Precipitation Of Mn Research Articles

Hydrothermal sulfide-bearing Fe-Si oxyhydroxide deposits from the Coriolis Troughs, Vanuatu backarc, southwestern Pacific

The Coriolis Troughs located in the Vanuatu backarc of the southwestern Pacific consist of the Vate Trough to the north, the Erromango Basin in the middle and the Futuna Trough to the south. Major hydrothermal activity responsible for the formation of Sulfides- and barite-bearing Fe-Si oxyhydroxide deposits as well as biological communities of living galatheid crabs and gastropods occurs at the caldera rim of a submarine volcano in the northern Vate Trough. The deposits consist of major nontronite and amorphous Fe-Si oxyhydroxides, minor sulfides and barite, and are associated with traces of Mn. Galena, chalcopyrite, pyrite, sphalerite, marcasite and covellite are the principle sulfide minerals. The deposits occur sporadically on pillow lavas and breccias as small chimneys, mounds, flat slabs and infillings in water depths between 1100 and 1400 m. The deposits are more than several tens of meters wide and extend about 1 km in one direction at the northwestern and eastern caldera rims. The appearance of shimmering water was observed in the holes between the pillow lavas and tubes, and was associated with a positive temperature anomaly by 0.2 °C relative to the ambient seawater. Dark back-scattered acoustic images obtained by side-scan sonar revealed the occurrence of mounds. This suggests the presence of several hydrothermal deposits on the eastern rim of the caldera. In the Futuna Trough, sulfide-bearing Fe-Si oxyhydroxide deposits are present as flat slabs and infillings of brownish fragments and reddish brown sediments and Mn precipitation on volcanic rocks near the summit of a submarine volcano on the northwestern flank in water depths between 940 to 980 m. The deposits are characterized by the presence of bacterial filaments, high concentrations of Fe (max. 42 wt%) and As (max. 2.7 wt%), major goethite and amorphous Fe-Si oxyhydroxides, birnessite, and sulfides. The hydrothermal nontronite deposits of the Vate Trough were formed under more reduced conditions than the goethite of the Futuna Trough. Sulfides and barite from the Vate deposits were formed as a result of the precipitation from hydrothermal fluids in the deposits whereas sulfides of the Futuna deposits were deposited from hydrothermal vents further away. The presence of sulfides in the Vate Fe-Si oxyhydroxide deposits suggests that extensive sulfide deposits may exist in the volcanic pile of the caldera. The Coriolis Troughs are still active, especially in the Vate Trough. There are a number of areas with the potential for hydrothermal precipitation of sulfide-bearing Fe-Si oxyhydroxide deposits in both the Vate and Futuna Troughs.

Precipitation of hydrothermal sediments on the active TAG mound: implications for ochre formation

Abstract Submersible and drilling studies of the active TAG hydrothermal mound (26°N, Mid-Atlantic Ridge) have led to new models of fluid flow and evolution within an active mineral deposit which has implications for Fe-oxide and ochre precipitation. Metalliferous sediments from the top of the hydrothermal mound accumulate from a combination of processes including slumping and oxidation of chimney material and in situ precipitation of low-temperature phases from fluids that percolate through the mound. Geochemical proxies of hydrothermal processes allow identification of the mode of formation of one sediment core from the southeastern periphery of the TAG mound. The Fe rich sediment is capped with a ∼5 cm thick kaolinite, illite, chlorite, smectite layer which formed from alteration and replacement of basalt and diagenetic reactions within the hydrothermal sediment. Underlying this layer is a ∼10 cm thick zone of Mn, Cu, Zn and Pb enrichment which is controlled by the sharp redox gradients in the core. The base of the core is characterized by Mn-poor, Fe-rich oxide that is dominated by goethite, haematite and amorphous Fe oxides equivalent to ochreous and gossan material. Rare earth element (REE) patterns from the different layers within the core allow interpretation of the modes of formation of sediment in the light of existing fluid flow models for TAG. The basal layer is dominated by in situ precipitation of Fe oxide phases from evolved fluids that result from significant within-mound anhydrite precipitation. The REE data for the upper part of the core demonstrate mixing with sea water which provides the oxidizing conditions for Mn precipitation along with Cu, Zn and Pb enrichment from the evolved fluid. Sea water ingress results in higher V/Fe and P/Fe ratios in the upper part of the core but no enhanced U/Fe ratio. The uppermost clay-rich layer hosts the majority of the REE inventory for the core and the significant positive Eu anomaly indicates recrystallisation of the phyllosilicate phases from the ochreous material during diagenesis. REE data from land-based ochre and gossan sediments demonstrate that the TAG model may be applicable to a wide variety of sites throughout the geological record.

