Nonequilibrium models for predicting forms of precipitated manganese oxides

Manganese oxides precipitated by bubbling air through 0.01 molar solutions of MnCl₂, Mn(NO₃)₂, MnSO₄, or Mn(ClO₄)₂ at a constantly maintained pH of 8.5 to 9.5 at temperatures of 25°C or higher consisted mainly of hausmannite, Mn₃O₄. At temperatures near 0°C, but with other conditions the same, the product is feitknechtite, βMnOOH, except that if the initial solution is MnSO₄ and the temperature is near 0°C the product is a mixture of manganite, γMnOOH and groutite, αMnOOH.All these oxides are metastable in aerated solution and alter by irreversible processes to more highly oxidized species during aging. A two-step nonequilibrium thermodynamic model predicts that the least stable species, βMnOOH, should be most readily converted to MnO₂. Some preparations of βMnOOH aged in their native solution at 5°C attained a manganese oxidation state of +3.3 or more after 7 months. Hausmannite aged at 25°C altered to γMnOOH. The latter is more stable than a or βMnOOH, and manganese oxidation states above 3.0 were not reached in hausmannite precipitates during 4 months of aging. Initial precipitation of MnCO₃ rather than a form of oxide is likely only where oxygen availability is very low.Composition of solutions and oxidation state and morphology of solids were determined during the aging process by chemical analyses, X-ray and electron diffraction and transmission electron micrographs.     

Oxidation of Mn(II): Initial mineralogy,oxidation state and ageing

The initial solid phase oxidation products formed during the oxidation of aqueous Mn(II) at 25°C were studied as a function of time. The analyses included morphology (TEM), mineralogy (x-ray diffraction), $\text{O}\text{Mn}$ ratio (iodometric method), oxidation state of manganese (XPS), and dissolved manganese. The initial solid formed under our conditions was Mn₃O₄ (hausmannite) which converted completely to γMnOOH (manganite) after eight months. βMnOOH (feitknechtite) appeared to be an intermediate in this transformation. The $\text{O}\text{Mn}$ ratio was initially 1.37 and increased to 1.49 over the same time span. Throughout the course of this study the XPS analyses showed that the surface of the solids (<50 Å) was dominated by Mn(III). The solution pH and dissolved manganese concentrations were consistent with disproportionation and oxidation reactions that favor the transformation of Mn₃O₄ to γMnOOH but not to γMnO₂.     

Ferromanganese oxide deposits from the Central Pacific Ocean,I. Encrustations from the Line Islands Archipelago

Chemical and mineralogical analyses of a well-controlled suite of ferromanganese encrustations from the Line Islands Archipelago (Central Pacific) suggest that they represent purely hydrogenous deposits—i.e. they have formed through the slow accumulation of trace metal-enriched oxides directly from the water-column. Mineralogically they consist predominantly of δMnO₂ and amorphous FeOOHxH₂O. Compositionally, they are similar to δMnO₂ nodules from adjoining basinal areas but are enriched in both Mn (mean = 20.4%, max = 29.3%) and Co (mean = 0.55%, max = 1.57%). δMnO₂ is the most important trace metal bearing phase; strong associations are noted between it and Co, Mo, Ni, Zn, and Cd, whilst only Be is associated specifically with FeOOH. V, Sr and Pb are partitioned between the authigenic oxide phases, whilst Ti most probably occurs as TiO₂xH₂O. Cu is contained in both aluminosilicate contaminant phases and Fe oxide phases. These relations are considered to reflect the differing scavenging behaviour of Mn and Fe oxides in the water column.Crusts from ~1–2 km are enriched in Mn and the Mn-related elements and exhibit higher $\text{Mn}\text{Fe}$ ratios than deeper crusts, which are compositionally constant. The higher $\text{Mn}\text{Fe}$ ratios may result from a supply of Mn from continental borderland sediments at these depths, which is transported horizontally by advective-diffusive processes. Since manganophile elements are enriched relative to Mn in the 1–2 km crusts, it is considered that the supply of Mn is scavenged by existing oxides, is oxidised and effectively occludes them. A higher proportion of oxide particles thus exhibit Mn oxide scavenging properties in the 1–2 km depth zone. The increased vertical flux of Mn resulting from the supply at ~1–2 km is not reflected by higher $\text{Mn}\text{Fe}$ ratios in deeper crusts, so that the vertical flux of oxides is not simply related to the standing crop. The $\text{Mn}\text{Fe}$ ratios of the crusts thus reflect the composition of suspended oxides at similar depths.     

