Iron Mineralogy Research Articles

Mössbauer study of the Fe mineralogy in two different Argentine soils

Two Argentine soils featuring different characteristics and compositions (mollisols and ultisols) have been studied by Mossbauer spectroscopy and X-ray diffraction. The first type has a weakly developed profile with a solum thickness of 40 cm; the Fe oxyhydroxides are present in low concentrations in mixtures with other slightly weathered minerals (e.g., quartz, feldspars, 2 : 1 phyllosilicates, etc.). The second one is a typic kandihumult, which is a highly weathered red coloured, deep soil. The Fe oxyhydroxides are abundant, mixed mainly with kaolinite clay minerals. Analyses of iron mineralogy show hematite and goethite in both soils. Their ratio is low in the first case and high in the last case. Magnetite–maghemite are also present in both situations, but in the mollisol their content is much lower than in the ultisol. The mineralogy found is related to the different lithological characteristics and processes of pedological evolution on both soils.

EVIDENCE FROM MOSSBAUER SPECTROSCOPY OF NEO-FORMATION OF MAGNETITE/MAGHEMITE IN THE SOILS OF LOESS/PALEOSOL SEQUENCES IN CHINA

Samples of four different loess/paleosol couplets of a loess sequence in Huangling (China) have been studied with 57Fe Mossbauer spectroscopy. Each sample was separated into strongly, weakly and very weakly magnetic fractions. The iron mineralogy of the strongly magnetic fractions of both loess and soils consists of magnetite/maghemite and hematite together with some silicates. The soils contain some additional small-particle maghemite. From the spectral behaviour a similarity in terms of morphology and crystal chemistry for hematite throughout the whole section could be inferred. The ratio of iron in magnetite and maghemite to that in hematite differentiates well between the loess and soil samples. These results strongly suggest the neo-formation of magnetite/maghemite in the soils.

Mineralogy of iron precipitates in a constructed acid mine drainage wetland : A. D. Karathanasis & Y. L. Thompson, Soil Science Society of America Journal, 59(6), 1995, pp 1773–1781

Mineralogy of iron precipitates in a constructed acid mine drainage wetland : A. D. Karathanasis & Y. L. Thompson, Soil Science Society of America Journal, 59(6), 1995, pp 1773–1781

Mineralogy of Iron Precipitates in a Constructed Acid Mine Drainage Wetland

AbstractPrecipitates forming at various stages of acid mine drainage treatment in a high metal load (≈1000 mg L−1 Fe) and low pH (≈3) constructed wetland were characterized by chemical dissolution, x‐ray diffraction, thermal analysis, and scanning electron microscopy. Minerals precipitating in flumes and in entry wetland cells lacking vegetation included poorly crystalline ferrihydrite, lepidocrocite, goethite, possibly an akaganéite‐like mineral, and high Fe/S ratio Fe oxyhydroxysulfates (similar to schwertmannite). Within vegetated wetland cells lined with crushed limestone, well‐crystallized gypsum, lepidocrocite, and Fe‐oxyhydroxysulfate minerals with low Fe/S ratios were accompanied by gradual reductions in ferrihydrite and akaganéite. The Fe/S molar ratios of Fe oxyhydroxysulfates in flume precipitates averaged 5.2 ± 0.3, while those of cell precipitates averaged ≈3.5 ± 0.5. The oxalate‐extractable (Feox) to total (Fet) Fe fraction of the precipitates was considerably higher in wetland cells (1.1 ± 0.3), where organic C was 10‐fold higher than in entry flumes (0.7 ± 0.1). Scanning electron micrographs of flume precipitates showed a fiber‐like morphology of densely aggregated spherical particles, 1.5 to 2.0 mm in diameter, with a closely packed microcrystalline matrix. Precipitates collected from vegetated wetland cells formed aggregates of somewhat smaller diameter spherical particles with grassy surfaces or finger‐like projections entangled with bacterial cells. The overall composition of the precipitates suggested that the Fe chemistry is controlled primarily by the solubility of Fe oxyhydroxides in flumes and by S‐enriched Fe oxyhydroxysulfates inside the wetland cells. Although jarosite and goethite are thermodynamically favored in the wetland cells, their formation appeared to be inhibited by the presence of organics and the precipitation of Fe oxyhydroxysulfates and gypsum.

