Search

Thirty seven floor sediment samples of Upper Pleistocene age from the Loutra Almopias Cave were collected from different beds and stratigraphic columns on the basis of their induration grade, grain distribution, and paleontological findings. Channel facies make up the bulk of the clastic sediments found in the cave passages. Slackwater facies compose the final layer of all the stratigraphic sections of the examined cave. The floor sediments are mineralogically immature, since they contain many ferromagnesian minerals, feldspars (especially plagioclase) and quartz. The extensive presence of silicate minerals means that the phyllites, gneisses, schists, ophiolitic rocks and the clastic Mariam Formation of the Almopia Zone are the main detrital load source, along with the flysch of the neighboring Pelagonian Zone. The presence of sand-sized grains, pebbles and cobbles of dolomitic or calcitic composition also designates the carbonate rocks of the Almopia and Pelagonian Zones as primary sources. The dolomite and calcite content of secondary chemical origin in the cave sediments is very limited. Kutnohorite, isomorphous with dolomite, was found for first time in a Greek cave. The provenance of the sediments is mixed; they are composed mainly of the weathering materials of the Alpine metamorphic basement and the carbonate rocks outcropping adjacent to the cave. The sediments were transported and deposited inside the cave, after rapid weathering and erosion of the surrounding rocks, under a tectonically active regime. The mineralogical variation in the stratigraphic columns demonstrates variations in the clastic load, due to the different weathering intensity periods. Most of the sediments are fluvial deposits, and one is considered a glacial deposit. Fossils of Lepus timidus (mountain hare) found within the cave represent the southernmost record of this species in Europe. Its presence signifies a cool phase at the end of the last glacial period before the onset of the warm Holocene Epoch.

The Datangpo Formation manganese deposits (DFMnD) in South China formed during the interglacial stage between the Sturtian and Marinoan glaciations of the Cryogenian period. These black shale-hosted deposits are composed of massive Mn-carbonates with microscopic laminae/laminations and cherty veins. To date, it has been thought that the DFMnD formed through inorganic processes, which were controlled by redox changes in the post-Sturtian Nanhua Rift Basin, South China. However, in this study, systematic petrographic, mineralogical, and geochemical analyses indicate a microbially mediated origin of the Mn ore deposits. Mineralized microbial woven micro-textures (observed at the μm scale) and microbial fossils are common in the laminated Mn-carbonate ores. We infer that microbial enzyme activity formed poorly crystallized Mn oxide/hydroxides and carbonaceous material, which transformed to rhodochrosite, kutnohorite, ankerite/dolomite, framboidal pyrite, and apatite via diagenesis. Some micro-scale quartz and K-feldspar may be detrital but most appears to have formed during diagenesis or through hydrothermal activity. A micro-mineralogical profile determined by 2500 spectra via high-resolution in situ micro-Raman spectroscopy also revealed cyclic laminations of Ca-rhodochrosite as microbialite (ankerite/dolomite) and quartz, indicating a mineralized biomat system. Ca-rhodochrosite transformed to kutnohorite under elevated temperatures, as indicated by the maturation level of organic matter (determined via Raman spectroscopy). Alternating micro-laminae denote cyclic changes in microbial groups (Mn- and Fe-oxidizing microbes versus cyanobacteria) during the formation of the Mn ore deposits. Our proposed model for the microbially mediated metallogenesis of Mn-carbonate deposits begins with enzymatic multi-copper oxidase processes associated with autotrophic microbial activity under obligatory oxic conditions, which results in the precipitation of Mn bio-oxides. Following their burial in organic-rich sediments, the Mn(IV) oxides and hydroxides are reduced, producing soluble Mn(II) via processes mediated by heterotrophic microbes under suboxic conditions, which in turn form the Mn-carbonates. This microbial metallogenesis model for the Cryogenian DFMnD in South China is similar to that proposed for the Jurassic Urkut Mn deposit in Hungary, indicating that a two-step microbially mediated process of Mn ore formation might be common throughout geological history.

The Eastern Mediterranean continental slope offshore Israel became a focus of exploration for, and production of, natural gas in recent years. The 2010–2011 Nautilus ROV expedition performed detailed video recordings and sampling in two areas offshore Israel: the Palmachim disturbance, southwest of Tel Aviv, and an area offshore Acre, north of Haifa. An analytical programme regarding the carbonate structures was carried out, examining the overall mineralogy, stable C and O isotopes, and Ca, Mg, and Mn concentrations. This provided information on their composition and as a result, an indication of the carbon sources and temperature of formation. The major authigenic minerals identified comprised magnesian calcite, dolomite, aragonite, and kutnohorite. The detrital minerals included quartz, clays, feldspars, and rare augite and enstatite, likely transported from the Nile estuary. The carbon isotope composition of aliquots taken from nineteen samples from these areas have an overall δ13C range from −62.0 to −0.1‰PDB, indicating a range of microbial/biogenic and thermogenic methane contributions. The range of δ18O from 2.7 to 7.0‰PDB reflects the range of temperatures of formation. The δ18O characteristics differ among areas. In general, high values; δ18O >5‰PDB are recorded from area N2 of the Palmachim disturbance, indicating low temperature of formation. Low values of δ18O (13C increase from −39.9 to −0.1‰(PDB), and δ18O decrease from 6.2 to 4.7‰(PDB). These values indicate a decrease in the methane-derived carbon contribution and an increase in temperature. Kutnohorite, Ca(Mn2+, Mg, Fe 2+)(CO3)2 is a major component in samples from Acre, and less so in the Palmachim disturbance. An exploratory investigation of the relationship between Mn/Ca, δ18O and δ13C revealed that samples having Mn/Ca < 0.1(wt./wt.) have δ13C 0.1 have δ13C between −35 and −22‰PDB suggesting a thermogenic origin. These results suggest that the mineralogical, isotopic δ13, δ18O, and chemical (Mn/Ca indicative of kutnohorite) characteristics of surficial carbonate structures can indicate and distinguish between deep and shallow methane sources in the Eastern Mediterranean.

International audience; Cleaved dolomite (CaMg(CO3)2) crystals were immersed in supersaturated, saturated, and undersaturated solutions with respect to zabuyelite (Li2CO3), and the growth process was characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). In all cases, overgrowths formed by 2D-nucleation could be clearly distinguished in the lateral force images recorded by AFM. With highly supersaturated solutions, AFM imaging was hindered by the formation of 3D crystal aggregates. When saturated solutions were used, the overgrowths reproduce the topography of the substrates, indicating that the lattice of the overgrown phase undergoes a compressive strain. Furthermore, we performed a nanotribological characterization by sliding a nanotip over both the 2D-overgrowths and dolomite substrates. Thus, friction coefficients, µ, could be easily quantified. As a result, a µ = 0.08 on the 2D-overgrowths was measured, which is one order of magnitude lower than the value of µ = 0.89 on the dolomite substrate. This friction reduction in the sliding of a nanotip on the overgrowth as compared to the friction observed on the dolomite substrate, together with the fact that atomic-scale stick-slip is observed on dolomite but not on the 2D overgrowths, point towards an anomalously weak interaction of Li2CO3 overgrowths with the AFM probe.