Rates of manganese oxidation in aqueous systems

The rate of crystal growth of Mn₃O₄ (hausmannite) and βMnOOH (feitknechtite) in aerated aqueous manganous perchlorate systems, near 0.01 M in total manganese, was determined at pH levels ranging from 7.00 to 9.00 and at temperatures from 0.5 to 37.4°C. The process is autocatalytic, but becomes psuedo first-order in dissolved Mn²⁺ activity when the amount of precipitate surface is large compared to the amount of unreacted manganese. Reaction rates determined by titrations using an automated pH-stat were fitted to an equation for precipitate growth. The rates are proportional to surface area of oxide and degree of supersaturation with respect to Mn²⁺. The oxide obtained at the higher temperature was Mn₃O₄, but at 0.5° C only βMnOOH was formed. At intermediate temperatures, mixtures of these solids were formed. The rate of precipitation of hausmannite is strongly influenced by temperature, and that of feitknechtite much less so. The difference in activation energy may be related to differences in crystal structure of the oxides and the geometry of polymeric hydroxy ion precursors.     

Reaction of Mn (hydr)oxides with oxalic acid, glyoxylic acid, phosphonoformic acid, and structurally-related organic compounds

Phosphonoformic acid, oxalic acid, glyoxylic acid, and 10 additional organic compounds that are structurally related to them have been reacted with synthetic MnO₂ (birnessite), consisting of 22% Mn^III and 78% Mn^IV, and synthetic MnOOH (manganite), consisting solely of Mn^III. Significant concentrations of dissolved Mn^III were detected in reactions of phosphonoformic acid with MnOOH, indicating that ligand-assisted dissolution took place. Reaction of phosphonoformic acid with MnO₂, and reaction of all other organic reactants with either MnOOH or MnO₂, yielded only Mn^II, indicating that reductive dissolution was predominant. As far as reductive dissolution reactions are concerned, MnO₂ yields a range of reactivity that is nearly 20-times greater than that of MnOOH. Oxidation converts phosphonoformic acid into orthophosphate ion, glyoxylic acid into formic acid, pyruvic acid into acetic acid, and 2,3-butanedione into acetic acid. When differences in surface area loading are accounted for, oxalic acid, pyruvic acid, and 2,3-butanedione yield virtually the same dissolution rates for the two (hydr)oxides. At pH 5.0, glyoxylic acid reacts 14-times faster with MnO₂ than with MnOOH. MnO₂ reacts more slowly than MnOOH by a factor of 1/16th with oxamic acid, 1/20th with lactic acid, and 1/33rd with dimethyl oxalate. Adsorptive, complexant, and reductant properties of the 13 organic reactants are believed responsible for the observed reactivity differences.     

Structural characterization of terrestrial microbial Mn oxides from Pinal Creek, AZ

The microbial catalysis of Mn(II) oxidation is believed to be a dominant source of abundant sorption- and redox-active Mn oxides in marine, freshwater, and subsurface aquatic environments. In spite of their importance, environmental oxides of known biogenic origin have generally not been characterized in detail from a structural perspective. Hyporheic zone Mn oxide grain coatings at Pinal Creek, Arizona, a metals-contaminated stream, have been identified as being dominantly microbial in origin and are well studied from bulk chemistry and contaminant hydrology perspectives. This site thus presents an excellent opportunity to study the structures of terrestrial microbial Mn oxides in detail. XRD and EXAFS measurements performed in this study indicate that the hydrated Pinal Creek Mn oxide grain coatings are layer-type Mn oxides with dominantly hexagonal or pseudo-hexagonal layer symmetry. XRD and TEM measurements suggest the oxides to be nanoparticulate plates with average dimensions on the order of 11 nm thick × 35 nm diameter, but with individual particles exhibiting thickness as small as a single layer and sheets as wide as 500 nm. The hydrated oxides exhibit a 10-Å basal-plane spacing and turbostratic disorder. EXAFS analyses suggest the oxides contain layer Mn(IV) site vacancy defects, and layer Mn(III) is inferred to be present, as deduced from Jahn-Teller distortion of the local structure. The physical geometry and structural details of the coatings suggest formation within microbial biofilms. The biogenic Mn oxides are stable with respect to transformation into thermodynamically more stable phases over a time scale of at least 5 months. The nanoparticulate layered structural motif, also observed in pure culture laboratory studies, appears to be characteristic of biogenic Mn oxides and may explain the common occurrence of this mineral habit in soils and sediments.     

