Geochemical Behavior of Manganese in Weathering Environments and Its Significance in Exploration

Under and climate conditions the chemical weathering of manganese ores is govermed by the fugacities of O2,CO2 and S2 in the atmosphere and soils.Manganese minerals exhibit solid phase transformations without migration of Fe and Mn.Under tropical and subtropical humid climate condi-tions low-valent Mn is instable and apt to be oxidized into high valency state.High-valent Mn miner-als are stable and easy to form secondary high-grade Mn ores.Secondary concentration is possible for Mn ores in carbonate formations,while those in clastic rocks tend to migrate and may be washed away.Such differences are the main obstacles in prospecting Mn ore deposits.     

High-resolution parallel electron energy-loss spectroscopy of Mn L2,3-edges in inorganic manganese compounds

Parallel electron energy-loss spectroscopy (PEELS) in a scanning transmission electron microscope (STEM) was used to record the Mn L_2,3-edges from a range of natural and synthetic manganese containing materials, covering valences 0, II, III, IV and VII, with an energy resolution of ca. 0.5 eV. The Mn L_2,3 electron-loss near-edge structure (ELNES) of these edges provided a sensitive fingerprint of its valence. The Mn²⁺ L_2,3-edges show little sensitivity to the local site symmetry of the ligands surrounding the manganese. This is illustrated by comparing the Mn L_2,3-edges from 4-, 6- and 8-fold coordinated Mn²⁺. In contrast, the Mn L₃-edges from Mn³⁺ and Mn⁴⁺ containing minerals exhibited ELNES that are interpreted in terms of a crystal-field splitting of the 3d electrons, governed by the symmetry of the surrounding ligands. The Mn L₃-edges for octahedrally coordinated Mn²⁺, Mn³⁺ and Mn⁴⁺ showed variations in their ELNES that were sensitive to the crystal-field strength. The crystal-field strength (10Dq) was measured from these edges and compared very well with published optically determined values. The magnitude of 10Dq measured from the Mn L₃-edges and their O K-edge prepeaks of the manganese oxides were almost identical. This further confirms that the value of 10Dq measured at the Mn L₃-edge is correct. Selected spectra are compared with theoretical 2p atomic multiplet spectra and the differences and similarities are explained in terms of the covalency and site symmetry of the manganese. The Mn L₃-edges allow the valence of the manganese to be ascertained, even in multivalent state materials, and can also be used to determine 10Dq.     

Geology and geochemistry of fault-hosted hydrothermal and sedimentary manganese deposits in the Oakover Basin,east Pilbara,Western Australia

Manganese mineralisation in the Oakover Basin is associated with Mesoproterozoic extension, basin formation and deposition of the Manganese Group. The underlying basement architecture of the Oakover Basin (a local half-graben geometry), inherited from the Neoarchean rifting event, plays an important role on the distribution, style and timing of manganese deposits. Fault-hosted manganese deposits are dominant along the ‘active’ faulted eastern margin, whereas flat-lying sedimentary deposits are dominant along the western ‘passive’ margin reflecting differences in ore-forming processes. The large number of significant manganese deposits in the Oakover Basin, previously thought to reflect a spatial association with Carawine Dolomite, more likely reflects the restricted nature of the Mesoproterozoic basin and development of a large reservoir of Mn²⁺ and Fe²⁺ in an anoxic zone of a stratified basin. Low O₂ conditions in the basin were caused by a paleotopographic high forming a barrier to open ocean circulation. The western margin sedimentary deposits formed later than the fault-hosted hydrothermal deposits along the eastern margin, once a significant reservoir of Mn²⁺ and Fe²⁺ had developed, and when there was sufficient subsidence to allow migration of the redox front onto the shallow shelf, with Mn precipitation on and within the seafloor sediments. The sedimentary manganese deposits are not uniformly distributed along the western edge of the basin; instead they are concentrated into discrete areas (e.g. Mt Cooke–Utah–Mt Rove, Bee Hill, Skull Springs and the Ripon Hills districts), suggesting a degree of structural control on their distribution. Fault-hosted manganese is observed beneath and adjacent to many of the sedimentary deposits. Marked geochemical differences are observed between the Woodie Woodie hydrothermal deposits and the sedimentary deposits. Woodie Woodie deposits display higher Ba, U, Mo, As, Sn, Bi, Pb, S and Cu than the sedimentary deposits, reflecting the composition of the hydrothermal fluids. The Al₂O₃ values of the Ripon Hills and Mt Cooke deposits are much higher than the Woodie Woodie deposits, reflecting the composition of the dominant host rock, as Al₂O₃ is typically <5 wt% in the Carawine Dolomite, but is >10 wt% in basal shale units of the Manganese Group. Highly variable Mn:Fe ratios (?5:1) in the hydrothermal manganese at Woodie Woodie reflects rapid deposition of Mn in and around fault zones. In contrast, slower accumulation of Mn oxides on and within the seafloor to form the large sedimentary deposits results in Mn:Fe ratios closer to 1:1 and elevated Co + Ni and REE values.     

