Formation Of Nontronite Research Articles

Electron microscopy investigation of the genetic link between Fe oxides/oxyhydroxides and nontronite in submarine hydrothermal fields

The two major components most frequently found in sediments near submarine hydrothermal fields are Fe oxides/oxyhydroxides and nontronite. Physico-chemical conditions in these environments vary from site to site and even within sites as the type and intensity of the hydrothermal activity change. The investigation of Fe oxides/oxyhydroxides and nontronite is helpful to constrain the specific conditions in which they formed or evolved, and thus to recreate the history of hydrothermal vents. Microbial activity is also involved in the formation of these mineral phases, which adds the interest of revealing processes at the interface of biological and inorganic phenomena. We investigated sediments from ten seafloor hydrothermal fields, most of them Recent sediments. They are rich in Fe oxides/oxyhydroxides and/or nontronite and/or Fe-talc. SEM analysis with energy-dispersive X-ray spectroscopy revealed many morphological types, among which vermicular habits are common, with chemical composition ranging frequently between Fe oxides/oxyhydroxides and nontronite. Smooth surfaces of the investigated particles correlates with high Fe/Si ratios, and rough surfaces with low Fe/Si ratios. This feature and the fact that many particles had chemical compositions (Si, Fe, Mn, Mg, Al) in a continuous line linking Fe oxide and nontronite are compatible with the in situ reaction of Fe oxides/oxyhydroxides as a frequent mechanism of nontronite formation. This process involves the reaction of Fe oxides/oxyhydroxides with either silica polymers adsorbed on their surface or Si-Fe amorphous phases precipitated on the same surface. However, it was not possible to establish a pseudomorphic replacement (of Fe oxides by nontronite) as no nontronite grains were found reproducing habits observed in grains with large proportion of Fe oxides/oxyhydroxides. All particle morphologies can have a range of surface roughness and composition. Chemical composition consistent with nontronite was represented mainly by small particles (<1μm) and flakes or veil-like habits. The frequent vermicular habit caused by precipitation on Fe-oxidizing bacteria was in line with previous findings indicating this as an important process of precipitating a wide range of Fe-Si phases. Overall, our data suggest that in situ replacement of Fe oxides/oxyhydroxides by nontronite takes place in all types of particles but that the replacement does not preserve the original particle morphology.

Hydrothermal nontronite formation associated with microbes from low‐temperature diffuse hydrothermal vents at the South Mid‐Atlantic Ridge

AbstractOceanic nontronite deposits have been identified to be closely related to low‐temperature hydrothermal activities. However, their formation mechanisms associated with microbes in diffuse hydrothermal vents still remain largely unknown. The friable deposits, collected from the low‐temperature diffuse flow at the Southern Atlantic Ridge, display a layered structure. Scanning electron microscope and transmission electron microscope analyses reveal that abundant filamentous, spherical, and rod‐shaped mineralized forms are preserved in the yellowish‐green layer of the deposits. These mineralized forms primarily consist of Si and Fe. Selected area electron diffraction patterns of the mineralized forms indicate that they are composed of nontronite. High intensities of 12C and 12C14N signals derived from cellular structures determined by nanosecondary ion mass spectrometry suggest the intimate relationship between nontronite and microbes. The results of 454 pyrosequencing analyses provide insights into the microbial communities involved in the biologically induced mineralization in the yellowish‐green layer. We propose an evolutionary model for establishing paragenetic sequences among nontronite, Mn oxide, and Fe oxyhydroxide in the deposits. This paragenetic sequence could be widespread in modern and ancient low‐temperature hydrothermal fields.

Influence of Aqueous Si and Fe Speciation on Tetrahedral Fe(III) Substitutions in Nontronites: A Clay Synthesis Approach