The formation of todorokite and birnessite in sea water pumped from under ground

Manganese oxides precipitated from aerated well sea water at the Marine Science Museum, Tokai University, have been analyzed chemically and mineralogically. The $\text{O}\text{Mn}$ ratios are lower in todorokite than in birnessite but these minerals have similar contents of minor transition metals, which can be taken up additionally from sea water after the precipitation of Mn oxides. On the basis of these results, the genesis of Mn minerals is discussed in relation to marine Mn nodules.     

Influence of Mn oxides on the reduction of uranium(VI) by the metal-reducing bacterium Shewanella putrefaciens

The potential for Mn oxides to modify the biogeochemical behavior of U during reduction by the subsurface bacterium Shewanella putrefaciens strain CN32 was investigated using synthetic Mn(III/IV) oxides (pyrolusite [β-MnO₂], bixbyite [Mn₂O₃] and K⁺-birnessite [K₄Mn₁₄O₂₇ · 8H₂O]). In the absence of bacteria, pyrolusite and bixbyite oxidized biogenic uraninite (UO₂[s]) to soluble U(VI) species, with bixbyite being the most rapid oxidant. The Mn(III/IV) oxides lowered the bioreduction rate of U(VI) relative to rates in their absence or in the presence of gibbsite (Al[OH]₃) added as a non-redox-reactive surface. Evolved Mn(II) increased with increasing initial U(VI) concentration in the biotic experiments, indicating that valence cycling of U facilitated the reduction of Mn(III/IV). Despite an excess of the Mn oxide, 43 to 100% of the initial U was bioreduced after extended incubation. Analysis of thin sections of bacterial Mn oxide suspensions revealed that the reduced U resided in the periplasmic space of the bacterial cells. However, in the absence of Mn(III/IV) oxides, UO₂(s) accumulated as copious fine-grained particles external to the cell. These results indicate that the presence of Mn(III/IV) oxides may impede the biological reduction of U(VI) in subsoils and sediments. However, the accumulation of U(IV) in the cell periplasm may physically protect reduced U from oxidation, promoting at least a temporal state of redox disequilibria.     

The influence of hydrous Mn–Zn oxides on diel cycling of Zn in an alkaline stream draining abandoned mine lands

Many mining-impacted streams in western Montana with pH near or above neutrality display large (up to 500%) diel cycles in dissolved Zn concentrations. The streams in question typically contain boulders coated with a thin biofilm, as well as black mineral crusts composed of hydrous Mn–Zn oxides. Laboratory mesocosm experiments simulating diel behavior in High Ore Creek (one of the Montana streams with particularly high Zn concentrations) show that the Zn cycles are not caused by 24-h changes in streamflow or hyporheic exchange, but rather to reversible in-stream processes that are driven by the solar cycle and its attendant influence on pH and water temperature (T). Laboratory experiments using natural Mn–Zn precipitates from the creek show that the mobilities of Zn and Mn increase nearly an order of magnitude for each unit decrease in pH, and decrease 2.4-fold for an increase in T from 5 to 20 °C. The response of dissolved metal concentration to small changes in either pH or T was rapid and reversible, and dissolved Zn concentrations were roughly an order of magnitude higher than Mn. These observations are best explained by sorption of Zn²⁺ and Mn²⁺ onto the secondary Mn–Zn oxide surfaces. From the T-dependence of residual metal concentrations in solution, approximate adsorption enthalpies of +50 kJ/mol (Zn) and +46 kJ/mol (Mn) were obtained, which are within the range of enthalpy values reported in the literature for sorption of divalent metal cations onto hydrous metal oxides. Using the derived pH- and T-dependencies from the experiments, good agreement is shown between predicted and observed diel Zn cycles for several historical data sets collected from High Ore Creek.     

Composition of manganese nodules and manganese carbonates from Loch Fyne,Scotland