Mineralogy and metamorphism conditions of manganese ore at the South Faizulino deposit, the southern Urals, Russia

The mineralogy of slightly metamorphosed manganese ore at the South Faizulino hydrothermalsedimentary deposit in the southern Urals has been studied; 32 minerals were identified. Quartz, hausmannite, rhodochrosite, tephroite, ribbeite, pyroxmangite, and caryopilite are major minerals; calcite, kutnahorite, alleghanyite, spessartine, rhodonite, clinochlore, and parsettensite are second in abundance. This mineralic composition was formed in the process of gradual burial of ore beneath the sequence of Middle Devonian-Lower Carboniferous rocks. The highest parameters of metamorphism are T ≈ 250°C and P ≈ 2.5 kbar. The relationships between minerals and their assemblages made it possible to reconstruct the succession of ore transformation with gradually increasing temperature and pressure. Manganese accumulated in the initial sediments as oxides and a gel-like Mn-Si phase. Rhodochrosite and neotocite were formed at the diagenetic stage. In the course of a further increase in temperature and pressure, neotocite was replaced with caryopilite; ribbeite, tephroite, pyroxmangite, and other silicates crystallized afterwards. In addition to the PT parameters, the formation of various metamorphic mineral assemblages was controlled by the Mn/(Mn + Si) ratio in ore and X _CO2 in pore solution. The latter parameter was determined by the occurrence of organic matter in the ore-bearing rocks. Ore veinlets as products of local hydrothermal redistribution of Mn, Si, and CO₂ were formed during tectonic deformations in the Middle Carboniferous and Permian.     

Geochemical Characteristics and Genetic Significance of Datangpo‐Type Manganese Ore Deposits during the Cryogenian Period

The Datangpo‐type manganese ore deposits, which formed during the Nanhuan (Cryogenian) period and are located in northeastern Guizhou and adjacent areas, are one of the most important manganese resources in China, showing good prospecting potential. Many middle‐to‐large deposits, and even super‐large mineral deposits, have been discovered. However, the genesis of manganese ore deposits is still controversial and remains a long‐standing source of debate; there are several viewpoints including biogenesis, hydrothermal sedimentation, gravity flows, cold‐spring carbonates, etc. Geochemical data from several manganese ore deposits show that there are positive correlations between Al₂O₃ and TiO₂, SiO₂, K₂O, and Na₂O, and strong negative correlations between Al₂O₃ and CaO, MgO, and MnO in black shales and manganese ores. U, Mo, and V show distinct enrichment in black shales and inconspicuous enrichment in Mn ores. Ba and Rb show strong positive correlations with K₂O in manganese ores. Cu, Ni, and Zn show clear correlations with total iron in both manganese ores and black shales. ∑REE of manganese ores has a large range with evident positive Ce anomalies and positive Eu anomalies. The Post Archean Australian Shale (PAAS) normalized rare earth element (REE) distribution patterns of manganese ores present pronounced middle rare earth element (MREE) enrichment, producing “hat‐shaped” REE plots. ∑REE of black shales is more variable compared with PAAS, and the PAAS‐normalized REE distribution patterns appear as “flat‐shaped” REE plots, lacking evident anomaly characteristics. δ¹³C values of carbonate in both manganese ores and the black shales show observable negative excursions. The comprehensive analysis suggests that the black shales formed in a reducing and quiet water column, while the manganese ores formed in oxic muddy seawater, which resulted from periodic transgressions. There was an oxidation–reduction cycle of manganese between the top water body and the bottom water body caused by the transgressions during the early Datangpo, which resulted in the dissolution of manganese. Through the exchange of the euphotic zone water and the bottom water, and episodic inflow of oxygenated water, the manganese in the bottom water was oxidized to Mn‐oxyhydroxides and rapidly buried along with algae. In the early diagenetic stage, Mn‐oxyhydroxides were reduced and dissolved in the anoxic pore water and then transformed into Mn‐carbonates by reacting with HCO₃^? from the degradation of organic matter or from seawater. In the intervals between transgressions, continuous supplies of terrigenous clastics and the high productive rates of organic matter in the euphotic zone resulted in the deposition of the black shales enriched in organic matter.     

