Iron Oxide Mineralogy Research Articles

Seasonal Changes In Mineralogy, Geochemistry and Microbial Community of Bacteriogenic Iron Oxides (BIOS) Deposited in a Circumneutral Wetland

Changes in the mineralogy, microbial community structure and aquatic geochemistry of bacteriogenic iron oxides (BIOS) deposited at a circumneutral groundwater seep were recorded seasonally. X-ray diffraction and X-ray absorption spectroscopy demonstrated that 2-line ferrihydrite dominated the iron-bearing mineralogy of the BIOS (>60%). Minor proportions of the more crystalline lepidocrocite were also detected in BIOS collected throughout the year (<30%), but goethite (<15%) was only identified in sediments collected during the spring, summer and fall months. Subsurface porewater profiles generally showed an increase in dissolved equilibrium concentrations of Fe(II), indicative of microbial Fe(III) reduction, and the depth at which appreciable levels of dissolved Fe(II) (and to a lesser extent manganese) arose, and sulfate concentrations declined, become shallower as the seasons progressed from spring to fall. Clone libraries constructed from 16S rRNA gene sequence analysis of surficial BIOS showed the presence of sequences closely affiliated to known Fe(II)-oxidizing and Fe(III)-reducing prokaryotes, pointing towards a tightly coupled microbial iron cycle. Clones that showed closest identity to Fe(II)-oxidizing bacteria in culture, including Gallionella spp. and Sideroxydans spp., made up a significant portion of the clone library, but their relative proportion declined through the winter (73%), spring (26%) and summer (9%). The dominance of the putative microaerophilic Fe(II)-oxidizing chemolithoautotrophs in the winter clone library may be partly due to the lower temperature and dissolved organic carbon concentrations observed during this season, conditions that could have limited mixotrophic or organotrophic growth by mesophilic bacteria. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.

Enriched Iron(III)-Reducing Bacterial Communities are Shaped by Carbon Substrate and Iron Oxide Mineralogy

Iron (Fe) oxides exist in a spectrum of structures in the environment, with ferrihydrite widely considered the most bioavailable phase. Yet, ferrihydrite is unstable and rapidly transforms to more crystalline Fe(III) oxides (e.g., goethite, hematite), which are poorly reduced by model dissimilatory Fe(III)-reducing microorganisms. This begs the question, what processes and microbial groups are responsible for reduction of crystalline Fe(III) oxides within sedimentary environments? Further, how do changes in Fe mineralogy shape oxide-hosted microbial populations? To address these questions, we conducted a large-scale cultivation effort using various Fe(III) oxides (ferrihydrite, goethite, hematite) and carbon substrates (glucose, lactate, acetate) along a dilution gradient to enrich for microbial populations capable of reducing Fe oxides spanning a wide range of crystallinities and reduction potentials. While carbon source was the most important variable shaping community composition within Fe(III)-reducing enrichments, both Fe oxide type and sediment dilution also had a substantial influence. For instance, with acetate as the carbon source, only ferrihydrite enrichments displayed a significant amount of Fe(III) reduction and the well-known dissimilatory metal reducer Geobacter sp. was the dominant organism enriched. In contrast, when glucose and lactate were provided, all three Fe oxides were reduced and reduction coincided with the presence of fermentative (e.g., Enterobacter spp.) and sulfate-reducing bacteria (e.g., Desulfovibrio spp.). Thus, changes in Fe oxide structure and resource availability may shift Fe(III)-reducing communities between dominantly metal-respiring to fermenting and/or sulfate-reducing organisms which are capable of reducing more recalcitrant Fe phases. These findings highlight the need for further targeted investigations into the composition and activity of speciation-directed metal-reducing populations within natural environments.

Iron oxide mineralogy and stable iron isotope composition in a Gleysol with petrogleyic properties

Purpose Properties of Fe oxides are poorly understood in soils with fluctuating water tables and variable redox conditions. The objective of this research was to (a) characterize the mineralogical composition of Fe oxides and (b) determine the relationship to the stable Fe isotope ratio in a soil with temporally and spatially sharp redox gradients.