Hematite, pyroxene, and phyllosilicates on Mars: Implications from oxidized impact melt rocks from Manicouagan Crater, Quebec, Canada

Visible and near‐IR reflectivity, Mössbauer, and X ray diffraction data were obtained on powders of impact melt rock from the Manicouagan Impact Crater located in Quebec, Canada. The iron mineralogy is dominated by pyroxene for the least oxidized samples and by hematite for the most oxidized samples. Phyllosilicate (smectite) contents up to ∼15 wt % were found in some heavily oxidized samples. Nanophase hematite and/or paramagnetic ferric iron is observed in all samples. No hydrous ferric oxides (e.g., goethite, lepidocrocite, and ferrihydrite) were detected, which implies the alteration occurred above 250°C. Oxidative alteration is thought to have occurred predominantly during late‐stage crystallization and subsolidus cooling of the impact melt by invasion of oxidizing vapors and/or solutions while the impact melt rocks were still hot. The near‐IR band minimum correlated with the extent of aleration (Fe3+/Fetot) and ranged from ∼1000 nm (high‐Ca pyroxene) to ∼850 nm (bulk, well‐crystalline hematite) for least and most oxidized samples, respectively. Intermediate band positions (900–920 nm) are attributed to low‐Ca pyroxene and/or a composite band from hematite‐pyroxene assemblages. Manicouagan data are consistent with previous assignments of hematite and pyroxene to the ∼850 and ∼1000 nm bands observed in Martian reflectivity spectra. Manicouagan data also show that possible assignments for intermediate band positions (900–920 nm) in Martian spectra are pyroxene and/or hematite‐pyroxene assemblages. By analogy with impact melt sheets and in agreement with observables for Mars, oxidative alteration of Martian impact melt sheets above 250°C and subsequent erosion could produce rocks and soils with variable proportions of hematite (both bulk and nanophase), pyroxene, and phyllosilicates as iron‐bearing mineralogies. If this process is dominant, these phases on Mars were formed rapidly at relatively high temperatures on a sporadic basis throughout the history of the planet. The Manicouagan samples also show that this mineralogical diversity can be accomplished at constant chemical composition, which is also indicated for Mars from analyses of soil at the two Viking landing sites.

Drying and oxidation effects on the magnetic properties of sulfidic material during oxidation

Laboratory experiments aimed at understanding the changes in magnetic properties of a sulfidic clay during its oxidation are described. Samples of a sulfidic clay from an acid sulfate soil were slowly oxidized and their magnetic susceptibility (χ) and remanence properties (ARM and SIRM) were measured over a 28 week period. Magnetic properties were measured for the undried material and for subsamples which had been dried by rapid air-drying or by extraction of water with dioxane. Magnetic susceptibility decreased during the first 12 weeks of oxidation and then increased. Samples dried in dioxane showed similar values to the undried material, but air drying caused decreases in χ in samples taken up to 7 weeks of oxidation. Thereafter, there was no effect of drying on χ. In contrast to susceptibility, the remanence properties decreased throughout the experiment, to 8% of their initial values. The results were interpreted in terms of rapid chemical oxidation of a labile magnetic compound (possibly greigite) during the early stages of oxidation and by air drying, and the biological oxidation of pyrite during later stages of oxidation. Associated with these reactions was the formation of a paramagnetic compound, probably ferrihydrite. The effects of changes in iron mineralogy during 28 weeks of oxidation on χ are shown schematically.

Speciation of Fe(II) and Fe(III) in Contaminated Aquifer Sediments Using Chemical Extraction Techniques

The iron mineralogy of aquifer sediments was described by chemical extraction techniques. Single-step extractions including 1 M CaCl 2 , NaAc, oxalate, dithionite, Ti(III)-EDTA, 0.5 M HCl, 5 M HCl, hot 6 M HCl, and a sequential extraction by HI and Cr II HCl were tested on standard iron minerals and nine aquifer sediments from different redox environments sampled in a landfill leachate plume. Ion-exchangeable Fe(II) is easily quantified by anaerobic CaCl 2 extraction. A rapid indication of the redox status of a sediment sample can be achieved by a 0.5 M HCl extraction. This extraction gives an indication of the content of amorphous Fe(III) and reduced Fe(II) species such as FeS and FeCO 3 , though the fractions are not quantified

Water table depths and color patterns in soils developed from red parent materials in Michigan, USA

