Mn-rich Carbonate Research Articles

Morphology of Transition Metal Carbonate Cathode Precursors

Increasing the capacity of cathode materials used for lithium-ion batteries is desirable, as it ultimately enhances the energy density. Due to their lower cost and reversible cycling capacity of 250 – 300 mAh/g, Li- and Mn-rich LMR-NMC oxides are strong candidates as next generation cathodes used in lithium-ion batteries. Apart from the atomic structure, morphology of the cathode particles also influence their performance. LMR-NMC cathode particles are usually constructed through a two-step cathode fabrication process, which involves initial coprecipitation of the Mn-rich carbonate based cathode precursors, and later calcination of these precursors with a lithium salt at elevated temperatures. The secondary particles generally maintain their as precipitated precursor morphologies even after high temperature calcination. Even though the primary particles do change their size during calcination, the rate of oxidation and lithiation experienced by the transition metal precursors depend substantially on the primary particle morphology. Hence, it is critical to understand and control both the primary and secondary particle morphologies obtained after the coprecipitation process. In the present context, carbonate based NMC cathode precursors containing only Mn, only Ni and only Co, is precipitated, along with equal amount of the transition metals (Ni0.33Mn0.33Co0.33CO3), using conventional batch reactors. NH4HCO3 is used as the source of carbonate anions during the coprecipitation process, and the entire reaction is conducted at 50°C. The obtained particle morphologies for different transition metals are shown in Figure 1(a) as visualized using high resolution TEM techniques. Except MnCO3, all other transition metals demonstrate aggregated morphologies, which most probably form through surface growth mechanisms. Competition between growth rate and surface energies that leads to the formation of single crystalline particles for MnCO3, and particulate features for other transition metals, are demonstrated in Figure 1(b). Multiscale computational methodologies are developed to elucidate the impact of reaction kinetics and thermodynamics on determining the overall primary and secondary particle morphologies. Influence of transition metal content and ammonia concentration in determining the final particle size and size distribution will be discussed as part of this study. Figure 1

Origin of Historical Ba-Rich Slags Related to Pb-Ag Production from Jihlava Ore District (Czech Republic)

The aim of this study was to characterize historical slags which originated during silver production from the Jihlava ore district, Czech Republic. The area was among the head producers of silver within the Lands of the Czech Crown in 13th–14th centuries. The mined ores had complex composition, being formed mostly by pyrite, sphalerite, galena, chalcopyrite, and accessory silver-rich minerals such as silver-bearing tetrahedrite (freibergite) or pyrargyrite, with gangue represented by quartz and Mn-rich carbonates or baryte. Large volumes of slags with contrasting composition were generated during the Pb-Ag production. Altogether, two main types of slags were identified in the district. The first type is characterized by high BaO contents (up to 34.5 wt.%) and dominancy of glass, minor quartz, and accessory amounts of Ba-rich feldspar (up to 93 mol.% of Cls), metal-rich inclusions, Ba-Pb sulphates and only rare pyroxene, wollastonite and melilite. The composition of the second group belongs to fayalitic slags containing glass, Fe-rich olivine, accessory pyroxene, feldspar, quartz, and inclusions of various metallic phases. Fluxes were derived from gangue (quartz, carbonates, baryte) or local host rocks for both types of slag. The calculated viscosity indexes reflect (with minor exceptions) medium-to-high effectivity of metal separation. Smelting temperatures were estimated from a series of ternary plots; however, more reliable estimates for both types of slags were obtained only from experimental determination of melting temperature and calculations using bulk/glass compositions (~1100–1200 °C).

Archean-Proterozoic unconformity on the Fennoscandian Shield: Geochemistry and Sr, C and O isotope composition of Paleoproterozoic carbonate-rich regolith from Segozero Lake (Russian Karelia)