The caves of Grottedal, Kronprins Christian Land, northeast Greenland, are found within the continuous Arctic permafrost zone. The caves contain several cryogenic features, including minerals and ice deposits. Convection loops within the air of the caves appear to create a sublimation horizon, above which increasing humidity leads to hoar frost formation. Below this sublimation horizon, dry and desiccating conditions lead to the leaching of CaCO3 and the formation of magnesian calcite and kutnohorite. Evidence of condensation corrosion in these caves may also be responsible for the formation of a new type of cryogenic speleothem (called cryogenic frostwork), which may form from freezing of this condensate on surfaces. Cave-air dynamics, driven by the cold temperatures (measured as low as −17.1°C) and climatic conditions of

Red discolorations are an effloresce phenomenon detect on the surface of stone objects and considered as damage factor in both esthetic and conservation points of view. It is very difficult to remove and seldom report about their weathering mechanism. Recently, numerous of red discolorations have affected the limestone objects of Lingyan Temple in Shandong province, one of the most important building materials in China. In order to set up the appropriate conservation remedy, it is essential to identify the origin, characteristics, composition, and the formation process of red discolorations. Several analytical and investigation techniques, such as X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), Micro-Raman spectroscopy (Raman), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS) were used to better understand the red discolorations. The results demonstrated that the discolorations on limestone surface have been caused by carbonic acid weathering process. The red discolorations consisted mainly of kutnohorite (Ca(Fe,Mg,Mn)CO3) and iron oxides. They showed tabular, lamellar, and granular morphologies, which originated from the in situ carbonic acid weathering of kutnohorite. After rainfall, the Ca, Mg, Mn ions with relatively high solubility were primarily leached from carbonatite phases. It was resulting in the sedimentation of red iron oxides through a chemical reaction and physical adhesion. Based on those analyses, a chelating agent (ethylenediamine tetraacetic acid disodium salt) was chosen to remove theses red discolorations on the stone object.

The Permian was a key period of manganese deposition in Southern China, as evidenced by a series of large-to medium-scale manganese deposits along the margin of the Yangtze Block. This study characterizes representative examples that occur in late Middle Permian silicon–mud–limestones in Zunyi and adjacent areas and examines their implications for manganese deposit genesis and sedimentary environment. The manganese deposits are typical carbonates deposits, with the main manganese-bearing mineral being rhodochrosite, followed by calcimangite, capillitite, manganocalcite, and kutnohorite in decreasing abundance. The ore bodies, hosted in the black shale series of the Late Permian Maokou Formation, are mainly stratiform or stratoid in shape, usually have lamellar, massive, banded, fragmental, or pisolitic structures, and contain paragenetic gangue minerals including rutile, barite, pyrite, and chalcopyrite, etc. These features are characteristic of hydrothermal exhalative sedimentary deposits, implying that this was their mode of genesis. The Mn-ores have higher Co, V, Ni, Cu, Sr and total REE content, lower Y/Ho ratios, and characterized by the Fe–Mn–(Cu + Co + Ni) × 10, lgTh vs. lgU, P2O5 vs. Y and Fe/Ti vs. Al/(Al + Fe + Mn) diagrams strongly suggest hydrothermal activity during manganese mineralization. The Th/U, V/Cr and V/(V + Ni) ratios of samples from the Zunyi manganese deposits are 0.38–3.27, 3.11–24.00 and 0.38–0.99, respectively, indicating the sedimentary environment was weakly oxidizing to reducing condition.

Neoproterozoic banded iron formations (BIF) and ironstones occur in the Urucum mining district (Pantanal, Brazil). Microbially mediated formation as microbialites has been proposed for these deposits. This study aims to provide a high resolution characterization of micro-textural features, mineral assemblage and distribution to gather evidence for two microbial ore forming systems: Fe- and Mn-oxidizing metabolic processes and cyanobacterial activity Optical- and cathodoluminescence rock microscopy, Fourier Transform Infrared (FTIR), and Raman spectroscopy allowed the identification of syngenetic and diagenetic minerals. Evidence from BIF and nodule samples points to low temperature, suboxic, neutral/slightly alkaline formation conditions, but rarely diagenetic acidic and anoxic local conditions occurred. Direct evidence for the action of microorganisms is: (a) microtextural evidence of microbially mediated formation of the ore beds, which occur as microbialites; (b) mineral types of syngenetic origin (ferrihydrite, todorokite, manganite) with various types of organic matter incorporated, (c) light isotope signal (cited negative hematite δ57Fe) and; (d) mineralized sedimentation cycles of Fe, Mn, Ca and Si. Two microbial ore forming systems are proposed as dual systems, characterized by the main Fe- and Mn-oxidizing metabolic processes and intense cyanobacterial activity. More ordered minerals, such as hematite, goethite and anatase, formed during diagenesis of Fe-rich biomat. Calcite-type (calcite, Mn-calcite, rhodochrosite) and dolomite-type carbonate structures (kutnohorite, ankerite, dolomite) occurred. Fine-grained carbonate is the common constituent, with some idiomorphic occurrences and dissemination in the laminae and nodules. The composition of the ankerite and the other carbonates is highly variable. The BIF’s gray laminae consist of hematite-rich part with minor quartz, scarce carbonate and apatite. The red laminae contain high contents of carbonate and hematite, with quartz and further diagenetic minerals (feldspar, apatite). Hematite occurs in considerable amount, but it is enough only for turning jasper reddish in color. Nodules were segregated from laminae by diapirism and/or microbial activity and underwent the same syngenetic and diagenetic changes as the microlayers from which they are segregated. Nodule compositions fit well with the diagenetic trends common in such ore forming systems. Most likely the differences between laminae and nodules are consequences of different initial organic matter contents and fine alternation of the oxygen supply of the system.

The industrial revolution was responsible for releasing over 1.5 trillion tonnes of carbon dioxide into the atmosphere resulting in a global temperature increase of ∼1 °C. It also produced billions of tonnes of alkaline materials that have the potential to react with some of that CO2, and this carbon capture potential may be used to help prevent climate change. Here we assess the carbon capture potential of legacy iron (Fe) and steel wastes in South Wales, the United Kingdom, and show that between 0 and 77% of total carbonation potential has been reached in the samples (equating to maximum potential between 40 and 608 kg CO2 /t of slag has been removed from the atmosphere by direct carbonation) across 10 historic works (out of 48 that operated in the region), even after > 140 years. The current work suggests that there is a capture potential of up to 17 Mt CO2 by direct carbonation which represents a potential carbon sink for future emissions. Iron and steel slag contains Ca silicate minerals, often dominated by melilite group phases (gehlenite, Ca2Al2SiO7 and akermanite, Ca2MgSi2O7), but also includes minor phases like olivine (fayalite, (Fe,Mn)2SiO4 and fosterite, Mg2SiO4). The presence of mineral carbonate phases such as calcite, magnesite, ankerite, and kutnohorite in the material demonstrates carbonation reaction has occurred after slag formation. Given that there is still carbonation potential to be realised, and that materials produced in the future from the iron and steel industry could play an important role in meeting our climate targets, then new management strategies are needed to maximise the use of this resource.