On-line precipitation/dissolution system for the preconcentration and determination of manganese traces by atomic absorption spectrometry

Flame atomic absorption spectrometry was used for the determination of Mn in biological material following preconcentration by precipitation. The proposed preconcentration method is based on the continuous precipitation of Mn(II) as hydrated Mn(IV) oxide in ammonia buffer and dissolution of the precipitate with hydrogen oxalate or dilute nitric acid. The sensitivity of the Mn determination is increased by the presence of hydrogen peroxide, which raises the rate of oxidation of Mn(II) to Mn(IV). By using a time-based technique (at a sample loading rate of 4 ml min −1) a concentration factor of up to 55 was obtained using 24 ml of sample. The effect of concurrent cations was investigated; the most adverse effect was exerted by Fe(III), which interfered at concentrations 50 times higher than that of Mn(II).

Active post-depositional oxidation of the most recent sapropel (S1) in sediments of the eastern Mediterranean Sea

Over a wide area of the eastern Mediterranean basin, two Mn-rich layers have been observed above the most recent sapropel (S1), one immediately above the sapropel top and one a few centimetres closer to the sediment surface. Different mechanisms have been proposed to explain the occurrence of these two Mn peaks: either both peaks have a diagenetic origin in which case the upper Mn peak is actively forming, or the lower peak is actively forming and the upper peak has a different formation mechanism. High-resolution porewater, including a gel sampler used for the first time in marine sediments, and solid phase data are now used to demonstrate that the oxidation front is located at the level of the lower Mn peak which, therefore, is presently being formed. A barium-organic carbon relationship is used to calculate the initial organic carbon profile of the S1 sapropel. The palaeoproductivity profiles generated by this method demonstrate that the original sapropel unit was bounded by the upper Mn peak. This implies that the interval between the two Mn peaks, where a low organic carbon content is now observed, was originally part of the sapropel. The initially deposited organic carbon has been oxidised by a progressive downwards-moving oxidation front. The penetration depth of this oxidation front, i.e., the distance between the two Mn peaks, is mainly determined by the organic carbon content, the sediment accumulation rate, and the bioturbation depth. The upper Mn peak appears to have formed as a result of either Mn precipitation upon oxygenation of previously anoxic eastern Mediterranean deep water, or preservation of a surficial Mn peak at the end of the high productivity episode. In either case the upper Mn peak marks the end of sapropel formation as indicated by the Ba profiles. This means that formation of the S1 sapropel ceased more recently than is indicated by radiocarbon dating of the visible top of S1.

MANGANESE AND TRACE METAL REMOVAL IN SUCCESSIVE ANAEROBIC AND AEROBIC WETLANDS

A microcosm study was conducted to evaluate the use of anaerobic wetlands preceding aerobic wetlands for removal of Mn, Cu, Ni, Zn, and Pb in wastewater. Initial concentrations for Mn, Cu, Ni, Pb, and Zn were 20, 2.0 1.5 2.1, and 2.0 mg/L, respectively. Each experimental unit consisted of three cattle-feeding troughs (cells) set in series, The first cell was anaerobic and the last two were aerobic. Water was delivered to the wetland cells at 20 mL/min for a period of 380 days. The anaerobic wetlands consisted of three treatments replicated two times. The aerobic wetlands consisted of two treatments replicated three times, one anaerobic treatment contained organic matter and limestone (SP). Another anaerobic treatment contained organic matter, limestone, and canarygrass (SP&CG). The third anaerobic treatment consisted of canarygrass planted in river gravel (RG). Water flowed into the top and was discharged from the bottom of each anaerobic wetlands The aerobic treatments consisted of reciprocating or not reciprocating water between two cells containing river gravel. The anaerobic troughs with organic matter were effective in reducing sulfate to sulfide and producing alkalinity in the range from 80 to 300 mg/L. Manganese removal in the anaerobic systems decreased with time with themore » effluent anaerobic waters near equilibrium with respect to MnS and MnCO{sub 3} toward the end of the experiment, Removal of Cu, Ni, Zn, and Pb was very effective in the anaerobic cells with organic matter due to precipitation of metal sulfides. Since Mn removal was ineffective in the long-term in the anaerobic system, aerobic wetlands would be necessary for further water treatment. Manganese removal in the reciprocating aerobic cells was quicker than in the nonreciprocating aerobic cells with removal due to precipitation of Mn oxides. Coupled anaerobic-aerobic wetlands appear to hold promise for removing trace metals via metal sulfide precipitation and Mn via Mn oxide precipitation.« less