应用电子探针技术研究桂西南下雷锰矿床锰钾矿的结构特征

八面体分子筛(OMS-2)具有2×2孔道结构,在离子交换、催化剂、能源和环境等方面具有非常重要的应用价值,然而天然OMS-2矿物材料——锰钾矿在典型结构的成分精细表征和成因研究等方面仍然缺乏.环带和核-边结构在锰氧化物矿物的结构中非常具有代表性,明确其矿物种属、探索其成分特征对于探究其成因、开拓锰氧化物的应用具有重要意...     

Origin and age of rift related fluorite and manganese deposits from the San Rafael Massif,Argentina

The San Rafael Massif is characterized by widespread fluorite and manganese epithermal ore deposits whose origin has been under debate to the present. Isotopic (Sm/Nd and K/Ar) and geochemical (trace elements and REE) data of fluorite and manganese ore allowed to establish the age and genesis of the deposits and to propose a regional genetic model. The fluorite deposits were formed during the Upper Triassic–Lower Jurassic as a result of the Triassic rifting that launched a hydrothermal activity at regional scale. The hydrothermal fluids had low T and high fO₂ with fluorine probably derived from a mantle source and REE scavenged from the volcanics of the Gondwanan Choiyoi Magmatic Cycle upper section. The manganese deposits were formed by oxidizing hydrothermal fluids that collected Mn from deep sources and also leached REE from the upper section of the Choiyoi Magmatic Cycle during two mineralization episodes. One episode was linked to the rift tectonic setting that remained active up to the Upper Cretaceous and the other was related to an Early Miocene back-arc extensional geodynamic setting. Both manganese and fluorite deposits were formed in extensional tectonic settings within an epithermal environment near the surface, and can be ascribed to the general model of detachment-related deposits.     

Manganese oxide mineralogy,petrography and genesis,Pilbara Manganese Group,Western Australia

Supergene manganese oxides, occurring in shales, breccias and dolomites of Proterozoic Age, in the Western Australian Pilbara Manganese Group, have Mn/Fe ranging from 1.9 to 254 and Mn⁴⁺ to Mn (Total) of 0.49–0.94. The manganese mineralogy is dominated by tetravalent manganese oxides, especially by cryptomelane, with lesser amounts of pyrolusite, nsutite, manjiroite, romanechite and other manganese oxide minerals. The manganese minerals are commonly associated with iron oxides, chiefly goethite, indicating incomplete separation of Mn from Fe during Tertiary Age arid climate weathering of older, manganiferous formations. These manganese oxides also contain variable amounts of braunite and very minor hausmannite and bixbyite. The braunite occurs in three generations: sedimentary-diagenetic, recrystallised sedimentary-diagenetic, and supergene. The mode of origin of the hausmannite and bixbyite is uncertain but it is possible that they resulted from diagenesis and/or low-grade regional metamorphism. The supergene manganese deposits appear to have been derived from manganiferous Lower Proterozoic banded iron formations and dolomites of the Hamersley Basin and overlying Middle Proterozoic Bangemali Basin braunite-containing sediments.     