Manganese deposits in the Early Carboniferous Baping Formation of central Guangxi, South China were formed in the Youjiang Basin on the southern passive margin of the South China Block. Hosted in the Second Member of the Baping Formation, the manganese ore layers were interbedded by thin-middle manganese carbonate and silicified carbonate. Previous studies provide limited information about this manganese deposit, and the sedimentary characteristics of ores and the metallogenic mechanism are not clear. In this study, integrated analyses of petrology, mineralogy, geochemistry, and carbon isotope reveal microbial metallogenesis in the Early Carboniferous manganese deposit of central Guangxi for the first time. Oncoid-bearing massive Mn carbonate and laminated Mn ore contain evidence of disseminated mineralized microbial biosignatures in millimeter scale. An in situ mineralogical study based on CL, FTIR, SEM-EDS and Raman spectroscopy has identified the syngenetic and diagenetic Mn and Fe systems during the formation of the manganese deposit. Both were formed by biomats and precipitated micro-laminae of microbiogenic minerals such as Mn-calcite, rhodochrosite, kutnohorite, and apatite. The metallogenic model of the manganese deposit in the Baping Formation can be divided into two main stages: (1) Sediment accumulation stage, where enzymatic Mn(II) oxidation controlled the accumulation of Mn oxides within the cyanobacterial organic and carbonate network, thus enriching the manganese; (2) Diagenetic stage, where Mn-metasomatism of microbial carbonate and the reaction between syngenetic manganese oxides and the organic matter led to the precipitation of proto-ore (Ca-rhodochrosite). A positive δ13C value (+1.53‰) within the Mn carbonate sample and negative δ13C values (−7.15‰ to −3.64‰) in Mn ore samples indicate the participation of organic carbon in the alkalinity source of Mn-bearing carbonates. The presence of kutnohorite and of Mn-sulfide minerals indicates a transitional zone between suboxic and anoxic conditions. Anatase represents the diagenetic mineral of Fe-biomat. After release of elements in the decomposed cells and EPS, a series of accessory minerals (feldspar, apatite, pyrite, etc.) formed.

The ability of Halomonas maura to bioprecipitate carbonate and sulphate crystals in solid media at different manganese concentrations has been demonstrated in this study for the first time. The precipitated minerals were studied by X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The precipitated minerals were different based on the manganese concentration present in the medium and the incubation time. In the absence of manganese, H. maura formed pseudokutnahorite crystals; in the presence of manganese, the concentration in the culture medium determined the precipitation carbonates, such as rhodochrosite and dolomites. However, in the presence of low concentrations of manganese chloride (MnCl2) (5 g/l), kutnohorite crystals were also formed. Finally, when H. maura was grown in the presence of manganese, small amounts of sulphate crystals (such as bassanite and gypsum) were detected. Our study of the precipitated minerals showed an active role of H. maura in the biomineralisation process, but the geochemical conditions, and the manganese concentrations in particular, were clearly influential.

High-manganese kutnohorite was found in the surface layer of bottom sediments of the South Caspian basin by the method of X-ray diffraction for the first time. Bottom sediments were collected by a multicorer during the 39th voyage of R/V Rift in 2012. Study of the samples by a complex of methods (X-ray diffraction, scanning electron microscopy, atomic absorption, and analysis of the carbon isotope composition) allowed us to attribute kutnohorite to autigenic minerals of the early stage of diagenesis.

This study applied a multianalytical approach to identify active early diagenetic processes during deposition of the Lower Jurassic Bachental bituminous marls (Western Tethys realm). Results point to a major redox control on organic matter (OM) preservation and early diagenetic modifications. Whereas Mn reduction was the main OM oxidation process causing a maximum OM loss of ~1.5% and a lowering of hydrogen index values by up to 400 mgHC/gTOC under suboxic conditions, sulfate reduction was the dominant degradation process under anoxic-euxinic conditions. OM oxidation processes caused an increase of pH values (up to >9) and alkalinity in pore waters. These changes initiated carbonate precipitation in all samples, with high Mn-contents and predominant kutnohorite mineralization in suboxic layers, and presence of Mn-bearing calcite in anoxic units. The actual occurrence of the distinct carbonate phases (dolomite, kutnohorite, Mn-bearing calcite and siderite) reflects the redox pattern established by previous studies. Dissolution of biogenic silica and incipient alteration of pyrite in response to changing pore water pH and alkalinity conditions was found limited to the anoxic-euxinic intervals. In summary, this study provides detailed insights into the early diagenesis of organic-rich marls in a semi-restricted basin setting under varying oxygen depletion.

We measured the bulk modulus and phase stability of a natural Mn-dolomite, kutnohorite, to 19 GPa. At room temperature, kutnohorite is stable in the rhombohedral dolomite phase up to 19 GPa, with an isothermal bulk modulus of 85(6) GPa ( K ′ = 4). The compressibility of kutnohorite is found to match well with both single and double carbonate trends with respect to bulk modulus and unit-cell volume. The thermoelastic properties measured in this study show that the Mn dolomite end-member fits well with the systematic of all the rhombohedral carbonates, both calcite (single carbonate) and dolomite (double carbonate) type.

An unusual sediment-hosted manganese deposit is described from the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. MnO contents up to 15 wt.% were observed in marine clastic and chemical sedimentary rocks. Mn enrichment is interpreted to have resulted from the hydrothermal alteration of manganiferous shale and BIF parent rocks, the primary MnO contents of which are as high as 8.5 wt.%. A detailed mineralogical and petrographic study shows that these parent rocks are characterized by manganoan siderite, ferroan rhodochrosite and other Mn–Fe-rich mineral phases, such as kutnohorite and Fe–Mn-chlorite. Their hypogene alteration gave rise to a diversification of mineral assemblages where ferroan tephroite, calcian rhodochrosite, rhodochrosite, pyrochroite, pyrophanite, cronstedtite, manganoan Fe-rich chlorite and manganoan phlogopite partially or totally replaced the previous mineral assemblage. Thermodynamic modeling performed on chlorite phases associated with the described mineral assemblages illustrates a decrease of average crystallization temperatures from ca. 310 °C during early metamorphic stages to ca. 250 °C during a hydrothermal stage. Mineral transformation processes were thus related to retrograde metamorphism and/or hydrothermal alteration post-dating metamorphism and gave rise to progressive Mn enrichment from unaltered parent to altered rocks. The timing of hypogene alteration was constrained by 40 Ar/ 39 Ar dating to between about 1500 and 1100 Ma ago, reflecting tectonic processes associated with the Namaqua-Natal orogeny along the southern Kaapvaal Craton margin. Manganiferous shale and BIF of the Mozaan Group may represent the oldest known examples of primary sedimentary Mn deposition, related to oxidation of dissolved Mn(II) by free oxygen in a shallow marine environment. Oxygenic photosynthesis would have acted as a first-order control during Mn precipitation. This hypothesis opens a new perspective for better constraining secular evolution of sediment-hosted mineral deposits linked to oxygen levels in the atmosphere-hydrosphere system during the Archean Eon.