Precambrian paleoweathering profiles provide insight into environmental changes and mark stratigraphic boundaries. However, the well-preserved and exposed stratigraphic boundaries, not affected by tectonics, are rare for the Precambrian. We report sedimentological, geochemical and Sr, C and O isotopic data for the Paleoproterozoic weathering profile in Chapanshary Island on Lake Segozero, Central Karelia, the eastern part of the Fennoscandian Shield. The Chapanshary Island weathering crust contains a unique carbonate caliche, rarely developed on the Archean granite-gneiss basement. The crust laterally grades into carbonate-cemented boulder conglomerate, which is overlain by sedimentary carbonate. The chemical index of alteration (CIA; 60–67) and plagioclase index of alteration (PIA; 53–82) values for the weathering crust correspond to weathered felsic/mafic substrate and to illite and K-rich composition of the regolith. The δ18O values of carbonate cement and sedimentary carbonate are highly negative (−22.3 to −17.6‰ V-PDB). The 87Sr/86Sr values of Fe- and Mn-rich carbonate of the regolith, boulder conglomerate cement and sedimentary carbonate are 0.7133 to 0.7242 and are exceptionally high for marine Paleoproterozoic carbonate. The high δ13C values (+3.9 to + 5.2‰ V-PDB) of carbonate cement and sedimentary carbonate indicate deposition during the ca. 2.22–2.06 Ga Lomagundi-Jatuli carbon isotope excursion of the early Paleoproterozoic. The Chapanshary Island weathering profile developed in a terrestrial setting with arid to semi-arid climate ca. 2.2–2.1 Ga ago under atmosphere with high CO2 content. The overlying sedimentary carbonates were deposited in a shallow-water setting (ephemeral ponds and lakes) evolved in the inner part of Fennoscandia.

The effects of process parameters on the properties of manganese-rich carbonate precursors: A study of co-precipitation synthesis using semi-batch reactors

The synthesis of precursors for lithium- and manganese-rich xLi2MnO3•(1-x)LiMO2 (M = Ni, Mn, Co) (LMR-NMC) materials is generally carried out via co-precipitation using a continuously stirred tank reactor (CSTR) under steady-state conditions. But during the early stages of research (e.g., when screening compositional spaces of interest), using a CSTR in steady-state mode can be time consuming and wasteful. An alternative is to operate the reactor in semi-batch mode, which shortens reaction times and reduces the amount of waste. However, the effect of this mode on the product is not well documented. The present work investigates how several process variables affect the physical properties of Mn-rich NMC carbonate precursors prepared under semi-batch operation of a CSTR. The process variables examined are pH, ammonia concentration of feed solution, process temperature, stirring speed, and reaction time. The results can help guide research-scale production to support efficient development of Mn-rich carbonates for cathode oxides.

Manganese-rich deposits in the Mesoproterozoic Gaoyuzhuang Formation (ca. 1.58 Ga), North China Platform: Genesis and paleoenvironmental implications

The redox state of the surface ocean–atmosphere system has long been considered to have been remarkably stable during the mid-Proterozoic (ca. 1800–800 Ma); however, there is a growing body of evidence that supports dynamic redox fluctuations during this time. Sedimentary Mn enrichment in the ca. 1.58 Ga Gaoyuzhuang Formation (North China) may represent an important archive of mid-Proterozoic paleoenvironmental conditions. We present a sedimentological and geochemical study of the Mn-rich carbonate succession in the Gaoyuzhuang Formation in order to further elucidate the genesis and paleoredox implications of this Mn-rich deposit. We suggest that the Mn-rich interval was deposited in deep subtidal environments close to storm wave-base. Manganese occurs either as millimeter-sized Mn-oxide (pyrolusite) nodules or as Mn-rich dolomite, which features moderately positive I/(Ca+Mg) values (0.5–1.7 μmol/mol) and Ce anomalies (1.1–1.2). In contrast, the underlying and overlying carbonates that lack Mn-oxides were deposited in a shallow subtidal environment above fair-weather wave-base. They have relatively low I/(Ca+Mg) values (<0.5 μmol/mol) and negligible Ce anomalies (average Ce/Ce* = 1.05 ± 0.05). Thus, we suggest that during the deposition of the highly Mn-enriched strata shallow seawaters were sufficiently oxygenated to drive active redox cycling of Mn, and that this interval was bracketed by less-oxygenated shallow marine conditions. Given the widespread distribution of the Mn-rich deposits across the North China platform, we argue that the Mn-rich deposits may have recorded a basin-scale, pulsed oxygenation. The association of this interval with enigmatic macrofossils in the overlying strata of the upper Gaoyuzhuang Formation warrants further investigation of the potential link between pulsed episodes of oxygenation and evolutionary advances in the early Mesoproterozoic.