AbstractMost dioctahedral 2:1 swelling clays in natural systems contain ferric iron, Fe(III), which can be located in both the tetrahedral and the octahedral sheets. The distribution of Fe(III) between octahedral and tetrahedral sites in nontronite depends on the Fe and Si speciation during nontronite synthesis. The role played by the chemical properties of solutions in the Fe(III) distribution between structural sites was studied through nontronite syntheses. A chemical series of Fe(III)-nontronites with variable tetrahedral [4]Fe(III) content (x) ([Si4-xFe(III)x]Fe(III)2O10(OH)2Nax) was synthesized at 150°C across a range of initial aqueous pH values between 11 and 14. The permanent layer charge, due to Fe(III)-for-Si(IV) tetrahedral substitutions only, ranged from 0.43 to as high as 1.54 per half-unit cell. A d063̄3 value of 1.562 Å was measured by X-ray diffraction (XRD) for the highest charged nontronite (x = 1.54). This high d063̄3 value has not been reported in the literature for a dioctahedral smectite until now. The [4]Fe(III) content (x) of the synthetic nontronites, estimated using Fourier-transform infrared spectroscopy (FTIR) through the wavenumber of the main stretching vSi-O band, was correlated with synthesis pH and its influence on calculated aqueous Si speciation. The increase in synthesis pH induced the increase in anionic aqueous Si species ratios (i.e. H3Si4(aq)− and H2Si4(aq)−), and favored the incorporation of Fe(III) in tetrahedral sites of synthesized nontronites. During nontronite formation in natural systems, the level of tetrahedral Fe(III)-for-Si(IV) substitutions may, therefore, be partly linked to the aqueous Si speciation and thus strongly dependent on the pH of the crystallization fluids.

Temperature Effects on the Crystallinity of Synthetic Nontronite and Implications for Nontronite Formation in Columbia River Basalts

AbstractThe formation conditions of the ferric smectite nontronite are not fully understood. The present study couples experimental and analytical data with field observations in an attempt to constrain the rate and temperature of formation of naturally occurring nontronites from Columbia River Basalt flows. Synthetic Fe-Al-Si gels were incubated at temperatures ranging from 4 to 150°C for 4 weeks. Samples were analyzed using Fe K-edge X-ray fluorescence spectroscopy (XAFS). Spectra of the synthesized nontronites were compared with spectra of natural samples collected from weathered Columbia River Basalt flows. Cation ordering in the synthetic samples increased with incubation temperature, but the synthetic clays did not approach the degree of crystal ordering of the natural nontronite samples. These observations suggest that highly ordered natural nontronites require longer crystallization times than are typically used in laboratory experiments. The natural samples were found filling open cracks near flow surfaces, indicating that the clays formed at temperatures below the boiling point of water. A comparison of experimental and field timescales with other estimates of nontronite growth rates suggests that natural nontronite crystallization in the region must have occurred at ambient, near-surface temperatures over timescales of up to millions of years.

Hydrothermal nontronite formation at Eolo Seamount (Aeolian volcanic arc, Tyrrhenian Sea)

A sediment core containing a yellowish-green clay bed was recovered from an area of extensive hydrothermal deposition at the SE slope of the Eolo Seamount, Tyrrhenian Sea. The clay bed is composed of pure nontronite (described for the first time in the Tyrrhenian Sea), which appears to be the most aluminous nontronite ever found among the seafloor hydrothermal deposits. The high Al content suggests precipitation from Al-containing hydrothermal solutions. The REE distribution of the Eolo nontronite has a V-shape pattern. The heavy REE enrichment is in part due to their preferential partitioning in the nontronite structure. This enrichment was possibly further enhanced by the HREE preferential sorption on bacterial cell walls. The light REE enrichment is the result of scavenging uptake by one of the nontronite precursors, i.e., poorly-ordered Fe-oxyhydroxides, from the hydrothermal fluids. Oxygen isotopic composition of the nontronite yields a formation temperature of 30 °C, consistent with a low-temperature hydrothermal origin. The relatively radiogenic Nd isotopic signature of the nontronite compared to the present-day Mediterranean seawater indicates that approximately half of Nd, and presumably the rest of the LREE, are derived from local volcanic sources. On the other hand, 87Sr/ 86Sr is dominated by present-day seawater Sr. Scanning electron microscopy investigation revealed that the nontronite is composed of aggregates of lepispheres and tube-like filaments, which are indicative of bacteria assisted precipitation. Bacteria inhabiting this hydrothermal site likely acted as reactive geochemical surfaces on which poorly-ordered hydrothermal Fe-oxyhydroxides and silica precipitated. Upon aging, the interactions of these primary hydrothermal precipitates coating bacterial filaments and cell walls likely led to the formation of nontronite. Finally, the well-balanced interlayer and layer charges of the crystal lattice of seafloor hydrothermal nontronite decrease its sorption capacity to zero. Thus the ubiquitous nontronite precipitation along the active plate boundaries and around the hot spots has no significant impact on oceanic trace element chemistry.

