceanic water column during this event.

Manganese oxides are important geomaterials, widespread in terrestrial and Martian environments. Characterisation of the oxidation state of Mn is a central issue in science; this task has been addressed up to the present by X‐ray spectroscopy or diffraction techniques. The former, however, requires access to synchrotron facilities, while the latter does not provide crystal‐chemical information at the local scale. In this work, we compare a large set of Raman data from well‐characterised samples, already published by the same authors of this paper or as found in the literature. We show a clear correlation between the oxidation state of Mn and the wavenumber of peculiar bands; octahedrally co‐ordinated Mn2+ is recognised by a band around 530 cm−1, Mn3+ by a band around 580 cm−1 and Mn4+ by a band around 630 cm−1, while tetrahedrally co‐ordinated Mn2+ is recognisable by a band around 650 cm−1. Strongly distorted Mn3+ octahedra are indicated by the appearance of Jahn–Teller modes. Our method allows a reliable, easily accessible tool to characterise the oxidation states of Mn in oxides, also suitable for microscale mapping. It provides a robust analytical basis for the use of these minerals as redox indicators in geology/geochemistry, in exoplanetary research or for monitoring technological processes.

Manganese oxides occur typically as cryptocrystalline and fine-grained mixtures of different Mn-phases, carbonates, silicates, and Fe oxides/hydroxides; thus their characterization by standard methods, such as X-ray diffraction, is extremely challenging. These materials have been widely used in various applications over the millennia, for example, in art works as pigments for pottery, mural paintings, stained glass, and recently, as nanostructured materials with very attractive physicochemical properties. Furthermore, they are important geomaterials that could play a key role in environmental applications, by controlling the partitioning of arsenic and heavy metals between rocks, soils, and aqueous systems. Raman spectroscopy, which is a punctual and nondestructive technique, has been widely used to characterize these materials. However, literature data are often conflicting and contradictory, usually not allowing a proper identification of the Mn species. In this work, we characterize the most common natural manganese oxides by combining X-ray powder diffraction, Fourier-transform infrared spectroscopy, and Raman spectroscopy. Our data show that some manganese oxides have characteristic Raman spectra and can be easily recognized by using Raman spectroscopy alone, whereas integration of Raman data with other techniques is mandatory to characterize minerals that have almost identical Raman spectra. With this respect, Raman spectroscopy is the only technique allowing an easy discrimination between hollandite [Ba(Mn-6(4+),Mn-2(3+))O-16] and cryptomelane [K(Mn-7(4+),Mn3+)O-16]. The final goal of this work is to provide reference Raman spectra, acquired on previously well-characterized Mn samples to facilitate the application of Raman spectroscopy in the study of these geomaterials.