The composition of minerals from stratiform ores of the Lengshuikeng deposit, China was studied using contemporary techniques. Barite, enargite, and kutnohorite typical of postvolcanic mineralization have been identified for the first time at the studied deposit. The fluid inclusion study shows that the stratiform ore was formed at a temperature of 340–140°C from homogeneous chloride solution with salinity of 0.2–11.7 wt % NaCl equiv. The O and C isotopic compositions of carbonates from the stratiform ore allowed us to calculate the oxygen isotopic composition of the H2O and the carbon isotopic composition of the CO2 in the ore-forming fluid. The data on ore minerals, fluid inclusions in quartz, and stable O and C isotopes show that two or more fluids derived from different sources could have mixed during formation of the stratiform ores.

Long term storage of CO2 in geologic formations is currently considered the most attractive option to reduce greenhouse gas emissions while continuing to utilize fossil fuels for energy production. Injected CO2 is expected to reside as a buoyant water-saturated supercritical fluid in contact with reservoir rock, the caprock system, and related formation waters. As reported by McGrail et al., experiments with basalts demonstrated surprisingly rapid carbonate mineral formation occurring with samples suspended in the supercritical CO2 (sc CO2) phase. Those experiments were limited to a few temperatures and CO2 pressures representing relatively shallow ( ≈ 1 k m ) reservoir depths. Because continental flood basalts can extend to depths of 5 km or more, in this paper we extend the earlier results across a pressure-temperature range representative of these greater depths. Different basalt samples, including well cuttings from the borehole used in a pilot-scale basalt sequestration project (Eastern Washington, U.S.) and core samples from the Central Atlantic Magmatic Province, were exposed to aqueous solutions in equilibrium with sc CO2 and water-rich sc CO2 at six different pressures and temperatures for select periods of time (30 to 180 days). Conditions corresponding to a shallow injection of CO2 (7.4 MPa, 34 °C) indicate limited reactivity with basalt; surface carbonate precipitates were not easily identified on post-reacted basalt grains. Basalts exposed under identical times appeared increasingly more reacted with simulated depths. Tests conducted at higher pressures (≥12 MPa) and temperatures (≥55 °C), reveal a wide variety of surface precipitates forming in both fluid phases. Under shallow conditions tiny clusters of aragonite needles began forming in the wet sc CO2 fluid, whereas in the CO2 saturated water, cation substituted calcite developed thin radiating coatings. Although these types of coatings were sparse, conditions corresponding to deeper depths showed increasing carbonate precipitation. Basalts exposed to aqueous dissolved CO2 (25.5 MPa, 116 °C) for 30 days were coated in tiny nodules of precipitate (∼100 μm in diameter) that were identified by micro x-ray diffraction as ankerite, [Ca(Fe,Mg)(CO3)2], a variety of dolomite commonly associated with hydrothermal and metamorphic environments. Surface characterization by SEM revealed well-developed round nodules composed of discrete individual platelets. In contrast, reaction products forming on the basalt in the corresponding wet sc CO2 phase had completely different morphology, appearing in an optical microscope as a surface coating instead of discrete nodules. Examination by SEM revealed layers of discrete platelets forming a cover over a few discrete nodules. Longer test durations (180 days) produced severe iron staining along with minerals structures similar to rhodochrosite and kutnohorite. These preliminary experiments show strong evidence of the faster rate of increase in mineralization reactions taking place in the sc CO2 phase, transformation reactions that are just beginning to be explored in detail.

This study presents far infrared spectra in the range 650–70 cm−1 of 18 common and rare carbonate minerals. Mineral samples of known provenance are selected and physically characterized to determine the purity of the crystalline phase and their composition. The fine ground mineral powders are embedded in polyethylene pellets, and their transmittance spectra are collected with a Fourier spectrometer. The far infrared spectra of different carbonate minerals from the same structural group have well-defined similarities. Observed shifts generally manifest the mass effect of the constituent metal cations. Remarkable spectral differences occur for different carbonates in the far IR region and may serve as fingerprints for mineral identification and are more useful identifiers of carbonate species than those in any other infrared range. For some of the minerals studied here, like kutnohorite, artinite, gaylussite, and trona, no far infrared spectra to that extend (up to 70 cm−1) have been found in literature.

The color of red willemite (Zn 2 SiO 4 ) from Franklin, New Jersey, has been attributed in the past to inclusions of franklinite. We have examined these inclusions, which are rod-shaped and 5–20 μm in diameter, and found them to contain multiple phases. Polarized-light microscopy shows the rods to be oriented uniformly parallel to the c axis of willemite, and to have hexagonal or pseudohexagonal cross-sections. Phases identified by standardless EDS analysis and electron back-scatter diffraction include franklinite, friedelite, zincian kutnohorite, serpentine and rhodonite, as well as a sparse, unidentified K-bearing phase. Possible explanations for the origin of the rods include exsolution, primary fluid inclusions, and epitactic growth contemporaneous with willemite crystallization. Although rod-like exsolution is known in some geological settings, and an original exsolved phase might have been altered to the present assemblage, some of the elements in that assemblage ( e.g., H, C, Cl, K, and Ca) are incompatible with the willemite structure at any temperature. An origin as fluid inclusions is supported by the shapes of the rods ( i.e. , negative crystals with a hexagonal cross-sections), the presence of phases containing OH, Cl, Ca, K, and CO 3 , and void space visible around solid phases. The tapered shapes of the rods seems inconsistent with epitaxy. We suggest that willemite-generating metasomatic fluids were preserved as inclusions rich in elements incompatible with willemite, and that subsequent conditions led to their crystallization, possibly by reaction with the host. Reaction ceased with complete consumption of the fluid, resulting in relict fluid inclusions that impart color on the macro scale.

Manganese is a common contaminant of mine water and other waste waters. Due to its high solubility over a wide pH range, it is notoriously difficult to remove from contaminated waters. Previous systems that effectively remove Mn from mine waters have involved oxidising the soluble Mn(II) species at an elevated pH using substrates such as limestone and dolomites. However it is currently unclear what effect the substrate type has upon abiotic Mn removal compared to biotic removal by in situ micro-organisms (biofilms). In order to investigate the relationship between substrate type, Mn precipitation and the biofilm community, net-alkaline Mn-contaminated mine water was treated in reactors containing one of the pure materials: dolomite, limestone, magnesite and quartzite. Mine water chemistry and Mn removal rates were monitored over a 3-month period in continuous-flow reactors. For all substrates except quartzite, Mn was removed from the mine water during this period, and Mn minerals precipitated in all cases. In addition, the plastic from which the reactor was made played a role in Mn removal. Manganese oxyhydroxides were formed in all the reactors; however, Mn carbonates (specifically kutnahorite) were only identified in the reactors containing quartzite and on the reactor plastic. Magnesium-rich calcites were identified in the dolomite and magnesite reactors, suggesting that the Mg from the substrate minerals may have inhibited Mn carbonate formation. Biofilm community development and composition on all the substrates was also monitored over the 3-month period using denaturing gradient gel electrophoresis (DGGE). The DGGE profiles in all reactors showed no change with time and no difference between substrate types, suggesting that any microbiological effects are independent of mineral substrate. The identification of Mn carbonates in these systems has important implications for the design of Mn treatment systems in that the provision of a carbonate-rich substrate may not be necessary for successful Mn precipitation.