Manganese nodules and manganese carbonate concretions occur in the upper 10–15 cm of the Recent sediments of Loch Fyne, Argyllshire in water depths of 180–200 m. The nodules are spherical, a few mm to 3 cm in diameter, and consist of a black, Mn-rich core and a thin, red, Fe-rich rim. The carbonate occurs as irregular concretions, 0.5–8 cm in size, and as a cement in irregular nodule and shell fragment aggregates. It partially replaces some nodule material and clastic silicate inclusions, but does not affect aragonitic and calcitic shell fragments.The nodules are approximately 75% pure oxides and contain 30% Mn and 4% Fe. In the cores, the principal mineral phase is todorokite, with a Mn/Fe ratio of 17. The rim consists of X-ray amorphous Fe and Mn oxides with a Mn/Fe ratio of 0.66. The cores are enriched, relative to Al, in K, Ba, Co, Mo, Ni and Sr while the rims contain more P, Ti, As, Pb, Y and Zn.The manganese carbonate has the composition (Mn_47.7 Ca_45.1 Mg_7.2) CO₃. Apart from Cu, all minor elements are excluded from significant substitution in the carbonate lattice.Manganese nodules and carbonates form diagenetically within the Recent sediments of Loch Fyne. This accounts for the high Mn/Fe ratios in the oxide phases and the abundance of manganese carbonate concretions. Mn concentrations in the interstitial waters of sediment cores are high (ca. 10 ppm) as also, by inference, are the dissolved carbonate concentrations.     

Oxidation of manganese by spores of a marine bacillus: Kinetic and thermodynamic considerations

The catalytic properties of spores of a marine Bacillus known to oxidize divalent manganese were used to perform laboratory Mn(II) oxidation experiments at environmental conditions of pH and Mn(II) concentration. We found that at pH 7.8 the initial kinetics of Mn(II) oxidation facilitated by the spores was four orders of magnitude greater than that which would be expected for abiotic autocatalysis on a colloidal MnO₂ surface. The rate progressively decreased as the spores became coated with manganese oxide, eventually becoming very near that predicted for abiotic surface catalysis. Transmission electron microscopic observations and oxidation state measurements of solids precipitated at pH 7.5 and [Mn(II)] < 50 nM indicated that the initial oxidation product was hausmannite (Mn₃O₄ or MnO_x where x = 1.33) which aged to more highly oxidized MnO₂ (x = 1.9) in the time scale of weeks. By utilizing spores to catalyze the oxidation rate, we were able to maintain our experimental system within the seawater range of pH and Mn(II) where highly oxidized manganese oxide precipitates are thermodynamically stable. In doing so we obtained, for the first time, laboratory precipitates with oxidation states similar to that found in marine particulate material. These results suggest that the concentration of manganese in seawater and the oxidation state of marine manganese oxides are controlled by the rapid precipitation of Mn₃O₄, which can be microbially mediated, followed by the disproportionation to MnO₂.     

Deposition of deep-sea manganese nodules

Manganese at equilibrium in seawater occurs dominantly as Mn²⁺ and inorganic complexes at a concentration ratio of about 1:0.72; solubility decreases exponentially with increasing pH or Eh. However, the nodule oxides birnessite and todorokite are at least four orders of magnitude undersaturated relative to the Mn concentrations of seawater, and are metastable relative to hausmannite and manganite. This apparent lack of equilibrium is explicable by the mechanism of precipitation.Surfaces assist Mn precipitation by catalyzing equilibration between dissolved and reactive O₂ and simultaneously also by adsorbing ionic Mn species. The effective Eh at the surface becomes 200–400 mV above that of seawater; the oxidation rate of Mn increases about 10⁸ ×, and the activation energies for Mn oxidation decrease ~ 11.5 kcal/mole. Consequently, marine Mn nodules and crusts form by adsorption and catalytic oxidation of Mn²⁺ and ferrous ions at nucleating surfaces such as sea-floor silicates, oxyhydroxides, carbonates, phosphates and biogenic debris. The resulting ferromanganese surfaces autocatalyze further growth. In addition, Mn-fixing bacteria may also significantly accelerate accretion rates on these surfaces.Mn which accumulates in submarine sediments may be diagenetically recycled in response to steep solubility gradients causing upward migration from more acidic and reducing horizons toward the sea floor. In contrast, the concentrations of the predominant ferric complexes, Fe(OH)₃⁰ and Fe(OH)₄^?, are relatively less sensitive to the Eh's and pH's found in this environment; Fe is therefore not as readily recycled within buried sediments. Consequently, Fe is not so effectively enriched on the sea floor, although it precipitates more readily than Mn because seawater is saturated in amorphous Fe(OH)₃.The metastable, perhaps kinetically-related, Mn oxides of nodules have a characteristic distribution: birnessite predominates in oxidizing environments of low sedimentation rate and todorokite where sedimentation rates and diagenetic Mn mobility are higher. Surface adsorption and cation substitution within the disordered birnessite-todorokite structure account for the high trace element content of Mn nodules.     