Nchwaningite, Mn (super 2+)2SiO3(PJ)2.H2O, a new pyroxene-related chain silicate from the N'chwaning Mine, Kalahari manganese field, South Africa

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

Effects of phytoplankton on metal partitioning in the lower River Rhine

Since algal growth has been shown to play a key role in determining the fate of metals in lakes and marine waters, we wondered if the Rhine phytoplankton, so much stimulated by nutrient input, would affect the partitioning of metals between the dissolved and particulate phases, thereby altering the retention of metals in the Rhine delta. In a seasonal study in which variations in the partitioning of Mn, Cu, Zn, Cd and Pb (expressed as log K p values) were correlated with phytoplankton parameters, it appeared that manganese occurred mainly in the particulate form during algal blooms, whereas dissolved manganese predominated during periods low in phytoplankton. Photosynthetic activity (up to 700 μg C l −1 h −1) correlated slightly better with the [log K p]-Mn than the chlorophyll a concentration (up to 140 μg l −1) and the pH (up to 8.35), suggesting that phytoplankton photosynthesis promotes Mn precipitation in the river. The variability in the partitioning of Cu, Zn, Cd and Pb in 1990 did not seem to be determined by the seasonal differences in phytoplankton and manganese partitioning, although increased values of [log K p]-Zn had been indicated for the summer of 1983, when metal concentrations had generally been higher than in 1990. The lack of effect of riverine phytoplankton blooms on partitioning of metals other than Mn contrasts with observations in stagnant waters. However, the low levels of cellular metal observed in cultures, along with the single growth pulse that phytoplankton shows during its short residence in the river, are consistent with the observations.

Effect of preheat temperature on the orientation relationship of (Mn,Fe)Al 6 precipitates in an AA 5182 Aluminium—Magnesium alloy

The precipitation of the (Mn,Fe)Al 6 phase during preheating of a commercial AA 5182 Al Mg alloy was studied by means of scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. The existence of two different morphologies with low and high aspect ratio (called here rhomboidal and platelike) was confirmed. The influence of preheating parameters on the precipitation was found to be close to the one known for Al-Mn alloys. It was found that the platelike dispersoids bear orientation relationships with the matrix of type [100] m||[2¯10] pp and (0¯11) m||(001) pp, which are not yet reported in the literature. On the other hand it was shown that rhomboidal precipitates do not follow any orientation relationship with the matrix. The more harmful influence on recrystallization and hot ductility of the rhomboidal precipitates compared to the platelike ones is discussed.

Temporary and definitive fixation of atmospheric lead in deep-sea sediments of the Western Mediterranean Sea

Most lead brought to the Mediterranean Sea has an anthropogenic origin and is mainly transported through the atmosphere. Atmospheric Pb was continuously collected at Cap Ferrat in 1986 and 1987. From this study, the estimation of the anthropogenic Pb flux on the whole Western Mediterranean was, averaged on 1986 and 1987 data, 4080 t. Assuming that the atmospheric anthropogenic Pb input varied in this course of time similarly to the consumption of Pb added to gasolines in France, the mean annual flux could be calculated: 3.95 kg km −2 yr −1, that is an annual input of 3360 t yr −1. Reaching the sea, this metal seems to become rapidly bound to phytoplankton. Grazing by zooplankton leads to the production of faecal pellets which frequently contain rather high metal concentrations. The sinking rate of pellets of various zooplankton species is high; within a few days pellets may reach deep-sea sediments. After deposition, Pb is released from this organic-rich material during early diagenesis. In most cases, it, therefore, returns to the overlaying water body by ascending diffusion. But, in a deep-sea area of approximately 80 000 km 2 where Mn oxide precipitation occurs in surficial sediments, Pb seems to remain stored by coprecipitation processes. By considering the lead stored in ‘excess’ in the surficial sediment of the deep-sea area, we estimate that a mean annual anthropogenic Pb amount ranging from 800 up to 1080 t was stored every year from 1950. On the same area, taking into account the Pb loss at the straits, the ‘direct’ atmospheric input to the sea bottom is, on average, 184 t yr −1. The remaining part, that is (800–1080)−184=(616–896) t yr −1, corresponds to an additional ‘indirect’ Pb flux in water due to Pb released from sediments of the surrounding areas where it does not remain stored.