温度影响溴和铷分配的热力学分析及在钾盐矿床中的应用

在海相蒸发岩矿床的地质-地球化学研究中,常应用盐类矿物中的微量元素来研究钾盐矿床的成因和成矿指示标志,Br、Rb就是其中很重要的2种微量元素。缺硫酸镁型钾盐矿床在成盐过程中会蒸发沉积形成氯化物型的盐类物质, Br、Rb按一定的规律分配到这些盐类物质中而不形成独立矿物。在盐类物质蒸发结晶过程中,Br、Rb在固_液相之间的分配主要受温度控制。文章通过Br和Rb的地球化学特征及微量元素分配的热力学分析,建立了Br、Rb在盐类矿物相中的分配系数与温度间的热力学函数模型:,并探讨了利用这些公式来计算出钾盐矿床结晶作用形成盐类物质时的古温度,这对研究钾盐矿床形成时的古环境、物质基础条件、物理化学条件等具有重要意义。     

Partition of cadmium and manganese between coexisting sphalerite and galena from some Japanese epithermal deposits

Sphalerites from Japanese epithermal Pb-Zn vein-type deposits, namely, Yatani, Oizumi and Hosokura, contain 2900–3400 ppm cadmium and 760–2100 ppm manganese. And galenas from the same deposits contain cadmium and manganese 19.2–26.9 ppm and 7.8–218.3 ppm, respectively. The temperatures, evaluated from the partition of cadmium between coexisting sphalerite and galena, are consistent within a total range of 150°C at maximum within an individual deposit. No systematic change with depth can be observed at the Shoko-hi vein, Hosokura mine and Hompi vein, Yatani mine. In several samples, the temperatures obtained from the partition of cadmium have been compared with those obtained from sulfur isotope fractionations. Iron content in sphalerite coexisting with pyrite indicates that the deposition of ore minerals at these deposits may have taken place from ore-forming solutions in which H₂S or HS^– was the predominant dissolved sulfur species, and the solutions may have been free from in situ oxidation or reduction.     

广西东平地区碳酸锰矿床Ga含量高异常的发现及成因初探

Ga是一种典型的稀有分散元素,主要产于铝土矿、闪锌矿及煤矿之中。最近,在广西东平地区下三叠统北泗组碳酸锰矿床中发现Ga高异常含量,w(Ga)介于5.16×10~(-6)~82.80×10~(-6)之间,平均为33.76×10~(-6),达到了Ga工业品位标准;锰矿层和围岩中w(Ga)平均分别为46.40×10~(-6)、19.31×10~(-6),高于国内外已报道的大部分锰矿床。文中根据北泗组碳酸锰矿床地球化学特征,揭示了该锰矿床为热水沉积;同时,结合现代大洋铁锰沉积有关Ga的最新报道,提出北泗组碳酸锰矿床中Ga的赋存与含锰矿物密切相关,其来源与海底热液活动有关。最后,文中还利用Mn/Fe-Ga、Co-Ga关系图判别了古代铁锰沉积的成因类型。     

Carbonate saturation and the effect of pressure on the alkalinity of interstitial waters from the Guatemala Basin

Interstitial water samples were collected from the Guatemala Basin using an in situ sampler and by centrifuging box core sediment samples. Results from these two sampling methods for Mn, Si, PO₄ agree well. There is a systematic difference in the alkalinity values, however, which suggests that CaCO₃ (s) precipitates from the box core samples when they are brought from in situ pressure at 1 atm. Thus the alkalinity on box core samples is less than that on samples collected in situ. The magnitude of the alkalinity decrease can be calculated using basic thermodynamic principles and the observed and predicted differences agree well.Both sampling methods show a sharp drop in pH just below the sediment water interface which can be explained by the oxidation of organic matter by O₂ in the absence of CaCO₃. Alkalinity increases during the reduction of MnO₂(s) and release of Mn²⁺ to the interstitial water. The result is that interstitial waters become undersaturated with CaCO₃ immediately below the sediment/water interface and then return to or nearly to saturation at depth.     