Improved methods for selective dissolution of Mn oxides: applications for studying trace element associations

The association of rare earth and other trace elements with Fe and Mn oxides was studied in Fe-Mn-nodules from a lateritic soil from Serra do Navio (Northern Brazil). Two improved methods of selective dissolution by hydroxylamine hydrochloride and acidified hydrogen peroxide along with a classical Na–citrate–bicarbonate–dithionite method were used. The two former reagents were used to dissolve Mn oxides without significant dissolution of Fe oxides, and the latter reagent was used to dissolve both Mn and Fe oxides. Soil nodules and matrix were separated by hand. Inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry after fusion with lithium metaborate, and X-ray diffraction were used to determine the elemental and mineralogical composition of the nodules and soil matrix. The latter was composed of kaolinite, gibbsite, goethite, hematite, and quartz. In the nodules, lithiophorite LiAl₂(Mn^IV₂Mn^III)O₆(OH)₆ was detected in addition to the above-mentioned minerals. The presence of hollandite (BaMn₈O₁₆) and/or coronadite (PbMn₈O₁₆) in the nodules is also possible. In comparison to the matrix, the nodules were enriched in Mn, Fe, K, and P, and relatively poor in Si, Al, and Ti. The nodules were also enriched in all trace elements determined. Phosphorus, As and Cr were associated mainly with Fe oxides; Cu, Ni, and V were associated with both Fe and Mn oxides; and Ba, Co, and Pb were associated mainly with Mn oxides. Distribution of rare earth elements indicated a strong positive Ce-anomaly in the nodules, compared to the absence of any anomaly in the matrix. Some of Ce was associated with Mn oxides. The improved methods achieved almost complete release of Mn from the sample without decreasing the selectivity of dissolution, i.e., without dissolving significant amounts of Fe oxides and other minerals, and provided reliable information on associations of trace elements with Mn oxides. These methods are thus proposed to be included in sequential extraction schemes for fractionation of trace elements in soils and sediments.     

贵州省松桃县道坨超大型锰矿床的发现及其成因探讨

贵州省松桃县道坨锰矿床是新发现的一个超大型全隐伏锰碳酸盐矿床。文章阐述了该矿床的发现概况及基本的地质和地球化学特征,并应用锰矿石和含锰黑色页岩的元素和碳同位素地球化学特征对菱锰矿的形成机制提出了制约。道坨超大型锰矿床的发现是填图及对区域地质资料综合分析的结果。该矿床具有品位高、厚度大、呈层性好及展布广等特点。其锰矿石的主量元素特征为Al2O3、TiO2、Fe2O3含量较低,P2O5中等程度富集,MnO、MgO含量相对较高,Fe/Mn比值低。在微量元素方面,锰矿石显示出较为明显的Co富集,含锰黑色页岩则显示出较为明显的Co、Mo富集;在稀土元素方面,锰矿石具有较高的稀土元素总量,轻微的"帽式"稀土元素PAAS标准化配分模式及明显的Ce正异常,含锰黑色页岩的稀土元素总量与PAAS接近,其稀土元素PAAS标准化配分模式较为平坦,无Ce异常。碳同位素测试结果显示出菱锰矿富集碳的轻同位素,表明在菱锰矿形成过程中存在有机碳的参与。文章表明,道坨锰矿床内的锰是以氧化物或氢氧化物的形式沉淀,菱锰矿是在缺氧且富含有机物质的成岩环境中转化而成。     

Mineralogy, petrography, geochemistry and genesis of the Paleoproterozoic Birimian manganese-formation of Nsuta/Ghana

