Search Results (5)

A new occurrence of the Mn-Ba ore mineral, romanechite, has been discovered in a small paleo-doline of the Apulian karst on Mesozoic carbonate rock successions, characterized by reddish incrustations and nodules made essentially by Fe-bearing calcite. The conditions under which Mn-Ba ore minerals form represent an intriguing area of research, as these minerals can act as scavengers for heavy elements, impacting soils, surface sediments, and even associated aquatic systems. The genesis of romanechite is linked to the progressive interaction of silicate aqueous solutions enriched in Al, Si, and Fe with the limestone substrate. The findings provide new insights into the genetic processes responsible for the formation of reddish Mn incrustations, supporting their polygenetic origin because of the chemical alteration of limestone and allochthonous siliciclastic muds. Full article

In equatorial and tropical regions, supergene mineral deposits created during water/rock interactions are found. Simply put, these supergene deposits are formed through the accumulation of low solubility ions or through the preservation of primary minerals. The supergene manganese (Mn) deposits are examples of the economic importance associated with the chemical weathering processes. In Brazil, the Southern Brasilia Orogen (SBO) was generated during the collision between the Paranapanema Craton and the passive margin of the São Francisco Craton. In the southern Minas Gerais (MG), several supergene Mn occurrences are hosted in the SBO, which were originated during the chemical weathering of gondites belonging to the Amparo Complex. Here, we studied the supergene Mn occurrences in the southern MG, more specifically in the municipalities of Ouro Fino and Careaçu. The MnO contents ranged from 25.50 to 28.40 wt% at Ouro Fino and from 16.80 to 21.20 wt% at Careaçu. These supergene Mn deposits have a diverse mineral assemblage, being composed of spessartine, quartz, Mn-oxides, goethite and kaolinite. The various Mn minerals formed due to spessartine incongruent dissolution were hollandites, cryptomelanes, romanechites, pyrolusites and lithiophorites. Both study areas are relevant for the possible opening of mines for the commercialization of Mn. Full article

The Piauí laterite (NE Brazil) was initially evaluated for Ni but also contains economic concentrations of Co. Our investigations aimed to characterise the Co enrichment within the deposit; by understanding the mineralogy we can better design mineral processing to target Co recovery. The laterite is heterogeneous on the mineralogical and lithological scale differing from the classic schematic profiles of nickel laterites, and while there is a clear transition from saprolite to more ferruginous units, the deposit also contains lateral and vertical variations that are associated with both the original intrusive complex and also the nature of fluid flow, redox cycling and fluctuating groundwater tables. The deposit is well described by the following six mineralogical and geochemical units: SAPFE, a clay bearing ferruginous saprolite; SAPSILFE, a silica dominated ferruginous saprolite; SAPMG, a green magnesium rich chlorite dominated saprolite; SAPAL, a white-green high aluminium, low magnesium saprolite; saprock, a serpentine and chlorite dominated saprolite and the serpentinite protolith. Not all of these units are ‘ore bearing’. Ni is concentrated in a range of nickeliferous phyllosilicates (0.1–25 wt%) including serpentines, talc and pimelite, goethite (up to 9 wt%), magnetite (2.8–14 wt%) and Mn oxy-hydroxides (0.35–19 wt%). Lower levels of Ni are present in ilmenites, chromites, chlorite and distinct small horizons of nickeliferous silica (up to 3 wt% Ni). With respect to Co, the only significant chemical correlation is with Mn, and Mn oxy-hydroxides contain up to 14 wt% Co. Cobalt is only present in goethite when Mn is also present, and these goethite grains contain an average of 0.19 wt% Co (up to a maximum of 0.65 wt%). The other main Co bearing minerals are magnetite (0.41–1.89 wt%), chlorite (up to 0.45 wt%) and ilmenite (up to 0.35 wt%). Chemically there are three types of Mn oxy-hydroxide, asbolane, asbolane-lithiophorite intermediates and romanechite. Spatially resolved X-ray absorption spectroscopy analysis suggests that the Co is present primarily as octahedrally bound Co³⁺ substituted directly into the MnO₆ layers of the asbolane-lithiophorite intermediates. However significant levels of Co²⁺ are evident within the asbolane-lithiophorite intermediates, structurally bound along with Ni in the interlayer between successive MnO₆ layers. The laterite microbial community contains prokaryotes and few fungi, with the highest abundance and diversity closest to ground level. Microorganisms capable of metal redox cycling were identified to be present, but microcosm experiments of different horizons within the deposit demonstrated that stimulated biogeochemical cycling did not contribute to Co mobilisation. Correlations between Co and Mn are likely to be a relic of parent rock weathering rather than due to biogeochemical processes; a conclusion that agrees well with the mineralogical associations. Full article