Formation of Nontronite from Oxidative Dissolution of Pyrite Disseminated in Precambrian Felsic Metavolcanics of the Southern Iberian Massif (Spain)

AbstractThis paper describes a rare occurrence of nontronite associated with sulfide-bearing felsic metavolcanics, providing evidence of colloidal deposition in open spaces as result of a low-temperature water-rock interaction. Microbotryoidal masses of green nontronite with impurities of kaolinite, illite, barite, amorphous silica and iron oxyhydroxides are found as vein and cavity fillings in deeply kaolinized rhyolites and rhyolitic tuffs of Precambrian age, at Oliva de Merida in SW Spain. Clay mineral characterization has been carried out by X-ray diffraction, infrared spectroscopy, thermal analysis, analytical electron microscopy and stable isotope (oxygen and hydrogen) analysis. Nontronite was formed under low-temperature alteration conditions, from a continuous sequence of reactions and aqueous solution compositions, involving two basic processes that acted in concert: oxidative dissolution of pyrite and hydrolysis of K-feldspar. After acidity neutralization, dissolved silica released by incongruent dissolution of K-feldspar reacted with ferric sulfate derived from pyrite oxidation to form nontronite under oxidizing conditions, in the presence of relatively warm meteoric water.

Formation of Nontronite from Oxidative Dissolution of Pyrite Disseminated in Precambrian Felsic Metavolcanics of the Southern Iberian Massif (spain)

Formation of Nontronite from Oxidative Dissolution of Pyrite Disseminated in Precambrian Felsic Metavolcanics of the Southern Iberian Massif (spain)

Possible Role of Microbial Polysaccharides in Nontronite Formation

AbstractNontronite and microbes were detected in the surface layers of deep-sea sediments from Iheya Basin, Okinawa Trough, Japan. Nontronite, an Fe-rich smectite mineral, was embedded in acidic polysaccharides that were exuded by microbial cells and electron microscopy showed that the nontronite layers were apparently oriented in the polysaccharide materials. We propose that the formation of nontronite was induced by the accumulation of Si and Fe ions from the ambient seawater and that extracellular polymeric substances (EPS) served as a template for layer-silicate synthesis. Experimental evidence for this hypothesis was obtained by mixing a solution of polysaccharides (dextrin and pectin) with ferrosiliceous groundwater. After stirring the mixture in a sealed vessel for two days, and centrifuging, Fe-rich layer silicates were identified within the precipitate of both the dextrin and pectin aggregates, whereas rod-shaped or spheroidal Si-bearing iron hydroxides were found in the external solution. Microbial polysaccharides would appear to have affected layer-silicate formation.

Biogenic Nontronite from Marine White Smoker Chimneys

AbstractClay samples of greenish colour were collected from submarine hydrothermal chimneys of the Galapagos Rift and Mariana Trough. Mineralogical and chemical investigations of the clay by scanning and transmission electron microscopy, X-ray diffraction, differential thermal analysis, infrared-spectros-copy, X-ray fluorescence, and determination of specific surface area, and oxygen isotope composition identify it as a well crystallized nontronite. This nontronite of hydrothermal origin has a nearly monomineralic character, a low Al-content, and a formation temperature of 21.5 to 67.3°C. The most remarkable characteristic, however, of the nontronite deposit is its microstructure, a network of microtubes composed of fine frequently folded clay sheets. These delicate filaments show close similarity in size and form to sheath forming bacteria. The correlation between clay mineral and chemical characteristics, as well as biological conditions at marine hydrothermal smoker chimneys, let us suggest that Fe oxidizing, sheath forming bacteria are playing a decisive role in nontronite formation at these sites.