Mineralogy, geochemistry, and origin of hydrothermal manganese veins at Wadi Maliek, Southern Eastern Desert, Egypt

The present work deals with the geology, mineralogy, geochemistry, and origin of the metagabbroic-hosted manganese deposits at Wadi Maliek in the southern Eastern Desert of Egypt. The manganese veins are found in the shear zones and channel ways of the fault planes within the metagabbroic rocks pointing to those hydrothermal solutions carrying manganese and iron load penetrating along these fractures. These faults are striking N 80° E?CS 80° W with dipping 65°. These veins vary in thickness from 15?cm up to 125?cm wide; each vein may show difference in thickness from bottom to top. Microscopic examinations, X-ray diffraction, infrared spectral, differential thermal (DTA), thermogravimetric (TGA), and ESEM-EDAX analyses revealed that the manganese minerals consist mainly of pyrolusite, psilomelane, and ramsdellite. Goethite and hematite are the common iron minerals. Petrographically, the manganese deposits can be classified into three ore types based on the predominance of manganese and iron minerals: manganese, manganese?Ciron, and iron ore types. The geochemistry of Maliek deposits indicated that the total averages of some major oxides in manganese, manganese?Ciron, and iron ore types are respectively as follows: SiO₂ (15.64%, 11.52%, and 20.58%), MnO (39.9%, 17.81%, and 0.77%), FeO* (7.13%, 33.31%, and 37.08%), CaO (5.89%, 5.82%, and 5.32%), and Na₂O (1.04%, 1.61%, and 1.53%). With regard to trace elements, the Maliek manganese deposits are rich in Zn, Ba, Pb, Sr, and V. Based on the geological, mineralogical, and geochemical results, the studied manganese deposits are considered to be precipitated from hydrothermal solution.     

Genesis for Rare Earth Elements Enrichment in the Permian Manganese Deposits in Zunyi,Guizhou Province,SW China

The Zunyi manganese deposits, which formed during the Middle to Late Permian period and are located in northern Guizhou and adjacent areas, are the core area of a series of large-medium scale manganese enrichment minerogenesis in the southern margin and interior of the Yangtze platform, Southern China. This study reports the universal enrichment of rare earth elements(REEs) in Zunyi manganese deposits and examines the enrichment characteristics, metallogenic environment and genesis of REEs. The manganese ore bodies present stratiform or stratoid in shape, hosted in the silicon–mud–limestones of the Late Permian Maokou Formation. The manganese ores generally present lamellar, massive, banded and brecciated structures, and mainly consist of rhodochrosite, ropperite, tetalite, capillitite, as well as contains paragenetic gangue minerals including pyrite, chalcopyrite, rutile, barite, tuffaceous clay rock, etc. The manganese ores have higher ΣREE contents range from 158 to 1138.9 ppm(average 509.54 ppm). In addition, the ΣREE contents of tuffaceous clay rock in ore beds vary from 1032.2 to 1824.5 ppm(average 1396.42 ppm). The REEs from manganese deposits are characterized by La, Ce, Nd and Y enriched, and existing in the form of independent minerals(e.g., monazite and xenotime), indicating Zunyi manganese deposits enriched in light rare earth elements(LREE). The Ce_(anom) ratios(average-0.13) and lithofacies and paleogeography characteristics indicate that Zunyi manganese deposits were formed in a weak oxidation-reduction environment. The(La/Yb)_(ch), Y/Ho,(La/Nd)_N,(Dy/Yb)_N, Ce/Ce* and Eu/Eu* values of samples from the Zunyi manganese deposits are 5.53–56.92, 18–39, 1.42–3.15, 0.55–2.20, 0.21–1.76 and 0.48–0.86, respectively, indicating a hydrothermal origin for the manganese mineralization and REEs enrichment. The δ~(13) C_(V-PDB)(-0.54 to-18.1‰) and δ~(18) O_(SMOW)(21.6 to 26.0‰) characteristics of manganese ores reveal a mixed source of magmatic and organic matter. Moreover, the manganese ore, tuffaceous clay rock and Emeishan basalt have extremely similar REE fractionation characteristic, suggesting REEs enrichment and manganese mineralization have been mainly origin from hydrothermal fluids.     