Relationships between soil color patterns and depth and duration of water tables were studied using piezometers in three soil hydrosequences that had developed from different colored parent materials. Correlation coefficients relating color indices and percent of time saturated were less for the hydrosequences developed from red (5YR and 7.5YR) parent materials (5YR materials are lacustrine deposits, 7.5YR materials are till) than those developed from 10YR parent materials (till). A modified color index was a better predictor of water saturation in soils developed from 5YR parent materials. The presence of hematite in the soils developed from red parent materials is thought to cause these soils to be less gray than would be predicted from landscape position and piezometer data. Iron mineralogy, at least partially, explains why hue was not as important as chroma in predicting water saturation in these hydrosequences. Iron contents in high chroma mottles were greater, whereas, those of low chroma mottles were less than those in matrices, suggesting Fe has moved in saturated horizons of these soils. Insufficient low chroma colors was the primary reason for misclassification of soils developed from red parent materials and is thought to be related to the presence of hematite.

The nanophase iron mineral(s) in Mars soil

A series of surface-modified clays containing nanophase (np) iron oxide/oxyhydroxides of extremely small particle sizes, with total iron contents as high as found in Mars soil, were prepared by iron deposition on the clay surface from ferrous chloride solution. Comprehensive studies of the iron mineralogy in these "Mars-soil analogs" were conducted using chemical extractions, solubility analyses, pH and redox, x ray and electron diffractometry, electron microscopic imaging, specific surface area and particle size determinations, differential thermal analyses, magnetic properties characterization, spectral reflectance, and Viking biology simulation experiments. The clay matrix and the procedure used for synthesis produced nanophase iron oxides containing a certain proportion of divalent iron, which slowly converts to more stable, fully oxidized iron minerals. The clay acted as an effective matrix, both chemically and sterically, preventing the major part of the synthesized iron oxides from ripening, i.e., growing and developing larger crystals. The precipitated iron oxides appear as isodiametric or slightly elongated particles in the size range 1-10 nm, having large specific surface area. The noncrystalline nature of the iron compounds precipitated on the surface of the clay was verified by their complete extractability in oxalate. Lepidocrocite (gamma-FeOOH) was detected by selected area electron diffraction. It is formed from a double iron Fe(II)/Fe(III) hydroxy mineral such as "green rust," or ferrosic hydroxide. Magnetic measurements suggested that lepidocrocite converted to the more stable maghemite (gamma-Fe2O3) by mild heat treatment and then to nanophase hematite (alpha-Fe2O3) by extensive heat treatment. After mild heating, the iron-enriched clay became slightly magnetic, to the extent that it adheres to a hand-held magnet, as was observed with Mars soil. The chemical reactivity of the iron-enriched clays strongly resembles, and offers a plausible mechanism for, the somewhat puzzling observations of the Viking biology experiments. Their unique chemical reactivities are attributed to the combined catalytic effects of the iron oxide/oxyhydroxides and silicate phase surfaces. The reflectance spectrum of the clay-iron preparations in the visible range is generally similar to the reflectance curves of bright regions on Mars. This strengthens the evidence for the predominance of nanophase iron oxides/oxyhydroxides in Mars soil. The mode of formation of these nanophase iron oxides on Mars is still unknown. It is puzzling that despite the long period of time since aqueous weathering took place on Mars, they have not developed from their transitory stage to well-crystallized end-members. The possibility is suggested that these phases represent a continuously on-going, extremely slow weathering process.

A Mössbauer study of an impactite from the Monturaqui crater

The iron mineralogy of samples of the Monturaqui impactite has been studied by Mossbauer spectroscopy. Magnetite, maghemite, goethite and a ferrous glass phase were identified. In a magnetic separate a bcc-structured iron-nickel alloy was identified in addition to the oxide phases. The oxides have formed by weathering of iron-nickel alloys.

The mineralogy and morphology of iron and manganese oxides in an imperfectly-drained Scottish soil

The distribution, mineralogy and morphology of the iron and manganese oxides in an imperfectly-drained Scottish soil, with a diffuse Fe/Mn pan, have been examined using electron microscopy, X-ray diffraction and chemical dissolution. The Fe oxide mineralogy above and within the Fe-pan is dominated by lepidocrocite, with subsidiary amounts of ferrihydrite. In the Mn-pan itself, feroxyhyte is present and below this, goethite predominates. Vernadite is the main manganese oxide mineral within the Mn-pan. The conditions necessary for the synthesis of these minerals in the laboratory are broadly consistent with the soil conditions implied from their location within the soil profile.