Evolution of the subseafloor hydrothermal system associated with the Ming VMS deposit, Newfoundland Appalachians, and its controls on base and precious metal distribution

The ~ 487 Ma Ming volcanogenic massive sulfide (VMS) deposit consists of four subparallel, elongated, semi-massive to massive sulfides lenses (the 1807, 1806, Ming North, and Ming South zones) hosted in rhyodacite of the Rambler Rhyolite formation, Newfoundland Appalachians. A discordant Cu-rich Lower Footwall zone underlies the semi-massive to massive sulfide lenses. Alteration associated with mineralization can be divided into nine facies that formed in three paragenetic stages: (1) weak quartz–calcite ± spessartine, quartz–sericite, and quartz–sericite–chlorite alteration (stage 1); (2) quartz–chlorite, quartz–chlorite–sulfide, and quartz–chlorite–sericite assemblages (stage 2); and (3) quartz–sericite–sulfide and localized Mn-rich carbonate assemblages (stage 3). A thin syngenetic silica-rich layer immediately overlies part of the VMS deposit and likely formed during the early stages. The volcanic architecture and synvolcanic faults controlled the lateral distribution of extrusive rocks and hydrothermal alteration. Precipitation of the high temperature, discordant to semi-conformable Cu-rich chloritic assemblages (stockwork), was laterally restricted to one of these synvolcanic faults and the transition from coherent- to volcaniclastic-dominated lithofacies. Lower temperature, sericitic assemblages (stages 1 and 3) are controlled by the distribution of volcaniclastic rocks and generally form the immediate footwall to the semi-massive to massive sulfide lenses. Lithogeochemical mass balance calculations illustrate the alteration minerals and mineralization: chlorite-rich assemblages—gains in SiO2, Fe2O3t, MgO, Cr, Ni, and Cu and losses in Na2O, MnO, and CaO and sericite-rich assemblages—gains in K2O, Zn, and Ag and losses in MnO, MgO, CaO, Na2O, and Y. Calcium- and magnesium-rich alteration assemblages are restricted to the northwest fringe of the deposit, distal to the main chloritic and sericitic alteration, and have elemental gains in P2O5, Y, and losses in K2O. The late stage 3 quartz–sericite–sulfide assemblage overprints most assemblages, hosts sphalerite–galena–sulfosalt–Ag–Au-rich veins, and is spatially associated with coherent volcanic rocks. The less permeable nature of these rocks is interpreted to have acted as a physical barrier for ascending metal-rich hydrothermal fluids. Results from the detailed reconstruction of the hydrothermal architecture and paragenetic evolution of the Ming deposit suggest that precious metals were introduced during the waning stage of the hydrothermal system, associated with decreases in temperature and pH of the hydrothermal fluids.

Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea

Redox conditions in the marine realm prior to the Great Oxidation Event (GOE; ∼2.46–2.32 Ga ago), during which the atmospheric oxygen level rose dramatically for the first time, are still debated. Here, we present C, O, Fe, and Mo stable isotope systematics of Fe-, Mn-, and carbonate-rich shales, deposited at different water depths in association with iron formations (IFs) of the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. δ13C values between −22.3 and −13.5‰ VPDB, and δ18O values between −21.1 and −8.6‰ VPDB for Fe–Mn-rich carbonate minerals indicate their precipitation out of equilibrium with seawater. Instead, early diagenetic reduction of Fe–Mn-oxyhydroxide precursor minerals, along with microbially induced oxidation of organic matter (OM), formed these carbonates. δ56FeIRMM-014 values between −1.27 and 0.14‰ and δ98MoNIST3134+0.25 values between −0.46 and 0.56‰ co-vary with Mn concentrations and inferred water depth of deposition. This suggests that, despite the diagenetic origin of the Fe–Mn carbonates, the primary light Fe and Mo isotopic signature of Fe–Mn-oxyhydroxides that originally precipitated from seawater is still preserved. While isotopically light Mo implies that Mn(II) was oxidized to Mn(IV) due to the availability of free, photosynthetically produced O2, Mn enrichment suggests that the water column was redox stratified with a Mn-redoxcline situated at a depth below the storm wave base. A trend to highly negative δ56Fe values with increasing Mn/Fe ratios and decreasing depositional depth suggests progressive oxidation of Fe(II) as deep-waters upwelled across a redoxcline towards shallow, locally oxygenated waters where Mn(IV) oxyhydroxides precipitated. Combined δ56Fe and δ98Mo data indicate pervasive oxygenation of seawater with the O2 content in the photic zone likely reaching levels higher than the maximum value of 10 μM proposed for Archean oxygen oases. Since abiotic Mn(II) oxidation is kinetically very slow in marine environments, it is likely that Mn-oxidizing microorganisms catalyzed Mn-oxidation in the oxygenated Pongola surface waters during deposition of IFs. This implies that aerobic metabolism had evolved before the GOE in shallow, aquatic habitats, where it exerted a first-order control on the deposition of shallow-marine, Mn-rich iron formations.