The influence of channel morphology on the chemical partitioning of Pb and Zn in contaminated river sediments

The chemical partitioning of Pb and Zn was investigated in contaminated stream sediments from the River Ystwyth in mid-Wales, with changes in metal/substrate fixation being related to variations in channel morphology. Confinement of the channel by bedrock (chute zones) is sufficient to create very turbulent flow conditions facilitating the precipitation of Mn(IV) on surfaces of sand sized particles (2000-63 μm). Phase specific extraction of Pb (easily reducible fraction) from this particle size range indicates significant adsorption on precipitated forms of Mn. Conversely, under meandering conditions, association with the moderately reducible fraction (Fe-oxides) was more influential in the physico-chemical partitioning of Pb. Thus oxyhydroxide “precipitation zones” are created which affect heavy metal partitioning. These phenomena are, however, not observed on silt and clay sized particle coatings (<63 μm). The partitioning of Zn is not influenced by channel morphology due to a combined influence of the greater solubility of ionic species and subsequent transfer in solution and to the prevalence of total sediment Zn as sphalerite (ZnS).

Kinetics of the Precipitation of Mn2+–Cation Vacancy Dipoles in Doubly Doped KCI: Me2+, Mn2+ (Me = Eu2+, Mg2+, Ba2+) Crystals

Kinetics of the Precipitation of Mn²⁺–Cation Vacancy Dipoles in Doubly Doped KCI: Me²⁺, Mn²⁺ (Me = Eu²⁺, Mg²⁺, Ba²⁺) Crystals

Electron microscopic characterisation of iron and manganese oxide/hydroxide precipitates from agricultural field drains. 1

Scanning and transmission electron microscopic examination of drain precipitates revealed the presence of a slime/organic layer and fungi, bacteria (including filamentous and Fe bacteria), and possibly actinomycetes. Most of the filamentous structures were encrusted with Fe and Mn compounds. Treating the samples with acidified NH2OH.HCl and leucoberbelin blue revealed some structures similar to Hyphomicrobium and Pedomicrobium spp., yeast cells, cocci, fungal spores, and relics of diatoms and amoebae. Both, scanning and transmission electron microscope-energy-dispersive analysis of X-rays showed a clear association of microbial structures with Fe and Mn oxides. It was suggested that Fe and Mn were being precipitated in the drains. However, the precipitates were not stable under natural conditions, and therefore we concluded that these precipitated oxides were also undergoing reductive dissolution. It thus appeared that precipitation of Fe and Mn, particularly Mn, had been mediated microbiologically in the drains.

High-temperature deformation of solution-treated Al-0.6% Si-1% Mn-0.7 Fe alloy

The tensile properties of Al-0.6% Si-1% Mn-0.7% Fe alloy were investigated in the temperature range 295–773 K, to assess the effect of the precipitation of Si, Mn and Fe during ageing and deformation on the mechanical properties. The alloy showed a pronounced drop in ductility at elevated temperatures. Elongation-to-fracture versus temperature-of-deformation curves are evaluated as a function of the strain rate. The elevated-temperature yield and ultimate tensile strength (UTS), ductility, strain-rate sensitivity and strain-hardening exponent have a strain-rate dependence. The minimum in the strain-rate sensitivity versus temperature curve is coincident with the elongation minimum temperature. At low and high temperature ranges the flow could be represented by the constitutive equations σ = K2 en and σ = K3 em, respectively. There is also a discussion of the activation energy for deformation in the vicinity of the ductility minima and from plotting the logarithm of the strain rate versus the reciprocal absolute temperature at a constant yield strength (18 MN m−2). A tentative model based on the diffusion of Si, Fe and Mn in Al and the subsequent precipitation of Si, FeAl3, MnAl6 and α-Al12Mn3Si is postulated to explain the loss in ductility at high temperatures and the corresponding change in strength. An attempt is made to correlate strength, ductility and structural changes at elevated temperatures.

Role of organic acid chelators in manganese regulation of lignin degradation by Phanerochaete chrysosporium.