Differential dissolution of hematite and goethite in dithionite and its implication on soil yellowing

This paper describes a modification of the deferrification method of Mehra and Jackson based on the parallel utilization at low temperature (20°C and occasionally 4°C) of the citrate-bicarbonate-dithionite (DCB) reagent on the one hand and citrate-bicarbonate (CB), on the other. The differences in amounts of Fe and Al solubilized by these two reagents during different periods of time were used for characterizing the chemical composition of fractions of free iron undergoing reduction. Simultaneously, the mineralogy of the same fractions was monitored by a differential X-ray diffraction method (DXRD) and the changes in colour of the residues of deferrification were recorded. With reddish samples containing the hematite-goethite pair, the application of the reducing reagent led to the preferential dissolution of Al-poor hematites over Al-rich goethites and reproduced in vitro the yellowing process often observed in the environments from which the samples were selected. It is noted, however, that the deferrification may not be perfectly selective for the re-pigmenting hematitic minerals but can affect also that part of the goethitic population made of Al-poor goethites. These observations suggest that, depending on the mineralogy of free iron, two types of yellowing are likely to occur in situ, the first one solubilizing hematite and leaving goethite unaffected and the second one mobilizing both hematite and Al-poor goethite. In both cases, the yellowing must be regarded as a process promoting net iron losses and relative alumina accumulations.

Mössbauer study of illitic clays containing iron-rich impurities

The iron mineralogy of nineteen illitic clays from eastern Bavaria was studied by Mossbauer spectroscopy and X-ray powder diffraction. Mossbauer spectra of the <2 μm fraction were taken at RT, 120 K and 4.2 K. The clays contain both paramagnetic Fe3+ and Fe2+. Superparamagnetic oxides are frequently present. The Fe2+ quadrupole splitting and the ratio of Fe3+ at 4.2 K to Fe2+ at 120 K are correlated and define two groups distinguished by their mineral content. The samples were heated systematically for 48 h up to 1250°C in steps of 50°C. One clay which is rich in chlorite and Fe(II) was studied in detail after firing in air and following a reduction for 3 h at 800°C with charcoal. The transformations of the mineral phases with temperature as shown by X-ray diffraction are also evident in the Mossbauer spectra.

Reactive iron in marine sediments

A combined field/laboratory study has been undertaken to explore the mineralogy, concentrations and reactivity (towards sulfide) of iron in marine sediments. Also considered is the importance of bacterial Fe liberation in regulating pore-water chemistry. Two contrasting marine environments are included; one is the relatively Fe-poor FOAM site and the other is the Fe-rich sediment of the subaqueous Mississippi Delta. Results show that oxide minerals are the most important Fe phases in early diagenetic pyrite formation. However, viewed separately, lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite. When Fe oxides are present in relatively high concentrations, dissolved sulfide is nearly absent from sediment pore waters (with concomitant high concentrations of dissolved Fe), even in the presence of active sulfide production by sulfate reduction. A combination of experimental results and diagenetic modelling shows that in some sediments pore water Fe finds it origin in the bacterial reduction of iron oxides. This seems the case even though greater amounts of Fe are reduced by reaction of sulfide with iron oxides. It appears that distinct microenvironments may exist in marine sediments, where, in one microenvironment, sulfide reacts with Fe oxides locally precipitating Fe sulfide minerals. In another, Fe reduced and solubilized by microorganisms migrates freely into solution.

Iron mineralogy in sediments. A Mössbauer study

Sequential chemical extraction is used to show that the Mössbauer spectrum of a lacustrine sediment is deceptively simple. Only two pairs of doublets arise when at least four chemically distinct forms of Fe are present. Recognition of this effect increases the power of Mössbauer spectroscopy as a tool in studies of the redox chemistry of Fe in sediments.

Mössbauer Spectroscopic Study of the Iron Mineralogy of Post-Glacial Marine Clays

AbstractThree post-glacial marine clays from eastern Canada and one marine clay from Japan have been studied by Mössbauer spectroscopy to ascertain their iron mineralogy. Small amounts of hematite (in two samples) and magnetite (in one sample) were found in the Canadian clays, and hematite was detected in the Japanese clay. The major spectral components were ferrous and ferric doublets, consistent with X-ray powder diffraction results that show chlorite, mica, and amphibole in the Canadian samples and smectite in the Japanese sample. Citrate-dithionite extraction removed hematite and most of the magnetite from these samples. Acid-base extraction also removed chlorite and some mica from the Canadian samples. Samples treated by these extractions had appreciably lower geotechnical yield stresses at given water contents.