Hk-Ordering in Aluminous Nontronite and Saponite Synthesized Near 90°C: Effects of Synthesis Conditions on Nontronite Composition and Ordering

AbstractIn studies on the fate of aluminium in the environment, nontronite and saponite have been obtained by synthesis in reducing alkaline conditions close to those prevailing in poorly drained soils developed from limestones. The two minerals obtained have different structures and organizations corresponding to two different growth and/or maturation mechanisms. High-resolution transmission electron microscopy of ultrathin sections of a synthetic aluminous nontronite embedded in resin showed the presence of crystallites consisting of two to ten co-terminating parallel layers, indicating synchronous growth. Electron diffraction showed that the individual crystallites had hk-ordering, i.e., orientation of layers with respect to the six-fold pseudosymmetry of the unit cell. Deposits of a synthetic saponite included hk-ordered crystallites and crinkled films with turbostratic stacking. The two saponite phases had slightly different b dimensions. Lattice fringe images of sections of saponite embedded in resin showed a high angular disorientation of the layers in the stacking direction, suggesting multiple nucleation and growth of individual layers, subsequently aggregated with imperfect parallelism.Exploration of the synthetic conditions of the aluminous nontronite indicated that calcium was essential for an hk-ordered product. Syntheses using potassium or sodium hydroxides and carbonates for pH control gave poorly organized nontronites. Hydrazine was not essential for nontronite formation, but better crystallized products—judging by their IR spectra—were obtained in its presence by maintaining reducing conditions in the early stages of synthesis. Attempts to prepare ferruginous beidellites under similar conditions to those in which aluminous nontronites formed were unsuccessful.

Oxygen isotope geothermometry and origin of smectites in the Atlantis II Deep, Red Sea

The sediments underlying the hot brine pool of the Atlantis II Deep, a localised area of geothermal activity in the Red Sea, comprise a diversity of facies characterised by combinations of one or more of five species assemblages, sulphide, sulphate, silicate, oxide and carbonate, each including several mineral phases. The silicate mineral assemblage is dominated by geothermal authigenic smectites. Previous studies of these smectites have reported iron-rich varieties only, nontronite in particular, and only one environment of formation. In three cores from the Southwest Basin of the Atlantis II Deep, of the present study, three smectites comprising two species have been distinguished [10,21] evidently from three different environments of formation. Two of these smectites are nontronites, one from sulphide/silicate/amorphous facies, the other from silicate/carbonate/oxide facies. The third is a montmorillonite/beidellite from sulphate/sulphide/silicate/oxide facies. The oxygen isotopic compositions of samples of the three smectites have been determined from which formation temperatures have been calculated. Six samples of the “anoxic” nontronite have formation temperatures in the range 90–140°C. A single sample of the “oxic” nontronite has a formation temperature of about 80°C. Four samples of the montmorillonite/beidellite have formation temperatures in the range 160–200°C. The formation temperature range of the two nontronites is intermediate between the temperature of the brine at or prior to discharge (up to 250°C [12]) and the temperature of the brine pool in the Deep (about 50–60°C [13,14]). The nontronite formation temperature range reflects genesis by combination of isotopically light silica supplied by the incoming brine and isotopically heavier iron oxyhydroxide settling from the upper layers of the brine pool. Evidently, the “anoxic” nontronite forms at greater depth (hotter) in the brine pool than the “oxic” nontronite resulting in a relatively greater contribution from silica but diminished contribution from iron oxyhydroxide in the former compared to the latter. The wide range of the formation temperatures for the “anoxic” nontronite is related to the different actual locations of the samples in the sulphide/silicate/amorphous facies. The formation temperature range of the montmorillonite/beidellite is approaching the estimated temperature of the brine at or prior to discharge. The montmorillonite/beidellite formation temperature range reflects genesis by combination of isotopically light silica and aluminium, both supplied by the incoming brine, at the site of an active discharge vent. The wide range of the formation temperatures for the montmorillonite/beidellite may in part reflect a possible thermal event at the brine source, likely to have occurred during deposition of the sulphate/sulphide/silicate/oxide facies and which, it appears, has contributed to the formation of this facies [10,20].

The geochemistry of transitional sediments recovered from the Galapagos Hydrothermal Mounds Field during DSDP Leg 70—implications for mounds formation

A total of 58 sediment samples have been analysed from holes drilled in the Galapagos Hydrothermal Mounds Field during Leg 70 of the Deep Sea Drilling Project. The sediments were selected, on the basis of physical and chemical characteristics, as being transitional in nature between pure nontronite of hydrothermal origin and typical pelagic sediment. Analyses were carried out for Li, Be, K, Mg, Ca, Si, Al, Mn, Fe, Sr, Ti, V, Cr, Co, Ni, Cu, Zn, Mo and Pb. The variations in chemical composition of the samples indicate that substantial volume changes have occurred in the pelagic sediment blanket during nontronite formation, a process which occurs predominantly within pre-existing sediments by precipitation over a redox gradient and dissolution of the biogenic component of the pelagic sediment. The elements Ca, Sr, Mn, Ni and Zn are lost overall from the pelagic sediment blanket whilst K, Mg, Fe, Si and possibly Mo and Pb also, are added. Li, Be, Al, Ti, V, Cr and Co remain immobile during the process of nontronite formation. Mass balance calculations for nontronite formation show that the overall morphology of the mounds can be explained satisfactorily in terms of volume changes which have occurred due to dissolution of biogenic sediment from and addition of nontronite to the sediment blanket.