Nitrogen, carbon, and manganese are potent regulators of lignin degradation, but although nitrogen and carbon elicit a generalizated response when cells are starved, manganese is a relatively specific regulator of lignin and manganese peroxidase (LiP and MnP, respectively). At high manganese levels, MnP is induced, and LiP is repressed. At low Mn levels, MnP is repressed, and LiP is induced. Organic acid chelators are very important in attaining LiP repression with high Mn. Both mineralization and lignin depolymerization are regulated by manganese in the presence of organic acid chelators. As long as the chelators keep Mn(II) and Mn(III) in solution, repression is observed, but eventually, dismutation reactions cause the formation and precipitation of Mn (IV) as MnO2. Repression is immediately relieved, and depolymerization and mineralization proceed at a high rate.

Manganese scavenging and oxidation at hydrothermal vents and in vent plumes

Hydrothermal vents provide a major source of dissolved Mn(II) to the oceans, where concentrations range from 5 mM within the 350°C hot smokers to just above ambient seawater concentration in far field vent plumes. The Mn(II)-rich environments within warm vents and vent plumes provide a suitable habitat for Mn(II) oxidizing bacteria. In order to compare rates of scavenging and oxidation of Mn(II) proximally within vent fields (<30 m from venting water and temperatures <16°C) and distally within vent plumes, and to determine the relative contribution of microbes, incubation experiments using 54Mn as a radiotracer were conducted in situ and on collected water samples from three hydrothermal vent locations: the Guaymas basin (GB), the Galapagos spreading center (GA), and the Endeavor Ridge of the Juan de Fuca spreading center (JDF). Both the adsorbed and oxidized fractions of the total 54Mn scavenged were determined and found to often be significant (as high as 65 and 74%, respectively). Manganese scavenging rates were generally higher in in situ incubations than in incubations conducted on board ship. Inhibition of 54Mn scavenging by sodium azide provided evidence for microbially mediated Mn(II) uptake and oxidation in waters both proximal (GA and GB) and distal to the vents (GA and JDF), even at distances as great as 17 km from the ridge axis at JDF. The highest manganese scavenging rates were observed within the vent fields (up to 2.5 nM/day). The residence times of dissolved Mn(II) were shorter in the GB and GA vent fields (26 and 28 days) than in the JDF vent field (1.4 years). This difference may be due to different mechanisms of Mn(II) precipitation in operation. At the GA vent field Mn(II) precipitation was often strongly inhibited by sodium azide and therefore apparently due to microbial activity. In contrast, Mn(II) scavenging within the JDF vent field was not significantly affected by sodium azide. Because 54Mn scavenging in the JDF vent field was dependent on the presence of oxygen and a much larger fraction of the total 54Mn scavenged was adsorbed than oxidized, manganese scavenging appears to occur primarily by an abiological mechanism, perhaps coprecipitation with iron oxyhydroxides. In comparison to the vent fields, Mn(II) scavenging rates were lower within the vent plumes (<0.6 nM/ day for GA and <0.2 nM/day for JDF), whereas residence times were not significantly different (as low as 34 days for GA and 1.0 years for JDF). The short residence times (90 and 118 days) and high microbial activity measured in bottom waters beneath the vent plumes at GA and JDF probably resulted from enhanced scavenging by manganate-coated bacteria that settled out from the vent plume and accumulated near the bottom. Therefore, bacteria not only enhance the scavenging of Mn within vent waters, but also facilitate Mn deposition to the sediments.

Mineralogy, paragenesis and genesis of the braunite deposits of the Mary Valley Manganese Belt, Queensland, Australia

The Mary Valley manganese deposits exhibit mineralogy and textures characteristic of at least four parageneses. The deposits consist mainly of isolated occurrences of braunite, together with a number of lower and higher valency manganese oxides, and manganese silicates, in bedded radiolarian cherts and jaspers of Permian age. The parageneses are: (a) Braunite — quartz (primary), (b) Braunite — hausmannite — spessartine — tephroite — quartz (metamorphic). (c) Hydrated manganese silicates — barite — braunite — hausmannite (hydrothermal veins), (d) Tetravalent manganese oxides (pyrolusite, cryptomelane, manjiroite, nsutite) (supergene). The primary mineralisation is interpreted as the result of the geochemical separation of Mn from Fe in a submarine exhalative system, and the precipitation of Mn as oxide within bedded radiolarian oozes and submarine lavas. During diagenesis this hydrothermal manganese oxide reacted with silica to produce primary braunite. The later geological of evolution of this volcanogenicsedimentary deposit involved metamorphism, hydrothermal veining by remobilised manganese, and supergene enrichment.