Étude expérimentale de la réactivité du CO2 supercritique vis-à-vis de phases minérales pures. Implications pour la séquestration géologique de CO2

Carbon dioxide sequestration in deep aquifers and depleted oilfields is a potential technical solution for reducing green-house gas release to the atmosphere: the gas containment relies on several trapping mechanisms (supercritical CO₂, CO_2(sc), dissolution together with slow water flows, mineral trapping) and on a low permeability cap-rock to prevent CO_2(sc), which is less dense than the formation water, from leaking upwards. A leakproof cap-rock is thus essential to ensure the sequestration efficiency. It is also crucial for safety assessment to identify and assess potential alteration processes that may damage the cap-rock properties: chemical alteration, fracture reactivation, degradation of injection borehole seals, etc. The reactivity of the host-rock minerals with the supercritical CO₂ fluid is one of the potential mechanisms, but it is altogether unknown. Reactivity tests have been carried out under such conditions, consisting of batch reactions between pure minerals and anhydrous supercritical CO₂, or a two-phase CO₂/H₂O fluid at 200?°C and 105/160 bar. After 45 to 60 days, evidence of appreciable mineral-fluid reactivity was identified, including in the water-free experiments. For the mixed H₂O/CO₂ experiments, portlandite was totally transformed into calcite; anorthite displayed many dissolution patterns associated with calcite, aragonite, tridymite and smectite precipitations. For the anhydrous CO₂ experiments, portlandite was totally carbonated to form calcite and aragonite; anorthite also displayed surface alteration patterns with secondary precipitation of fibrous calcite. To cite this article: O. Regnault et al., C. R. Geoscience 337 (2005).     

Geochemical and mineralogical aspects of sulfide mine tailings

Tailings generated during processing of sulfide ores represent a substantial risk to water resources. The oxidation of sulfide minerals within tailings deposits can generate low-quality water containing elevated concentrations of SO₄, Fe, and associated metal(loid)s. Acid generated during the oxidation of pyrite [FeS₂], pyrrhotite [Fe_(1−x)S] and other sulfide minerals is neutralized to varying degrees by the dissolution of carbonate, (oxy)hydroxide, and silicate minerals. The extent of acid neutralization and, therefore, pore-water pH is a principal control on the mobility of sulfide-oxidation products within tailings deposits. Metals including Fe(III), Cu, Zn, and Ni often occur at high concentrations and exhibit greater mobility at low pH characteristic of acid mine drainage (AMD). In contrast, (hydr)oxyanion-forming elements including As, Sb, Se, and Mo commonly exhibit greater mobility at circumneutral pH associated with neutral mine drainage (NMD). These differences in mobility largely result from the pH-dependence of mineral precipitation–dissolution and sorption–desorption reactions. Cemented layers of secondary (oxy)hydroxide and (hydroxy)sulfate minerals, referred to as hardpans, may promote attenuation of sulfide-mineral oxidation products within and below the oxidation zone. Hardpans may also limit oxygen ingress and pore-water migration within sulfide tailings deposits. Reduction–oxidation (redox) processes are another important control on metal(loid) mobility within sulfide tailings deposits. Reductive dissolution or transformation of secondary (oxy)hydroxide phases can enhance Fe, Mn, and As mobility within sulfide tailings. Production of H₂S via microbial sulfate reduction may promote attenuation of sulfide-oxidation products, including Fe, Zn, Ni, and Tl, via metal-sulfide precipitation. Understanding the dynamics of these interrelated geochemical and mineralogical processes is critical for anticipating and managing water quality associated with sulfide mine tailings.     