The pressure–temperature conditions of the reactions of the double carbonates CaM(CO3)2, where M = Mg (dolomite), Fe (ankerite) and Mn (kutnohorite), to MCO3 plus CaCO3 (aragonite) have been investigated at 5–8 GPa, 600–1,100°C, using multi-anvil apparatus. The reaction dolomite = magnesite + aragonite is in good agreement with the results of Sato and Katsura (Earth Planet Sci 184:529–534, 2001), but in poor agreement with the results of Luth (Contrib Mineral Petrol 141:222–232, 2001). The dolomite is partially disordered at 620°C, and fully disordered at 1,100°C. All ankerite and kutnohorite samples, including the synthetic starting materials, are disordered. The P–T slopes of the three reactions increase in the order M = Mg, Fe, Mn. The shallower slope for the reaction involving magnesite is due partly to its having a higher compressibility than expected from unit-cell volume considerations. At low pressures there is a preference for partitioning into the double carbonate of Mg > Fe > Mn. At high pressures the partitioning preference is reversed. Using the measured reaction positions, the P–T conditions at which dolomite solid solutions will break down on increasing P and T in subduction zones can be estimated.

Background Mn oxides occur in a wide variety of geological settings and exert considerable influences on the components and chemical behaviors of sediments and soils. Microbial reduction of Mn oxides is an important process found in many different environments including marine and freshwater sediments, lakes, anoxic basins, as well as oxic-anoxic transition zone of ocean. Although the pathway of Mn anaerobic reduction by two model bacteria, Geobacter and Shewanella, has been intensively studied, Mn bio-reduction is still the least well-explored process in nature. Particularly, reduction of Mn oxides by other bacteria and in the presence of O2 has been fewly reported in recent publishes. Results A series of experiments were conducted to understand the capability of Dietzia DQ12-45-1b in bioreduction of birnessite. In anaerobic systems, Mn reduction rate reached as high as 93% within 4 weeks when inoculated with 1.0 × 1010 cells/mL Dietzia DQ12-45-1b strains. Addition of AQDS enhanced Mn reduction rate from 53 to 91%. The anaerobic reduction of Mn was not coupled by any increase in bacterial protein concentration, and the reduction rate in the stable stage of day 2–14 was found to be in good proportion to the protein concentration. The anaerobic reduction of birnessite released Mn(II) either into the medium or adsorbed on the mineral or bacteria surface and resulted in the dissolution of birnessite as indicated by XRD, SEM and XANES. Under aerobic condition, the reduction rate was only 37% with a cell concentration of 1.0 × 1010 cells/mL, much lower than that in parallel anaerobic treatment. Bacterial growth under aerobic condition was indicated by time-course increase of protein and pH. In contrast to anaerobic experiments, addition of AQDS decreased Mn reduction rate from 25 to 6%. The reduced Mn(II) combined with carbon dioxide produced by acetate metabolism, as well as an alkaline pH environment given by cell growth, finally resulted in the formation of Mn(II)-bearing carbonate (kutnohorite), which was verified by XRD and XANES results. The system with the highest cell concentration of 1.0 × 1010 cells/mL gave rise to the most amount of kutnohorite, while concentration of Mn(II) produced with cell concentration of 6.2 × 108 cells/mL was too low to thermodynamically favor the formation of kutnohorite but result in the formation of aragonite instead. Conclusion Dietzia DQ12-45-1b was able to anaerobically and aerobically reduce birnessite. The rate and extent of Mn(IV) reduction depend on cell concentration, addition of AQDS or not, and presence of O2 or not. Meanwhile, Mn(IV) bioreduction extent and suspension conditions determined the insoluble mineral products. Electronic supplementary material The online version of this article (doi:10.1186/s12932-015-0026-0) contains supplementary material, which is available to authorized users.

The objective of this study is to present the thermal emission spectra of various anhydrous, calcite- and dolomite-series carbonate minerals to illustrate the effect of the structural cation (Ca 2+ , Mg 2+ , Fe 2+ , Mn 2+ , Zn 2+ ) on shifting the positions of the carbonate absorption bands. All of the carbonate mineral emission spectra included in this study exhibit three absorption features related to three specific vibrational modes of the carbonate anion (CO 3 2- ). These anion vibrations include the out-of-plane bend, the asymmetric stretch, and the in-plane bend (i.e., the ν 2 , ν 3 , and ν 4 modes, respectively). The positions of the absorption-band emissivity minima are unique for each carbonate chemistry and are thus diagnostic of mineralogy. The average ν 2 , ν 3 , and ν 4 wavenumber positions for the various carbonate minerals are as follows: calcite (CaCO 3 ) 883, 1523, and 712 cm -1 ; magnesite (MgCO 3 ) 901, 1572, and 748 cm -1 ; siderite (FeCO 3 ) 876, 1523, and 736 cm -1 ; rhodochrosite (MnCO 3 ) 877, 1535, and 726 cm -1 ; smithsonite (ZnCO 3 ) 882, 1509, and 742 cm -1 ; dolomite (CaMg(CO 3 ) 2 ) 894, 1547, and 728 cm -1 ; and kutnahorite (CaMn(CO 3 ) 2 ) 882, 1526, and 716 cm -1 . Carbonates as a general mineral class crystallize in a variety of geological environments; however, each specific carbonate mineralogy typically is limited to a narrow range of geologic settings in which it forms. Thermal infrared emission data to be received from the Mars Global Surveyor thermal emission spectrometer will contain the spectral signature of carbonates if they are present above the detectibility limit on the surface of Mars. This study presents the spectral information necessary to recognize carbonate as a mineral class as well as identify the specific type of carbonate from thermal emissivity data. Knowledge of distinct carbonate mineralogy will be useful for interpreting the environmental conditions that were present on Mars during the carbonate formation. The result of this study is that the major carbonate mineral species (calcite, dolomite, magnesite, siderite, and smithsonite) can be identified from thermal infrared emission data, provided moderate (10 cm -1 ) spectral sampling. Because of the similarity of absorption band positions between kutnahorite and calcite, high (2 cm -1 ) spectral sampling is required to distinguish kutnahorite. Moderate spectral sampling data are also sufficient to determine the amount of Mg and Fe in Mg-Fe solid solution minerals to within 3-5% of the cation abundance.