Manganese precipitates in the karst terrain of Grand Cayman, British West Indies

Ten morphologically distinct forms of Mn precipitates (probably birnessite) occur in a wide variety of paleokarst features that characterize dolostones of the Bluff Formation on Grand Cayman. These Mn-rich precipitates, which commonly contain variable amounts and selections of Al, Si, Fe, K, Ti, Ni, Na. Mg, and Ca, are not evenly distributed throughout the Bluff Formation of Grand Cayman. Thus, some areas have abundant Mn precipitates, whereas other areas are devoid of such precipitates. Mn precipitates, of variable morphology, have teen found in stalactites, karst breccia, caymanite, terrestrial oncoids, and root calcretes and along fractures and cavities in the dolostone. There does not appear to be any correlation between the morphology and composition of the Mn precipitate and its host substrate.The Mn and its associated elements may have been derived from the terra rossa and (or) swamps that formed on the surface of the Bluff Formation at various times. The scattered distribution of the soils and swamps, with respect to time and space, accounts for the patchy occurrence of the Mn precipitates presently found in the Bluff Formation. Although some of the Mn precipitates are abiogenic in origin, others appear to have formed through the direct or indirect intervention of various microbes.Paragenetic analysis of the Mn precipitates and their associated cements shows that Mn precipitation has not been a constant, ongoing process. Indeed, Mn precipitation does not appear to be occurring at the present day. Available evidence suggests that Mn precipitation occurred at various times when climatic conditions were suitable for Mn mobilization.

Geochemistry of Mn and Fe in lake sediments in relation to lake acidity

Concentrations of easily reducible Mn (ER Mn, Mn oxides), easily reducible + reducible Fe (ERR Fe, Fe oxides), and alkaline extracted Mn and Fe (org Mn and Fe, organically bound) were determined in sediments of three circumneutral (CN) and four acidic lakes. Up to 80% of total sediment Fe (total = ERR + org Fe) was removed as ERR Fe in the CN vs. acidic lakes. Acidic lakes contained twice the total Fe compared to CN lakes, which was distributed equally between the ERR Fe and org Fe sediment fractions. Mn was recovered as ER Mn from all seven lakes. Two of four acidic lakes contained little ER Mn (due to the pH‐dependent decrease in particulate Mn precipitation). However, two acidic lakes contained surficial concentrations that were equivalent to CN lakes. ER Mn in sediments of acidic lakes is attributed to sediment diagenesis. Differences in ERR Fe and org Fe in sediments of acidic vs. CN lakes may be due to the pH‐induced precipitation of Fe humic complexes.

Microbial precipitation of manganese by bacteria and fungi from desert rock and rock varnish

Abstract Precipitation of manganese by fungi and chemoorganotrophic bacteria isolated from desert rock was quantitatively shown to be dependent on increasing NaCl concentrations in vitro. Many fungi were able to precipitate Mn(IV) under conditions suggested to be similar to those found on desert rock. However, Mn(IV) precipitation in bacterial cultures took place mostly at pH values above 8. Our results lend farther support to the hypothesis on the microbial genesis and stabilization of rock varnish and the patination of ancient petroglyphs.

Seasonal biogeochemical cycles in riverborne groundwater

The behavior of dissolved (<0.45 μm) inorganic species and changes in relevant properties of anthropogenically polluted river water were investigated during infiltration and movement in a hydraulically connected saturated aquifer. Water from the river and from several sampling wells was analyzed over 5 years for temporal and spatial chemical changes.At the early stage of infiltration, a drastic decrease in pH and in the concentrations of O2 and NO3− occurs. The gradients in these properties are most significant in the interstitial water of the river sediments. They are the result of the degradation of aquatic biota. The amount and grain morphology of river sediments varies seasonally and depends also on water flow conditions.Temperature-related variations in microbiological activity in the river water and in the sediments of the riverbed induce pronounced annual cycles in the aquifer for several of the investigated properties. Each summer, under anoxic conditions, manganese (hydr) oxides dissolve, and trace metals (Cu, Zn, Cd) are mobilized. The main sources for manganese and other trace elements were found within the river sediments; degradation of algae and other aquatic biota and dissolution of calcite contribute to the observed effects. Higher winter concentrations of O2 and NO3− result in precipitation of Mn and other redox sensitive elements in the aquifer. Zinc and Cd are retarded by interactions with the aquifer material, whereas Cu is mobile, probably as an organic complex.