加蓬某地锰矿的降磷工艺研究

通过加蓬某地锰矿物质成分研究查明其中磷的赋存状态,结果未见磷的独立矿物,却发现磷与铁的关系非常密切.采用溶出铁并保留锰相的方法无法彻底除磷,研究发现其主要原因是锰相为基质,铁受其包裹,在不破坏基质锰相的情况下,不可能将铁除去.采用了物理选矿和化学选矿2种方法进行降磷试验,结果表明,采用焙烧-酸浸的工艺可有效降低该锰矿中...     

Coupled alkali feldspar dissolution and secondary mineral precipitation in batch systems – 2: New experiments with supercritical CO2 and implications for carbon sequestration

In order to evaluate the extent of CO₂–water–rock interactions in geological formations for C sequestration, three batch experiments were conducted on alkali feldspars–CO₂–brine interactions at 150–200 °C and 300 bars. The elevated temperatures were necessary to accelerate the reactions to facilitate attainable laboratory measurements. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. SEM, TEM and XRD analysis of reaction products showed extensive dissolution features (etch pits, channels, kinks and steps) on feldspars and precipitation of secondary minerals (boehmite, kaolinite, muscovite and paragonite) on feldspar surfaces. Therefore, these experiments have generated both solution chemistry and secondary mineral identity. The experimental results show that partial equilibrium was not attained between secondary minerals and aqueous solutions for the feldspar hydrolysis batch systems. Evidence came from both solution chemistry (supersaturation of the secondary minerals during the entire experimental duration) and metastable co-existence of secondary minerals. The slow precipitation of secondary minerals results in a negative feedback in the dissolution–precipitation loop, reducing the overall feldspar dissolution rates by orders of magnitude. Furthermore, the experimental data indicate the form of rate laws greatly influence the steady state rates under which feldspar dissolution took place. Negligence of both the mitigating effects of secondary mineral precipitation and the sigmoidal shape of rate–ΔG_r relationship can overestimate the extent of feldspar dissolution during CO₂ storage. Finally, the literature on feldspar dissolution in CO₂-charged systems has been reviewed. The data available are insufficient and new experiments are urgently needed to establish a database on feldspar dissolution mechanism, rates and rate laws, as well as secondary mineral information at CO₂ storage conditions.     

Manganese(III) binding to a pyoverdine siderophore produced by a manganese(II)-oxidizing bacterium

The possible roles of siderophores (high affinity chelators of iron(III)) in the biogeochemistry of manganese remain unknown. Here we investigate the interaction of Mn(III) with a pyoverdine-type siderophore (PVD_MnB1) produced by the model Mn(II)-oxidizing bacterium Pseudomonas putida strain MnB1. PVD_MnB1 confirmed typical pyoverdine behavior with respect to: (a) its absorption spectrum at 350-600 nm, both in the absence and presence of Fe(III), (b) the quenching of its fluorescence by Fe(III), (c) the formation of a 1:1 complex with Fe(III), and (d) the thermodynamic stability constant of its Fe(III) complex. The Mn(III) complex of PVD_MnB1 had a 1:1 Mn:pvd molar ratio, showed fluorescence quenching, and exhibited a light absorption spectrum (A_max = 408-410 nm) different from that of either PVD_MnB1-Fe(III) or uncomplexed PVD_MnB1. Mn(III) competed strongly with Fe(III) for binding by PVD_MnB1 in culture filtrates (pH 8, 4°C). Equilibration with citrate, a metal-binding ligand, did not detectably release Mn from its PVD_MnB1 complex at a citrate/PVD_MnB1 molar ratio of 830 (pH 8, 4°C), whereas pyrophosphate under the same conditions removed 55% of the Mn from its PVD_MnB1 complex. Most of the PVD_MnB1-complexed Mn was released by reaction with ascorbate, a reducing agent, or with EDTA, a ligand that is also oxidized by Mn(III). Data on the competition for binding to PVD_MnB1 by Fe(III) vs. Mn(III) were used to determine a thermodynamic stability constant (nominally at 4°C) for the neutral species MnHPVD_MnB1 (log K = 47.5 ± 0.5, infinite dilution reference state). This value was larger than that determined for FeHPVD_MnB1 (log K = 44.6 ± 0.5). This result has important implications for the metabolism, solubility, speciation, and redox cycling of manganese, as well as for the biologic uptake of iron.     