THE KONDAROS-KATSIMOUTI INTERMEDIATESULFIDATION EPITHERMAL PB-ZN-AG-MN MINERALIZATION, WESTERN MILOS, GREECE: NEW MINERALOGICAL AND GEOCHEMICAL DATA

The metallic mineralization in Kontaros-Katsimouti area is an epithermal Pb-Zn Ag mineralization located along the NW-trending Kondaros-Katsimouti-Vani fault, NW Milos Island, Greece. It is hosted within propylitically and argillically altered dacitic flow dome and volcaniclastic sandstone and shows features typical of intermediate sulfidation deposits like colloform banding, cockade breccias and gangue adularia, Mnrich carbonates and amethystine quartz. The Kondaros-Katsimouti system evolves at higher elevation into the Vani Ag-Pb mineralization, which occurs proximal to the Vani manganese deposit. The metallic mineralogical assemblage at Kondaros-Katsimouti includes mainly galena and sphalerite and minor pyrite. Silver is present in the form of Ag-(Cd) rich tetrahedrite (up to 23.1 wt. % Ag) and polybasite included in galena. Bulk ore analyses indicate enrichment in W (up to 424 mg/kg) and Mo (up to 24 mg/kg), similarly to the other neighboring mineralizations in western Milos (e.g. Vani, TriadesGalana). This enrichment suggests a magmatic-hydrothermal contribution to the ore fluids, probably from a buried granitoid at depth. Boiling, in addition to mixing processes between magmatic- and seawater, resulted in pH increase, oxidation and temperature decrease, and resulted into ore deposition.

Fluid evolution in a volcanic-hosted epithermal carbonate–base metal–gold vein system: Alto de la Blenda, Farallón Negro, Argentina

Alto de la Blenda is a ∼6.6-Ma intermediate-sulphidation epithermal vein system in the Farallon Negro Volcanic Complex, which also hosts the 7.1-Ma porphyry–Cu–Au deposit of Bajo de la Alumbrera. The epithermal vein system is characterised by a large extent and continuity (2 km × 400 m open to depth × 6 m maximum width) and an average gold grade of ∼8 g/t. The vein is best developed within an intrusion of a fine-grained equigranular monzonite, interpreted as the central conduit of a stratovolcano whose extrusive activity ended prior to porphyry–Cu–Au emplacement at Bajo de la Alumbrera, which is in turn cut by minor epithermal veins. The Alto de la Blenda vein consists predominantly of variably Mn-rich carbonates and quartz, with a few percent of pyrite, sphalerite, galena and other sulphide and sulphosalt minerals. Four phases of vein opening, hydrothermal mineralisation and repeated brecciation can be correlated between different vein segments. Stages 2 and 3 contain the greatest fraction of sulphide and gold. They are separated by the emplacement of a polymictic breccia containing clasts of quartz feldspar porphyry as well as basement rocks. Fluid inclusions in quartz related to stages 2 to 4 are liquid rich with 2–4 wt% NaCl(eq). They homogenise between 160 and 300 °C, with very consistent values within each assemblage. Vapour inclusions are practically absent in the epithermal vein. Quartz fragments in the polymictic breccia contain inclusions of intermediate to vapour-like density and similar low salinity (∼3 wt% NaCl(eq)), besides rare brine inclusions containing halite. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) analyses of epithermal inclusions indicate high concentrations of K, Fe, As, Sb, Cs, and Pb that significantly vary within and through subsequent vein stages. Careful consideration of detection limits for individual inclusions shows high gold concentrations of ∼0.5 to 3 ppm dissolved in the ore fluid, which contains variably high sulphur concentrations in excess over Fe and other chalcophile metals. Compositional variations are interpreted to reflect cooling and contraction of lower-density magmatic fluids at depth, like those preserved in porphyry clasts that were mechanically transported up by the polymictic breccia. Ore mineral precipitation from the magmatic fluid occurred by further cooling and possibly minor mixing with surface-derived water, leading to sulphide saturation, de-sulphidation of the magmatic fluid and consequent gold precipitation. The absence of flash boiling and/or reduction by carbonaceous host rocks has led to relatively modest but constant gold grades in the carbonate–base metal–gold veins of Alto de la Blenda.