Pore water chemistry of the Mounds Hydrothermal Field, Galapagos Spreading Center: Results from Glomar Challenger Piston Coring

On DSDP Leg 70, Glomar Challenger piston cored hydrothermal MnO2‐encrusted nontronite mounds and adjacent pelagic sediments through to basement. Pore waters were collected by centrifuging, squeezing, and in situ sampling; analyses are presented here for Ca, Mg, Si, NH3, Mn, and Fe. Our results confirm Maris and Bender's (1982) conclusions that hydrothermal solutions enriched in Ca by 1–2 mM and depleted in Mg by ∼2 mM are upwelling through the mounds and the surrounding pelagic sediments. Si, NH3, and Mn2+ concentrations generally increase upcore, reflecting addition of products of metabolic reactions to upwelling hydrothermal solutions. Pore water iron concentrations decrease upcore, probably as a result of oxidation and precipitation of upwelling hydrothermal iron. The formation of nontronite (Fe(III)4Si8O20(OH)4) involves oxidation of dissolved Fe2+. Several models, constrained by the electron balance, are proposed to explain the process of nontronite formation. The stratigraphy of the mounds (thick nontronite covered by a thin MnO2 crust) may be explained by postulating Fe2+ oxidation by MnO2 and replacement of MnO2 by nontronite at the base of the MnO2 crust, followed by upward migration of Mn2+ and precipitation of MnO2 at the sediment water interface.

Elemental and isotopic compositions of some metalliferous and pelagic sediments from the Galapagos mounds area, DSDP Leg 70

Nontronite, the main metalliferous phase of the Galapagos mounds, occurs at a subsurface depth of ∼ 2–20 m; Mn-oxide material is limited to the upper 2 m of these mounds. The nontronite forms intervals of up to a few metres thickness, consisting essentially of 100% nontronite granules, which alternate with intervals of normal pelagic sediment. The metalliferous phases represent essentially authigenic precipitates, apparently formed in the presence of upwelling basement-derived hydrothermal solutions which dissolved pre-existent pelagic sediment. Electron microprobe analyses of nontronite granules from different core samples indicate that: (1) there is little difference in major-element composition between nontronitic material from varying locations within the mounds; and (2) adjacent granules from a given sample have very similar compositions and are internally homogeneous. This indicates that the granules are composed of a single mineral of essentially constant composition, consistent with relatively uniform conditions of solution Eh and composition during nontronite formation. The Pb-isotopic composition of the nontronite and Mn-oxide sediments indicates that they were formed from solutions which contained variable proportions of basaltic Pb, introduced into pore waters by basement-derived solutions, and of normal-seawater Pb. However, the Sr-isotopic composition of these sediments is essentially indistinguishable from the value for modern seawater. On the basis of 18 O 16 O ratios, formation temperatures of ∼20–30°C have been estimated for the nontronites. By comparison, temperatures of up to 11.5°C at 9 m depth have been directly measured within the mounds and heat flow data suggest present basement-sediment interface temperatures of 15–25°C.

Peloidal nontronite formation in Recent sediments of Lake Chad

Chemical analyses, electron-microprobe and X-ray diffraction data indicate that peloidal oolitic sand-size grains composed basically of Fe oxides are transformed into the Fe(III)-smectite nontronite in sediments of Lake Chad. The process begins in the interior of the grain and proceeds outward. Morphology of the grains suggests that the process results in an increase in the volume of the grain. Comparison is made with chamosite (ferro-aluminous 7 Å chlorites of sedimentary origin) and glauconite formation from peloidal material. It would appear that silica activity in solution should be low and pH must be near neutral conditions and Eh near zero for the formation of nontronite from Fe oxide.