Sedimentary manganese metallogenesis in response to the evolution of the Earth system

The concentration of manganese in solution and its precipitation in inorganic systems are primarily redox-controlled, guided by several Earth processes most of which were tectonically induced. The Early Archean atmosphere-hydrosphere system was extremely O₂-deficient. Thus, the very high mantle heat flux producing superplumes, severe outgassing and high-temperature hydrothermal activity introduced substantial Mn²⁺ in anoxic oceans but prevented its precipitation. During the Late Archean, centered at ca. 2.75 Ga, the introduction of Photosystem II and decrease of the oxygen sinks led to a limited buildup of surface O₂-content locally, initiating modest deposition of manganese in shallow basin-margin oxygenated niches (e.g., deposits in India and Brazil). Rapid burial of organic matter, decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the Archean-Proterozoic transition. Concurrently, a massive drawdown of atmospheric CO₂ owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused Paleoproterozoic glaciations (2.45-2.22 Ga). The spectacular sedimentary manganese deposits (at ca. 2.4 Ga) of Transvaal Supergroup, South Africa, were formed by oxidation of hydrothermally derived Mn²⁺ transferred from a stratified ocean to the continental shelf by transgression. Episodes of increased burial rate of organic matter during ca. 2.4 and 2.06 Ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere. Black shale-hosted Mn carbonate deposits in the Birimian sequence (ca. 2.3-2.0 Ga), West Africa, its equivalents in South America and those in the Francevillian sequence (ca. 2.2-2.1 Ga), Gabon are correlatable to this period. Tectonically forced doming-up, attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric CO₂ drawdown causing glaciation on the Neoproterozoic Rodinia supercontinent. Tectonic rifting and mantle outgassing led to deglaciation. Dissolved Mn²⁺ and Fe²⁺ concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation. During the Sturtian glacial-interglacial event (ca. 750-700 Ma), interstratified Mn oxide and BIF deposits of Damara sequence, Namibia, was formed. The Varangian (≡ Marinoan; ca. 600 Ma) cryogenic event produced Mn oxide and BIF deposits at Urucum, Jacadigo Group, Brazil. The Datangpo interglacial sequence, South China (Liantuo-Nantuo ≡ Varangian event) contains black shale-hosted Mn carbonate deposits. The Early Paleozoic witnessed several glacioeustatic sea level changes producing small Mn carbonate deposits of Tiantaishan (Early Cambrian) and Taojiang (Mid-Ordovician) in black shale sequences, China, and the major Mn oxide-carbonate deposits of Karadzhal-type, Central Kazakhstan (Late Devonian). The Mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans. During the Early Jurassic OAE, organic-rich sediments host many Mn carbonate deposits in Europe (e.g., Úrkút, Hungary) in black shale sequences. The Late Jurassic giant Mn Carbonate deposit at Molango, Mexico, was also genetically related to sea level change. Mn carbonates were always derived from Mn oxyhydroxides during early diagenesis. Large Mn oxide deposits of Cretaceous age at Groote Eylandt, Australia and Imini-Tasdremt, Morocco, were also formed during transgression-regression in greenhouse climate. The Early Oligocene giant Mn oxide-carbonate deposit of Chiatura (Georgia) and Nikopol (Ukraine) were developed in a similar situation. Thereafter, manganese sedimentation was entirely shifted to the deep seafloor and since ca. 15 Ma B.P. was climatically controlled (glaciation-deglaciation) assisted by oxygenated polar bottom currents (AABW, NADW). The changes in climate and the sea level were mainly tectonically forced.