Mössbauer Effect Studies of Iron in Kaolin. II. Surface Iron

Abstract57Fe Mössbauer spectra have been used to interpret the effects of different cleaning processes on the iron mineralogy of a Weipa, Australia, kaolin. A magnetically separated fraction contained 28% of its iron as hematite likely of secondary origin. An initial centrifugal size separation was shown to give an improved final product, and oxalic acid was found to be more efficient at removing Fe from the kaolinite surface than dithionite bleach. The Mössbauer spectra clearly show that beneficiation steps which give a substantial increase in kaolin brightness result in only minor changes in the clay iron mineralogy. Similar results were also obtained for two commercially available kaolins.

A mössbauer study of the iron mineralogy of acid-leached bauxite

The removal of iron from bauxite via some form of leaching step is particularly desirable because of the greatly enhanced value of low-iron bauxite in the refractory, abrasives and chemical industries, Previous investigations have shown that the iron minerals in Weipa bauxite are selectively dissolved under certain conditions, but that the extent of iron removal is limited unless total extraction of other mineral components takes place. In an effort to understand this behaviour, Mössbauer effect measurements have been made on samples of Weipa bauxite both before and after leaching with hydrochloric acid. Because of the finely divided state of the goethite present it has been necessary to take measurements at both 300 K and 4.2 K in order to fully characterize the iron mineralogy of the bauxites. Prior to leaching, the majority of the iron is present as hematite, which may be aluminium substituted by up to about 10%. Smaller quantities of aluminous goethite (usually about 15% substituted) and iron present within the structure of kaolin associated with the bauxite have also been found. About 95% of the hematite is removed during leaching, together with some of the gibbsite and boehmite present. A significant precipitation of goethite, containing a variable amount of aluminium, takes place during the leaching. From the limited number of samples investigated it appears that at higher acid concentrations the degree of alumiuium substitution of the precipitated goethite may decrease. Other studies of the forms of iron precipitated in aciiic chloride media have shown β-FeOOH (akaganéite) to be the principal product. It is believed that the finely divided goethite initially present in the bauxite has nucleated the precipitation thus leading to the formation of goethite rather than akaganéite.

A Mössbauer spectroscopic and X-ray diffraction study of the iron mineralogy of some sediments from the Southwestern Pacific Basin

The results of a 57Fe Mössbauer spectroscopic and X-ray diffraction study of four pelagic clay sediments from the Southwestern Pacific Basin collected at increasing distances from New Zealand, on a New Zealand—Rarotonga transect, are presented. These show that, with increasing distance from New Zealand, the Fe 3+/Fe 2+ ratio of the sediment increases as a result of the increasing contribution of the X-ray amorphous secondary hydrous iron oxides, notably ferrihydrite, due to the longer sedimentation periods and consequent greater degree of oxidation of the sediments. This increase in the Fe 3+/Fe 2+ ratio is correlated with the change in colour of the sediments from pale yellowish to dusky brown, as well as a number of other sediment parameters. Apart from a slight increase in the relative abundance of montmorillonite with increasing distance from New Zealand, the abundances of the other clay minerals, illite and kaolinite, and of chlorite in the sediments are approximately equal. Quartz and feldspar with minor augite are the dominant detrital minerals. Calcite is present in minor amounts in some of the sediments.

Mössbauer investigation of the transformations of the iron minerals in oil shale during retorting

Iron phases in raw and retorted shales have been studied from the TOSCO II, Paraho, and Lawrence Livermore Laboratory (LLL) 125 kg retorting processes by Mössbauer effect spectroscopy. Using the Mössbauer effect, changes in the iron mineralogy during retorting were monitored. Results show that the pyrite fraction in the TOSCO II, and LLL hydrogen run S-9 samples underwent changes during retorting whereas the iron-containing carbonates did not. As in other pyrolysis processes, FeS 2 breakdown is affected by the presence of organic matter. Hydrogen released by the indigenous kerogen acts to reduce pyrite to the magnetic sulphide pyrrhotite. In all of the retorts containing oxygen, carbonate breakdown was observed prior to pyrite oxidation. Free oxygen is introduced to several retorts. The iron minerals within these systems eventually become a mixture of the oxides hematite and magnetite. This study demonstrates that alteration of specific iron minerals during retorting may be controlled by varying the internal retorting conditions.