The manganese deposit of Nsuta, in the Ashanti Belt of Southern Ghana, is sandwiched between Birimian metasedimentary rocks. The metasedimentary rocks contain interbedded carbonate-rich layers, which exhibit a characteristic banded appearance near the contact with the orebody. The orebody is a carbonate-type manganese-formation and in terms of origin is considered here as a Mn-analogue of the volcanogenic-exhalative Algoma type iron-formation. The protolith of the orebody (chemical sediment including Fe-bearing rhodochrosite and alabandite) is envisioned to have been formed in a marine basin with relatively high CO₂ activity and Eh-pH conditions were extremely low (Eh 1 to −0.6 Volt and pH 8 to 11) during Birimian times (2170–2180 Ma). These conditions occurred immediately below the shelf break in a shallow-marine environment. Subsequent submarine weathering (halmyrolysis) followed later by metamorphism of Eburnian age (2100 Ma) led to the formation of Mg-Ca-Fe-bearing rhodochrosite, the dominant mineral in the orebody. Other minerals of the orebody are: sulfides (e.g. two generations of alabandite sphalerite, pyrite, millerite, niccolite, gersdorffite, and molybdenite), oxides and hydroxides (vanadium-bearing jacobsite, galaxite; brucite, Mn²⁺-todorokite), Mn-silicates and an unknown boron mineral. Pyrochroite, possibly preceded by manganosite, occurs as a retrograde mineral. This mineral assemblage forms the protore of the Nsuta deposit. Opaque Mn⁴⁺-todorokite replacing Mn²⁺-todorokite, manganite, manganomelane, pyrolusite and nsutite which formed at the expense of rhodochrosite, are of supergene origin and represent the economic part of the deposit. The orebody is interleaved between the associated pelitic-psammitic metasedimentary rocks suggesting that its protoliths was deposited over a time interval during the sedimentation of the latter. Both units underwent subsequent processes (submarine weathering and metamorphism) together. The compositional differences between the orebody with high Mn and CO₂ and low Si and Al contents relative to the metasedimentary rocks are explained by a model involving the continuous sedimentation of continent-derived materials (protolith of the metasedimentary rocks). During this time a pulsatory phase of submarine volcanism and consequent precipitation of materials of essentially volcanogenic-exhalative origin occurred (protolith of the orebody). From the exhalations, the carbonate minerals in both the manganese-rich sediments and the metasedimentary host-rocks (in the latter in the form of layers and disseminations leading to relatively high concentrations of Mn, Ca and CO₂) were precipitated. Received: 18 April 1997 / Accepted: 16 July 1998     

Experimental Study of Gold and Platinum Solubility in a Complex Fluid under Hydrothermal Conditions

Abstract: The solubility of gold was studied in water and aqueous NaCl (1– 5 m) solutions under oxygen and sulfur buffered conditions between 300–500C at a constant pressure 1 kb. Two buffer assemblages HMP and PPM were used. Analysis of the scatter in measured values in log m_Au–m_NaCl–T frame fixed linear dependence between log m_Au and T at any studied iso‐pleth (m_NaCl) in the form of log m_Au = a. T(C) + b. Coefficients of the equation were calculated for water and NaCl (1, 3, 5 m) solutions. The maximum solubility characterizes the NaCl‐free system in the presence of HMP. In the case, Au solubility increases from (log m_Au) –6. 72 to –5. 04 at 300 and 500C, respectively. In the presence of PPM, maximum of Au solubility was obtained for the 5 mNaCl solution. In a similar manner solubility rises from –6. 54 to –5. 77 at 300 and 500C, accordingly. In studied f_O2/f_S2 area the behavior of Au solubility testified that: (i) – a composite interaction between chloride and hydrosulfide speciation of gold affects its total solubility; (ii) – in addition of NaCl up to about 1. 5 m the solubility decreases, more pronounced in the presence of HMP; (iii) – the contribution of chloride in total Au solubility is more for PPM despite of lower f_O2value, than for HMP. The solubility of platinum was studied in the Pt–Cl–S–H₂O system between 300 and 500C, 1 kb. PPM solid buffer controlled oxidation state, pH and sulfur activity of solutions (H₂O, 1 mNaCl and 0. 1 mHCl). Under the conditions, PtS precipitated from the solutions with increasing temperature and acidity. The PtS solubility in the 0. 1 m_HCl solutions lowers slightly in the range of 300–500C from –5. 30 to –5. 60 (in log m_Pt) that is typical to the hydrosulfide species. It was deduced that reducing media, regulated by the PPM assemblage, suppress activity of chloride species of Pt. More oxidizing conditions were modeled in runs using mixtures of Mn(II), Mn(III) and Mn(IV) oxides to buffer the aqueous‐chloride solutions between 300 and 500C, 1 kb. It was found that MnO tends to oxidize at T below 400C forming intermediate Mn‐hydroxides (β–MnOOH, Mn (OH)₂ and Mn₂(OH)₃Cl). These phases are metastable and transfer to Mn₃O₄ with increasing duration. Generation of the Mn‐hydroxides leads to a change of physical‐chemical parameters of the solutions, such as water activity, pH and Eh. The last results in abrupt increase in the noble metals dissolution. At stable existence of only Mn₃O₄, the solubility of both Pt and Au lowers to equilibrium values. Essential catalysis effect of Pt on intensity and rate of Mn(II) oxidation was found. The dominant role of chloride of Pt and Au was defined under most oxidized conditions, specified by Mn₂O₃–MnO₂ buffer. So at 400C, dissolved Au (log m_Au) increases from –4. 40 in water to –1. 00 in 0. 1 m_HCl, and ones of Pt (log m_Pt) from –4. 80 to –2. 90 accordingly. Thus, mixing of hydrosulfide and chloride solutions, as well as transformation of the systems to the stable state act upon total solubility of the noble metals.     

Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy

The local structures of divalent Zn, Cu, and Pb sorbed on the phyllomanganate birnessite (Bi) have been studied by powder and polarized extended X-ray absorption fine structure (EXAFS) spectroscopy. Metal-sorbed birnessites (MeBi) were prepared at different surface coverages by equilibrating at pH 4 a Na-exchanged buserite (NaBu) suspension with the desired aqueous metal. Me/Mn atomic ratios were varied from 0.2% to 12.8% in ZnBi and 0.1 to 5.8% in PbBi. The ratio was equal to 15.6% in CuBi. All cations sorbed in interlayers on well-defined crystallographic sites, without evidence for sorption on layer edges or surface precipitation. Zn sorbed on the face of vacant layer octahedral sites (□), and shared three layer oxygens (O_layer) with three-layer Mn atoms (Mn_layer), thereby forming a tridentate corner-sharing (TC) interlayer complex (Zn-3O_layer-□-3Mn_layer). ^TCZn complexes replace interlayer Mn²⁺ (Mn_inter²⁺) and protons. ^TCZn and ^TCMn_inter³⁺ together balance the layer charge deficit originating from Mn_layer⁴⁺ vacancies, which amounts to 0.67 charge per total Mn according to the structural formula of hexagonal birnessite (HBi) at pH 4. At low surface coverage, zinc is tetrahedrally coordinated to three O_layer and one water molecule (^[IV]TC complex: (H₂O)-^[IV]Zn-3O_layer). At high loading, zinc is predominantly octahedrally coordinated to three O_layer and to three interlayer water molecules (^[VI]TC complex: 3(H₂O)-^[VI]Zn-3O_layer), as in chalcophanite (^[VI]ZnMn₃⁴⁺O₇·3H₂O). Sorbed Zn induces the translation of octahedral layers from −a/3 to +a/3, and this new stacking mode allows strong H bonds to form between the ^[IV]Zn complex on one side of the interlayer and oxygen atoms of the next Mn layer (O_next): O_next…(H₂O)-^[IV]Zn-3O_layer. Empirical bond valence calculations show that O_layer and O_next are strongly undersaturated, and that ^[IV]Zn provides better local charge compensation than ^[VI]Zn. The strong undersaturation of O_layer and O_next results not only from Mn_layer⁴⁺ vacancies, but also from Mn³⁺ for Mn⁴⁺ layer substitutions amounting to 0.11 charge per total Mn in HBi. As a consequence, ^[IV]Zn,Mn_layer³⁺, and Mn_next³⁺ form three-dimensional (3D) domains, which coexist with chalcophanite-like particles detected by electron diffraction. Cu²⁺ forms a Jahn-Teller distorted ^[VI]TC interlayer complex formed of two oxygen atoms and two water molecules in the equatorial plane, and one oxygen and one water molecule in the axial direction. Sorbed Pb²⁺ is not oxidized to Pb⁴⁺ and forms predominantly ^[VI]TC interlayer complexes. EXAFS spectroscopy is also consistent with the formation of tridentate edge-sharing (^[VI]TE) interlayer complexes (Pb-3O_layer-3Mn), as in quenselite (Pb²⁺Mn³⁺O₂OH). Although metal cations mainly sorb to vacant sites in birnessite, similar to Zn in chalcophanite, EXAFS spectra of MeBi systematically have a noticeably reduced amplitude. This higher short-range structural disorder of interlayer Me species primarily originates from the presence of Mn_layer³⁺, which is responsible for the formation of less abundant interlayer complexes, such as ^[IV]Zn TC in ZnBi and ^[VI]Pb TE in PbBi.     