Synthesis and electrochemical characterization of Mg-doped Li-rich Mn-based cathode material

Mg-doped Mn-rich carbonate precursors (Ni0.15Co0.15Mn0.7CO3) and Li-rich Mn-based cathode materials (Li1.2Ni0.12Co0.12Mn0.56O2) have been synthesized by a carbonate precipitation method. The influence of Mg substitution for Ni, Co and Mn on the structural and electrochemical performance of Li1.2Ni0.12Co0.12Mn0.56O2 has been investigated. XRD results of Mg-doped Li1.2Ni0.12Co0.12Mn0.56O2 powders show that they exhibit similar XRD patterns as those of Li-rich Mn-based cathode materials. Electrochemical charge/discharge measurements in lithium-ion cells indicate the Mg-doped samples to exhibit lower discharge capacity but superior rate capability. The Mg substituted for Mn with 0.03 is found to be the most effective in improving the electrochemical performance. Li1.2Ni0.12Co0.12Mn0.536Mg0.024O2 (Li1.5Ni0.15Co0.15Mn0.67Mg0.03O2.5) displays a high reversible capacity of 205.4mAhg−1 at 0.1 C. EIS results show that Mg doping decreases the charge-transfer resistance and enhances the reaction kinetics, which is considered to be the major factor for higher rate performance.

EMP and SIMS studies on Mn/Ca and Fe/Ca systematics in benthic foraminifera from the Peruvian OMZ: a contribution to the identification of potential redox proxies and the impact of cleaning protocols

Abstract. In this study we present an initial dataset of Mn/Ca and Fe/Ca ratios in tests of benthic foraminifera from the Peruvian oxygen minimum zone (OMZ) determined with SIMS. These results are a contribution to a better understanding of the proxy potential of these elemental ratios for ambient redox conditions. Foraminiferal tests are often contaminated by diagenetic coatings, like Mn rich carbonate- or Fe and Mn rich (oxyhydr)oxide coatings. Thus, it is substantial to assure that the cleaning protocols are efficient or that spots chosen for microanalyses are free of contaminants. Prior to the determination of the element/Ca ratios, the distributions of several elements (Ca, Mn, Fe, Mg, Ba, Al, Si, P and S) in tests of the shallow infaunal species Uvigerina peregrina and Bolivina spissa were mapped with an electron microprobe (EMP). To visualize the effects of cleaning protocols uncleaned and cleaned specimens were compared. The cleaning protocol included an oxidative cleaning step. An Fe rich phase was found on the inner test surface of uncleaned U. peregrina specimens. This phase was also enriched in Al, Si, P and S. A similar Fe rich phase was found at the inner test surface of B. spissa. Specimens of both species treated with oxidative cleaning show the absence of this phase. Neither in B. spissa nor in U. peregrina were any hints found for diagenetic (oxyhydr)oxide or carbonate coatings. Mn/Ca and Fe/Ca ratios of single specimens of B. spissa from different locations have been determined by secondary ion mass spectrometry (SIMS). Bulk analyses using solution ICP-MS of several samples were compared to the SIMS data. The difference between SIMS analyses and ICP-MS bulk analyses from the same sampling sites was 14.0–134.8 μmol mol−1 for the Fe/Ca and 1.68(±0.41) μmol mol−1 for the Mn/Ca ratios. This is in the same order of magnitude as the variability inside single specimens determined with SIMS at these sampling sites (1σ[Mn/Ca] = 0.35–2.07 μmol mol−1; 1σ[Fe/Ca] = 93.9–188.4 μmol mol−1). The Mn/Ca ratios in the calcite were generally relatively low (2.21–9.93 μmol mol−1) but in the same magnitude and proportional to the surrounding pore waters (1.37–6.67 μmol mol−1). However, the Fe/Ca ratios in B. spissa show a negative correlation to the concentrations in the surrounding pore waters. Lowest foraminiferal Fe/Ca ratios (87.0–101.0 μmol mol−1) were found at 465 m water depth, a location with a strong sharp Fe peak in the pore water next to the sediment surface and respectively, high Fe concentrations in the surrounding pore waters. Previous studies found no living specimens of B. spissa at this location. All these facts hint that the analysed specimens already were dead before the Fe flux started and the sampling site just recently turned anoxic due to fluctuations of the lower boundary of the OMZ near the sampling site (465 m water depth). Summarized Mn/Ca and Fe/Ca ratios are potential proxies for redox conditions, if cleaning protocols are carefully applied. The data presented here may be rated as base for the still pending detailed calibration.