Rippite K₂(Nb,Ti)₂(Si₄O₁₂)(O,F)₂, a new K-Nb-cyclosilicate, has been discovered in calciocarbonatites from the Chuktukon massif (Chadobets upland, SW Siberian Platform, Krasnoyarsk Territory, Russia). It was found in a primary mineral assemblage, which also includes calcite, fluorcalciopyrochlore, tainiolite, fluorapatite, fluorite, Nb-rich rutile, olekminskite, K-feldspar, Fe-Mn–dolomite and quartz. Goethite, francolite (Sr-rich carbonate–fluorapatite) and psilomelane (romanèchite ± hollandite) aggregates as well as barite, monazite-(Ce), parisite-(Ce), synchysite-(Ce) and Sr-Ba-Pb-rich keno-/hydropyrochlore are related to a stage of metasomatic (hydrothermal) alteration of carbonatites. The calcite–dolomite coexistence assumes crystallization temperature near 837 °C for the primary carbonatite paragenesis. Rippite is tetragonal: P4bm, a = 8.73885(16), c = 8.1277(2) Å, V = 620.69(2) ų, Z = 2. It is closely identical in the structure and cell parameters to synthetic K₂Nb₂(Si₄O₁₂)O₂ (or KNbSi₂O₇). Similar to synthetic phase, the mineral has nonlinear properties. Some optical and physical properties for rippite are: colorless; Mohs’ hardness—4–5; cleavage—(001) very perfect, (100) perfect to distinct; density (meas.)—3.17(2) g/cm³; density (calc.)—3.198 g/cm³; optically uniaxial (+); ω = 1.737-1.739; ε = 1.747 (589 nm). The empirical formula of the holotype rippite (mean of 120 analyses) is K₂(Nb_1.90Ti_0.09Zr_0.01)[Si₄O₁₂](O_1.78OH_0.12F_0.10). Majority of rippite prismatic crystals are weakly zoned and show Ti-poor composition K₂(Nb_1.93Ti_0.05Zr_0.02)[Si₄O₁₂](O_1.93F_0.07). Raman and IR spectroscopy, and SIMS data indicate very low H₂O content (0.09–0.23 wt %). Some grains may contain an outermost zone, which is enriched in Ti (+Zr) and F, up to K₂(Nb_1.67Ti_0.32Zr_0.01)[Si₄O₁₂](O_1.67F_0.33). It strongly suggests the incorporation of (Ti,Zr) and F in the structure of rippite via the isomorphism Nb⁵⁺ + O²⁻ → (Ti,Zr)⁴⁺ + F¹⁻. The content of a hypothetical end-member K₂Ti₂[Si₄O₁₂]F₂ may be up to 17 mol. %. Rippite represents a new structural type among [Si₄O₁₂]-cyclosilicates because of specific type of connection of the octahedral chains and [Si₄O₁₂]⁸⁻ rings. In structural and chemical aspects it seems to be in close with the labuntsovite-supergroup minerals, namely with vuoriyarvite-(K), K₂(Nb,Ti)₂(Si₄O₁₂)(O,OH)₂∙4H₂O. Full article

The Sanbao Mn–Ag (Zn-Pb) deposit located in the Laojunshan ore district is one of the most important deposits that has produced most Ag and Mn metals in southeastern Yunnan Province, China. Few studies are available concerning the distribution and mineralization of Ag, restricting further resource exploration. In this study, detailed mineralogy, chronology, and geochemistry are examined with the aim of revealing Ag occurrence and its associated primary base-metal and supergene mineralization. Results show that manganite and romanèchite are the major Ag-bearing minerals. Cassiterite from the Mn–Ag ores yielded a U–Pb age of 436 ± 17 Ma, consistent with the Caledonian age of the Nanwenhe granitic pluton. Combined with other geochemical proxies (Zn-Pb-Cu-Sn), the Sanbao Mn–Ag deposit may originally be of magmatic hydrothermal origin, rather than sedimentary. The Ag-rich (Zn-Pb (Sn)-bearing) ore-forming fluids generated during the intrusion of the granite flowed through fractures and overprinted the earlier Mn mineralization. Secondary Ag (and possibly other base-metals) enrichment occurred through later supergene weathering and oxidation. Full article