The sedimentary geochemistry of manganese is dominated by the redox control of its speciation, higher oxidation states (Mn (super 3+) and (super 4+) ) occurring as insoluble oxyhydroxides in well-oxygenated environments and the lower oxidation state (Mn (super 2+) ) being much more soluble in oxygen-deficient settings. Its geochemical behavior is therefore quite different in oxic and anoxic environments, and where oxic and anoxic conditions are juxtaposed, Mn is recycled between the two environments. In modern marine sediments, Mn is present above its crustal abundance as an oxyhydroxide in all slowly accumulating (pelagic) sediments of the deep ocean and in surficial deposits of continental margin environments. Diagenetic recycling of Mn in the latter causes surficial deposits to have larger Mn enrichments than in many pelagic sediments. Bottom sediments of permanently anoxic basins show no enrichments and have Mn concentrations that are controlled solely by the aluminosilicate fraction. Manganese carbonates (kutnohorite and calcic rhodochrosite) are found only in anoxic sediments accumulating beneath surface oxic horizons (and therefore under oxygenated bottom waters) in many nearshore environments. Such enrichments are due to delivery of Mn by burial of surface oxyhydroxides into the subsurface anoxic environment where they are dissolved. Pore-water Mn levels can reach saturation with respect to a mixed Mn-Ca carbonate phase in such sediments. The diagenetic origin for these phases is shown by their carbon isotope compositions, which typically indicate a carbon source from decomposing organic matter. The presence of Mn carbonates therefore signifies that the host sediment must have accumulated under oxygenated bottom waters.On the basis of this information it is proposed that, in contrast to several current explanations for the formation of Mn carbonates (kutnohorite and rhodochrosite) in ancient organic-rich shales, limestone, and marl sequences and in many Mn ore deposits, the occurrence of these mineral phases indicates that the sediments originally accumulated beneath oxygenated bottom waters. By analogy with the present, Mn carbonates could not have formed in the bottom waters of anoxic basins. These diagenetic phases, however, did form where Mn was supplied at a high rate, namely, by the burial of oxyhydroxide-enriched surface sediments, to a subsurface anoxic environment. This situation could only have occurred under oxygenated bottom waters. The presence of Mn carbonates in ancient black shales (and in some carbonate-rich rocks) lends strong support to the notion that these rocks did not necessarily form in anoxic basins but owe their carbon richness to a high supply of organic matter to sediments deposited under oxygenated bottom waters, probably in continental margin settings.

Nchwaningite, Mn~+Si03(OH)2' H20, is an orthorhombic chain silicate [space group Pea2" Z = 4, a = 12.672(9), b = 7.217(3), e = 5.341(2) A], occurring as aggregates shaped like pin cushions, together with calcite, bultfonteinite, and chlorite at N'chwaning mine, located in the Kalahari manganese field, northern Cape Province, South Africa. The aggregates consist of light brown, transparent needles, which average 1.0 x 0.1 x 0.05 mm in size. Nchwaningite is named after the mine N'chwaning II, where it was found first. Nchwaningite is biaxially negative with the refractive indices ex= 1.681(2), fJ = 1.688(2), 'Y= 1.690(2), 2 ~ = 54.4(4)°. The optical orientation is X = b, Y = a, Z = c. Nchwaningite has two perfect cleavages parallel to (010) and (100). The calculated density is 3.202 glcm3. The chemical composition, as determined by electron microprobe analyses, indicates minor substitutions of Mg for Mn. The crystal structure, including H positions, was solved and refined from X-ray singlecrystal data to R = 2.14%, Rw = 2.91%. The nchwaningite structure consists of double layers of laterally linked so-called truncated pyroxene-building units formed by a double chain of octahedra, topped with a Zweier single chain of Si tetrahedra. Symmetry-equivalent units are linked laterally but turned upside down. This yields a double-layer structure with H bridges linking the layers. A striking feature of the structure is that one Mn06 comer is formed by a H20 molecule. The tetrahedral chain and octahedral distortion of the new mineral is compared with pyroxenes having MnH in Ml [synthetic MnSi03 (P2/e clinopyroxene) andjohannsenite CaMn(Si03)2]' To test a complete Ca and Mg substitution for Mn in the new nchwaningite structure type, distance least-square refinements were performed. It was found that Ca and Mg analogues would also yield reasonable interatomic distances and polyhedra. OCCURRENCEAND ORIGIN sedimentation of the distinct Fe and Mn formations resulted from the different conditions of solubility and preThe Kalahari manganese field is the largest continental cipitation of Fe and Mn, related to cyclic sea-level changes. Mn deposit on the Earth, located in the northern Cape Furthermore, Beukes and Gutzmer (personal communiProvince, South Africa. Three Mn layers are interbedded cation) assumed a primary precipitation of braunite towith Fe formations of the Hotazel Formation of the gether with calcium manganese carbonates, whereas Bau Transvaal Sequence. These layers cover an area of about (personal communication), using REE-data, postulated a 35 x 15 km, and the ore varies in thickness from 6 to primary MnH precipitate with braunite as a secondary 45 m, with reserves of about 1.3 billion tons (Taljaardt, product. Using trace-element profiles in the Ongeluk la1982). vas, Schuette and Cornell (1992) postulated an early alSeveral controversial theories about the primary sedi- teration of fresh andesites caused by heated sea water, mentary origin of the Mn ores exist: Beukes (1983) and leading to the precipitation of Fe and Mn. Nel et al. (1986) postulated a volcanogenic or volcano- The majority of the ore, the so-called Mamatwan type, exhalative source for Fe and Mn, related with submarine is a fine crystalline sedimentary laminated braunite and volcanism in a basin west of the Kapvaal craton. The kutnohorite lutite of relatively low grade, containing a precipitation of Fe and Mn resulted from an upwellingof maximum of -38 wt% Mn. A small portion of the ore 02-undersaturated, Fe- and Mn-rich bottom waters on is developed as coarse crystalline massive Wessels-type the shelf in a photic zone with the first O2 producers. The ore of relatively high grade with Mn contents between 42

The Molango manganese deposit in Mexico is by far the largest known Mn deposit in North America. Mn carbonate mineralization is hosted by a finely laminated Upper Jurassic marine sedimentary sequence that forms the base of the Chipoco facies of the Taman Formation (Kimmeridgian). Manganese enrichment occurs over a total stratigraphic thickness of approximately 50 m, and a length of over 50 km along strike and serves as the protore for supergene Mn oxide ore. The carbonate ore zone comprises the basal 1 to 10 m of the Chipoco facies.The carbonate ore bed consists of fine-grained rhodochrosite and dispersed organic matter, magnetite, and maghemite but generally only trace quantities of pyrite. Fine laminations and clotted textures suggest deposition in a restricted marine environment. Manganese content is highest at the base of the mineralized interval, drops abruptly to subore grade, Mn-rich carbonates, and then gradually decreases upward to background levels (i.e., trace Mn in calcite). Chemical and mineralogical studies indicate that the predominant mineralogy changes upward from rhodochrosite in the ore zone, to mixed carbonates (Mn calcite, kutnohorite, + or - rhodochrosite), to Mn calcite.Several geologic and geochemical processes resulted in the formation of Mn carbonate by the early diagenetic reduction of Mn oxides through the oxidation of organic matter and iron sulfide. Mn oxides were concentrated at the margins of a dysaerobic to mildly anoxic stratified basin and became an important oxidizing agent of organic matter in the sediment pile. Effects of organic matter oxidation reactions included production of manganous ion (Mn (super +2) ), MnCO 3 precipitation, oxidation of iron monosulfide (precursor to pyrite), and formation of Fe oxide. The latter two processes explain the anomalously low pyrite and abundant magnetite content in the ore zone. The observed vertical variation of Mn carbonate mineralogy and the mineral compositions reflect variations in the concentrations of reduced manganese in the pore waters. Thus, Mn carbonate precipitated from pore water and did not form as a chemical sediment in the water column. The ultimate source of the manganese is uncertain but may have been fluvial-sediment loads or hydrothermal activity associated with the rifting of the Gulf of Mexico.