Eh ph Diagrams for mn, fe, co, ni, cu and as Under Seawater Conditions: Application of two new Types of eh ph Diagrams to the Study of Specific Problems in Marine Geochemistry

E_H pH diagrams have been calculated using the PHREEQC programme in order to establish the predominance fields of Mn, Fe, Co, Ni, Cu and As in bottom waters from the Angola Basin. Predominance fields are presented separately for both aquatic species and solid mineral phases in order to simplify interpretation of the data. The diagrams show significant differences from standard E_H pH diagrams for these elements calculated for freshwater at 25 °C and 1 bar which assume an element concentration of 10^-6 M. In particular, our diagrams show that Mn²⁺ and NiCO ₃ ⁰ are the predominant aquatic species for Mn and Ni in bottom seawater and FeOOH, Fe₂O₃, Fe₃O₄, CoFe₂O₄, CuFe₂O₄, CuFeO₂, and Ba₃ (AsO₄)₂ the predominant solid phases for Fe, Co, Cu and As, respectively. Mn and Ni are therefore undersaturated and Fe, Co, Cu and As supersaturated in bottom seawater from the Angola Basin. Neither rhodochrosite (MnCO₃) nor siderite (FeCO₃) can form in this marine environment in equilibrium with seawater. A mixed Mn-Ca carbonate is therefore formed within the pore waters of reducing sediments. The high Ni/Cu ratios in cobalt-rich manganese crusts formed adjacent to the oxygen minimum zone may be explained by the change from Cu²⁺ to CuCl ₃ ^2- as the dominant aquatic species of Cu in seawater at an E_H of +0.48 V.     

Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted basin: examples from the Gorge Creek Group in the Pilbara Block

The >3·0 Ga chert sequence of the Gorge Creek Group is exposed at Ord Ranges about 50 km east of Port Hedland in the Pilbara Block. The chert sequence examined in this study is 15 m thick and consists of oxide-rich laminated chert, grey chert (silicified clastic rock), carbonaceous black chert and carbonate-rich laminated chert. Although the cherts have undergone postdepositional silica enrichment, such as cementation and metasomatic silicification, primary precipitation of silica at the site of deposition is indicated by abundant microstructures (mosaic and spherulitic structures). Other primary to early diagenetic components were carbonates, sulphates (gypsum and anhydrite) and organic matter. Although these mineral associations, on the whole, correspond to those of modern marine evaporites, they are different from modern equivalents with respect to abundant precipitation of amorphous silica and presumed primary precipitation of iron-carbonate (siderite). This feature is a possible manifestation of peculiar physicochemical conditions in the water mass from which the chemical sediments were precipitated; compared with modern ocean waters, the concentrations of Fe and Si were significantly higher and the pH value might have been lower. These conditions could be obtained by contributions of Fe- and Si-enriched hydrothermal solutions and continental run-off to the site of deposition. Grey cherts contain detrital quartz and altered Fe–Ti oxides and were formed in a period of input of terrigenous detrital materials. They are characterized by higher concentrations of TiO₂, Al₂O₃, Cr, Ni, Zn, Rb and Zr compared with the other types of chert and by very low (< 4) Al₂O₃/TiO₂ values. These features are attributed to the supply of terrigenous detrital materials that contain abundant Fe–Ti oxides (ilmenite and titanomagnetite) and fine TiO₂ particles. Such detrital materials might have been formed by extensive chemical alteration of source rocks and residual enrichment of Ti relative to Al.     

A 200–300 year cyclicity in sediment deposition in the Gotland Basin,Baltic Sea,as deduced from geochemical evidence

During the EU funded project BASYS (Baltic Sea System Study) short (Niemistö-type) and long (box and piston cores) sediment cores were taken which cover sedimentation during the past 8 ka. The uppermost part of the sedimentary sequence was chosen for a detailed geochemical study and freeze dried samples were analysed for about 20 elements but only the elements Mn and Ca are discussed. An age model was constructed using radiometric dating results by ²¹⁰Pb/¹³⁷Cs and ¹⁴C AMS. Significant correlation exists along the cores between very high Mn and moderately high Ca due to occrrences of the mineral rhodochrosite (kuthnahorite), a complex Mn(Ca) carbonate. This mineral is thought to be produced when salt water meets the pool of dissolved Mn at the bottom of the Gotland Basin. During favourable hydrographic conditions, e.g. strong northwesterly winds, salt water from the North Sea invades even the deepest parts of the central Baltic. Mn²⁺ which is produced mainly by the dissolution of ferromanganse oxides/oxyhydroxides in the water colum and in the course of destruction of organic matter in the sediments, combines with HCO₃^2- and Ca²⁺ in the seawater to form rhodochrosite. After burial, this mineral stays in the sediment and is seen as light-coloured layers. A certain cyclicity in the upper 1.5 m of the cores was observed in that about 200–300 a periods of elevated Mn–Ca are followed by periods with lower Mn–Ca of similar duration. An explanation for the observed cyclicity may be sea level variations: during rising sea level (transgression) more and more saline water is pushed into the deep basin of the Baltic Sea and if conditions are favourable (high dissolved Mn) the mineral rhodochrosite is precipitated.