Spinels and other oxides in Mn-rich rocks from the Hutter Mine, Pittsylvania County, Virginia, U.S.A.: Implications for miscibility and solvus relations among jacobsite, galaxite, and magnetite

The Hutter Mine locality, Pittsylvania County, Virginia, is a metamorphosed magnetite deposit, with substantial development of subsidiary manganoan marble, that occurs within Latest Precambrian or Early Paleozoic sillimanite-grade pelitic schists. Manganese oxides and spinels at the Hutter Mine include manganosite (MnO) (coexisting with hausmannite and jacobsite) as well as spinels rich in jacobsite (FeMn2O4), magnetite (Fe3O4), and galaxite (MnAl2O4), and a variety of intermediate solid solutions between these three end-members. Several samples contain spinels that exhibit substantial miscibility along the jacobsite-galaxite and jacobsite-magnetite joins. Magnetite-galaxite solid solution is, by comparison, very limited. Coexisting manganoan spinels within the jacobsite-galaxite-magnetite ternary system include jacobsite-rich varieties with galaxite 75. However, the wide range of spinel compositions at the Hutter Mine largely reflects compositional variability in the host rock. In a skarn reaction zone between Fe-rich, quartz-bearing amphibolites and Si-poor, Mn-rich marbles, the galaxite content of spinel drops from 60% to near zero as silica activity increases over a 5 mm interval. In this same reaction zone, magnetite content of spinel increases from about 10 to 95%, but over a narrower interval (about 2 mm). Total variation in spinel composition in this reaction zone is nearly the same as that seen over the entire suite of Hutter Mine samples. Both regional metamorphic geology and thermobarometry on local pelite samples indicates that T max at the Hutter Mine was 550–600 °C. Manganosite formed by the decarbonation of Mn-rich carbonate in the presence of a CO2-poor ( X CO2≤ 0.01) fluid having log a SiO2 < −3.0. Oxygen fugacity in the manganosite-bearing sample was buffered by coexisting manganosite and hausmannite, placing f O2 within the magnetite stability field at peak T . This result is consistent with the occurrence of magnetite as the principal ore at Hutter. The extensive miscibility observed along the jacobsite-galaxite join requires reexamination of miscibility gaps proposed in previous studies. We suggest that the wide compositional gaps found in previous studies reflect a variety of chemical factors of which silica activity is the most critical. In particular, the large range of silica activities observed in Hutter Mine rocks stabilizes spinels with a wide range in galaxite content. The crests of both the jacobsite-galaxite and jacobsite-magnetite two-phase regions appear to occur at relatively low temperatures, probably below 600 °C.

Manganese carbonate mineralization in the Molango District, Mexico

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.

Occurrence and geochemical nature of phosphatic rocks and Mn-rich carbonate rocks in the Toyoman Series, Kitakami Mountains, Northeastern Japan.

Various phosphatic and carbonate rocks are intercalated within black shale of the Upper Permian Toyoman Series. They are divided into four types on the basis of their occurrence and lithology, i. e., 1) nodule, 2) A type: phosphate bearing calcareous shale, 3) B type: coarse grey coloured carbonaceous sandstone, 4) C type: massive greyish sparitic limestone. Petrological and chemical features of four types are transitional with each other. Phosphate is dominant in the nodules and A type rocks, while carbonate is dominant in B and C type rocks. The analytical results of these rocks show the following characteristics: P2O5 and F contents of analysed rocks have wide ranges from 17.73 to 0.09% and 14, 000 to 85 ppm, respectively. Strong correlations between P2O5-F, FeO* (total iron as FeO)-MgO, CaO-CO2, and MnO-CaO are worthy of note. Phosphate-rich rocks are composed mainly of cryptocrystalline fluorapatite. Carbonate in all types contain several% of MnCO3 as solid solution. Judging from an important role of upwelling, the existence of sedimentary phosphatic rocks and Mn-rich carbonate rocks indicates that the sedimentary environments of the Toyoman Series were not a closed inland sea, but a continental shelf or slope.