The two most important types of manganese deposits in China are sedimentary and supergene, which account for 71 and 15 percent of the total manganese reserves, respectively. According to the host-rock associations, sedimentary manganese deposits can be classified into (1) a mud-rock type, (2) a black shale series type, (3) a carbonate rock type, and (4) a chert-limestone-mudstone type. The black shale series-type manganese deposits account for 48 percent of the total manganese reserves and are large- or medium-sized deposits.The black shale series or dark-colored sedimentary association consists of dark-colored mudrock-chert-carbonate rocks rich in sulfides and organic matter (C organic > or =1%). The term, black shale series-type manganese deposits, is a general name for all manganese deposits hosted in black shale, black chert, black siliceous shale, dark gray limestone, etc.Black shale series-type manganese deposits are distributed in 13 stratigraphic horizons from middle Proteozoic to Early Triassic. They can be divided into three subtypes: (1) a black shale subtype, (2) a black chert-siliceous shale subtype, and (3) a dark gray limestone subtype. Subtype 1 consists mainly of primary Mn carbonate ores; subtypes 2 and 3 consist mainly of Mn-containing limestones. Weathering and leaching processes can change the Mn limestones into Mn oxide ores which form large and high-quality supergene deposits, for example, the Mugui and Dongxiangqiao deposits.Manganese carbonate ores hosted in the black shale series consist mainly of rhodochrosite, calcio-rhodochrosite, kutnohorite, manganocalcite, Mn-containing calcite, Mn-containing dolomite, etc. The ores are black, dark gray, light gray, yellowish, and pink in color and have banded, laminated, massive, oolitic, spotted, and brecciated structures.Commonly, changes in the ore-bearing sequences show, from bottom to top, (1) black shale series --> (2) Mn carbonate ore bed --> (3) light-colored rocks. This indicates that ore beds formed in the transition from reduced to slightly oxidized and from weakly alkaline to alkaline conditions. The evolution of the ore-bearing sequence indicates that manganese mineralization occurred during the early stage of regression.The important Mn carbonate deposits hosted in the black shale series resulted from three main processes during formation: concentration in seawater, precipitation from seawater, and enrichment in the sediments. Anoxic conditions are a basic factor for manganese concentration in seawater; a weak oxic and alkaline environment is favorable for Mn carbonate precipitation, and microbiological activities play an important role for early diagenetic enrichment of manganese in the deposits.

Distal sediments stratigraphically equivalent to sedex Zn-Cu-Pb deposits at the base of the Proterozoic Onwatin argillite formation, Sudbury Basin, are depleted in S and enriched in Mn, occurring as kutnohorite, manganosiderite and spessartine, and in Ba in the form of the K-Ba-feldspar, hyalophane. Fluid inclusion data indicate deposition of the distal sediments at ∼ 125°C and of the sedex carbonate gangue at ∼ 170°C. The overlying Onwatin argillites, deposited at ∼ 60°C, are characterized by a distinctive pattern of S C org ≈ 0.36 indicative of diagenetic S, alternating with S C org ⪢ 0.36 , probably reflecting episodic expulsion of S-bearing fluids into the water column during the waning stages of hydrothermal activity. Similar to the S/Corg pattern are patterns of rhythmically alternating low and high Zn, Pb, As, Re, Ni and Pd values in the Onwatin argillite. Coinciding with the onset of hydrothermal activity, δ34S-values in pyrite decrease abruptly from ∼ + 7‰ at the top of the Onaping Formation to + 3‰ at the base of the overlying Onwatin argillites. δ13C-values in the carbonate gangue of the sedex deposits, −8.9 to −2.4‰, lie in or close to the range for magmatic carbon, in contrast to values of −18.7 to −14.9‰ in carbonate derived by oxidation of organic carbon in the overlying and underlying rocks. Low δ34S-values in the sedex deposits and basal Onwatin rocks and enrichment of the Onwatin argillite in S, Ni, Re and Pd suggest derivation of these metals, at least part of the S and possibly Cu, Zn, Pb from the magmatic Ni-Cu ores and/or associated Sudbury Igneous Complex which probably also provided the energy to drive the convective-exhalative system responsible for the sedex mineralization. The model implies potential for further sedex deposits along the 110 km of the Onwatin-Onaping contact.

18 pages, 10 figures, 4 tables.-- Available online Dec 6, 2008.-- Issue title: "EUROMARGINS: Imaging, monitoring, and modelling the physical, chemical and biological processes in the European passive continental margins"., More than 500 Fe–Mn nodules were sampled during the Anastasya-01 cruise (TASYO project) along the continental margin of the Gulf of Cadiz (eastern Central Atlantic), at the confluence of the Mediterranean Sea with the Atlantic Ocean, where extensive nodule fields were discovered. Based on wide previous studies that included swath bathymetry, multi-channel and very high-resolution seismic reflection, gravimetry, magnetism and underwater photography surveys, nodules were collected at water depths ranging from 850 to 1000 m, associated with hydrocarbon-derived ankerite and dolomite chimneys and crusts. Thirty six selected samples among the various morphological types were used for the laboratory analysis of physical properties (morphology, colour, surface texture, sphericity, weight and size), mineralogy (XRD, optical and electronic microscopy) and geochemistry (XRF, AAS, ICP-MS, EPMA, GC-MS, S, C and O isotopes). Nodules show a wide range of sizes, densities, weights and morphologies. Nodules are formed of multiple millimetre-thick layers of Fe and Mn oxyhydroxides surrounding the nucleus composed of Early–Middle Miocene plastic marl. The textures developed by the Fe and Mn oxyhydroxides layers are massive, laminated, detrital and mottled to dendritic. Goethite, lepidocrocite, Mn-oxides (7 Å-manganates and 10 Å-manganates), quartz, and phyllosilicates are the main components. Accessory minerals are calcite, dolomite, siderite, rhodochrosite, kutnohorite, pyrite, chalcopyrite, potassium feldspar, zircon, rutile, ilmenite, and chlorite. Fe–Mn carbonates from the siderite–rhodochrosite continuous series are forming part of the marl nuclei. Framboidal, filamentous, and globular textures are observed in Fe–Mn oxides and pyrite suggesting biogenic origin. Nodules show a high mean abundance of Fe (38.6%) moderate Mn (6.0%), and low contents of trace metals and REEs compared to the average content of deep-seabed polymetallic nodules from other oceanic areas. The Mn/Fe ratio ranges from 0.07 to 0.25. The studied nodules hold in their oxide layers hydrocarbons (n-alkanes) derived from marine bacterial activity. Aromatic hydrocarbons in the form of phenanthrene, are also present which is characteristic of mature petroleum. The structure, mineralogy, and chemical composition in the studied nodules are similar to those of diagenetic–hydrogenetic continental margin nodules rather than deep-sea nodules. We suggest that the formation of this type of nodule occurred through a combined diagenetic–hydrogenetic growth process involving fluid venting from deep-seated hydrocarbon reservoirs, bio-mineralization processes, and bottom current erosion., This work has been funded thanks to a research fellowship of the Geological Survey of Spain within the framework of the European Science Foundation EuroCORE-EuroMARGINS projects: “MOUNDFORCE” (O1-LEC-EMA06F, REN-2002-11668-E-MAR) and “MVSEIS” (O1-LECEMA24F, REN-2002-11669-E-MAR).

A new Mn‐silicate mineral, coombsite, iso‐structural with zussmanite, has been found in a 50 cm thick rhodonite‐quartz lens at Watsons Beach, southeast Otago. The Mn‐rich lens is cut by thin carbonate veinlets consisting of rhodochrosite and rare kutnohorite. Stilpnomelane‐related Mn‐silicate, parsettensite, occurs in the lens as dark‐coloured aggregates and as dispersed crystals in the quartz often associated with coombsite. Rarely, the brown Mn‐silicate, caryopilite, occurs associated with concentrations of carbonate. Occurring in rhodonite and quartz is a new brown‐coloured fibro‐lamellar mineral, coombsite, the Mn analogue of zussmanite. The name is given in honour of Professor D.S. Coombs of the University of Otago. Coombsite is hexagonal (rhombohedral) with a 11.828, c 29.146 A, Dcalc 3.063, Dmeas 3.0, e 1.600, (0 1.619. TheX‐ray powder pattern of coombsite closely resembles that of zussmanite, and the axial constants accord well with the expected values of Mn‐substituted zussmanite. An ave...

Mineralogical and fluid-inclusion studies have been conducted on Mn-rich breccias from three low-sulfidation epithermal gold deposits, Cirotan (Indonesia), Orcopampa (Peru) and Cavnic (Romania). In these deposits, the manganiferous minerals occur within cockade breccias that precede most of the deposition of sulfide. Cockade breccias consist of a lithic or sulfide nucleus, a few centimeters across, encrusted by several concentric envelopes that can be subdivided into three successive zones. Each zone may be composed of several stacked sequences of deposition. The Mn minerals are mainly clustered within the internal pinkish zone close to the nucleus, that also comprises quartz, “adularia” and sparse sulfides. Within this Mn mineral zone, each sequence begins with the deposition of Mn silicate, close to the rhodonite end-member (MnSiO 3 ), that fully or partially grades into Mn carbonate, close to the rhodochrosite (MnCO 3 ) end-member, before the deposition of the next sequence. The intermediate zone (the quartz–sulfide zone) consists of quartz–sulfide sequences, with minor Mn silicates. Each sequence begins with fine-grained quartz, indicating a high rate of nucleation, which becomes coarser grained and irregular at the end of the sequence. Chlorite and sulfides are associated with quartz at the end of this zone. The external zone of the cockades is characterized by comb quartz, associated with illite, calcite and, sparingly, manganoan dolomite. The mineralogical zonation illustrated by the cockades expresses a general evolution of the depositional process with time, with a progressive Mn-depletion and Ca-enrichment. No kutnohorite has been observed. Fluid inclusions from the three zones are all two-phase aqueous inclusions (L + V). The fluid salinities are similar in all the deposits and do not exceed 2.6 wt% NaCl eq., the final comb quartz from Orcopampa excepted. Two isenthalpic boiling events are recorded in relation with brittle deformation. The replacement of rhodonite by rhodochrosite revealed in each sequence of the Mn mineral zone could result in a temperature drop in relation to these boiling events, which release CO 2 and increase pH.

A theoretical study of the system CaCO3-MnCO3-H2O provides evidence that solid solution is limited to three mole fraction ranges corresponding to Mn-calcite, kutnohorite and calcic-rhodochrosite. All these authigenic phases may control the concentration of manganese in suboxic marine pore waters. As a consequence the solubility of manganese cannot be adequately described by a single thermodynamic equilibrium constant.

Kutnohorite-quartz veins penetrate through a weakly metamorphosed Late Proterozoic sedimentary rhodochrosite carbonate in pyrite shales. Kutnohorite is pleochroic ranging from colourless (∈) to pinkish grey (ω), ω=1.735, ∈=1,543,d=3.066g·cm−3;a 0=4.852(1),c 0=16.219(7) A. A quantitative chemical analysis leads to the formula (Ca0.95Mn0.05)1.00(Mn0.64Mg0.23Fe0.13)1.00(CO3)2.04. An electron microprobe scanning reveals considerable microscale inhomogeneity in cleavage rhombohedrons taken from a coarsely grained aggregate. It is mostly of the character of periodic fluctuations of the main element contents around an average value. There is a strong prevalence of an Fe+Ca−Mn+Mg antagonism, the extremes of compositonal differences lying in distances of 0.0X mm. The fluctuation maxima amount to about 9% for FeO, 6% for MnO, 3% for CaO, and 1.5% for MgO. In one of the samples even a two phase character with the above antagonism between the two phases was detected by X-rays. The inhomogeneities are due, partly at least, to the Fe-metasomatic processes that followed the formation of the kutnohorite by Alpine-paragenesis hydrothermal metamorphism.

Physicochemical environments for the formation of quartz-free manganese oxide ores from the Hotazel Formation of the Kalahari manganese field, northern Cape Province, South Africa, are discussed quantitatively on the basis of the phase relations in the system Mn-Fe-Si-O and qualitatively in the system Ca-Mn-Fe-Si-O. The phase relations involve major oxide minerals of the ores, namely, braunite, braunite II, hausmannite, bixbyite, jacobsite, and hematite, and were constructed under quartz-deficient conditions, using existing and estimated thermody-namic data and mineral assemblages from the ores. Standard enthalpy delta HM 0 (sub f.298) and Gibbs free energy delta G 0 (sub f.298) values of braunite are estimated to be -1,011.9 + or - 5 and -937.3 + or - 5 kcal/ mole, respectively, on the basis of published experimental data and predicted entropy and heat capacity. The low-grade (diagenetic to very low grade metamorphic) Mamatwan-type ores, consisting mainly of braunite, hausmannite, jacobsite, hematite, kutnohorite, and calcite, are altered by hydrothermal solutions into the high-grade, high-temperature Wessels-type ores consisting of braunite II, bixbyite, hausmannite, hematite, calcite, and andradite with trace amounts of braunite. The peak temperature of formation of the high-grade ores appears not to have exceeded about 300 degrees to 400 degrees C. The ores are oxidized from oxygen fugacity levels defined by the assemblage hausmannite-jacobsite-hematite to that defined by the assemblage bixbyite-hausmannite, with increasing temperature. Under hydrothermal quartz-deficient conditions, the stability of braunite is enlarged to higher temperatures with increasing silica activity (a (sub SiO (sub 2 aq ) ) ). Because braunite is much less abundant in the high-grade Wessels-type ore than in the low-grade Mamatwan-type ore, the maximum a (sub SiO (sub 2 aq ) ) ) in the solutions could be estimated at 10 (super -4) to 10 (super -3) at 300 degrees to 400 degrees C from the stable assemblage bixbyite-hausmannite-braunite. Kutnohorite, and to some extent calcite, was a major source of Ca reacting with silica released from braunite to form braunite II and andradite.

X-ray powder diffraction data, unit cell parameters and chemical data published previously for kutnohorite, the manganese-rich double carbonate mineral, are critically evaluated and compared with new measurements on a specimen from Chvaletice, Czechoslovakia with a composition Ca (Mn0.64Mg0.23Fe0.13)(CO3)2. Data were collected with a computer controlled diffractometer and analysed with profile analysis techniques. The new powder diffraction data set yields much better data and unit cell parameters than the earlier ones given for kutnohorite on PDF 11-345 (Powder Diffraction File, 1987). A least-squares evaluation resulted in ao = 4.8518(3)Å and co = 16.217(2)Å.