Micromorphology, mineralogy and geochemistry of lateritic weathering over serpentinite in south-east Cameroon

A laterite profile developed on serpentinite in Lomié, south-east Cameroon, was studied to understand its structure and genesis. Micromorphological, mineralogical and geochemical characteristics were studied, using thin sections, SEM-EDS, X-ray diffraction and chemical analyses. In the lower saprolite, magnesite is dissolved and serpentine is gradually replaced by kaolinite, without loss of the lithic fabric (orientation of kaolinite, distribution of magnetite). This nearly iso-volumetric weathering, at the scale of the sample, can be considered as in situ replacement of serpentine by kaolinite. Iron oxide hypocoatings formed along planar voids, and also smectite formed from solution in the void system. Congruent dissolution of kaolinite starting in the mid saprolite eventually led to a collapse of the fabric and to a chaotic accumulation of fragments of the hypocoatings. In the overlying layer, this fabric is even more strongly expressed, and iron oxides are more concentrated. The weathering profile shows a severe loss of Mg and Si, and an accumulation of Al, Fe, Ti and some minor and trace elements. The overlying layers have fabrics and components different from those recognized for the saprolite, such as clastic quartz grains and iron oxide nodules with various lithic fabrics, pointing to an admixture of allochthonous materials. The appearance of mobile elements (e.g. Rb, Sr, Pb) and certain stable elements (Zr, Nb), as well as the removal of serpentinite-related elements clearly show the mixing with materials derived from micaschists in the vicinity. The presence of a discontinuity in the profile is also confirmed by variations in iron oxide mineralogy. At least three lithological discontinuities were identified.

Microbial and geochemical features suggest iron redox cycling within bacteriogenic iron oxide-rich sediments

The geochemistry, mineralogy and microbiology of bacteriogenic iron oxides (BIOS) deposited at a circumneutral groundwater seep were examined to evaluate the biogeochemical controls on the redox cycle of iron in such environments. X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that 2-line ferrihydrite dominated (60–70%) the BIOS mineralogy, with more crystalline lepidocrocite and goethite phases comprising the remainder, likely the product of Fe(II)-induced dissolution–reprecipitation pathways. Subsurface porewater profiles displayed an increase in dissolved (<0.2μm) equilibrium concentrations of Fe(II), also reflected in peaks in HCl-extractable sedimentary Fe(II) at depth, indicative of biological iron reduction. Interestingly EXAFS analysis of sediments collected from such depths revealed little mineralogical change from the surficial BIOS. Dissolved concentrations of Fe(III), likely present as nanoparticulates or organic complexes, also rose with depth. Scanning electron microscopy images of the surface BIOS samples showed a proliferation of sheath structures characteristic of Fe(II)-oxidizing Leptothrix spp. A 16S rRNA clone library obtained from the surficial BIOS included microorganisms closely associated to known Fe(II)-oxidizing (Gallionella spp., Sideroxydans spp.) and Fe(III)-reducing (Rhodoferax ferrireducens and Geothrix fermentans) bacteria. Analysis of subsurface sediment collected from an Fe(III)-reducing horizon revealed a drop in the proportion of β-Proteobacteria, which hosts numerous Fe(II)-oxidizing genera, and a rise in the δ-Proteobacteria, home to anaerobic metal-reducing prokaryotes. The recognition of clones with close identity to bacteria intimately involved in complementary iron redox transformations in the same surficial BIOS sediment suggests that microbial oxidation and reduction of iron may be tightly coupled in this, and other, shallow sedimentary environments.

Mapping iron oxides and the color of Australian soil using visible–near‐infrared reflectance spectra

Iron (Fe) oxide mineralogy in most Australian soils is poorly characterized, even though Fe oxides play an important role in soil function. Fe oxides reflect the conditions of pH, redox potential, moisture, and temperature in the soil environment. The strong pigmenting effect of Fe oxides gives most soils their color, which is largely a reflection of the soil's Fe mineralogy. Visible–near‐infrared (vis–NIR) spectroscopy can be used to identify and measure the abundance of certain Fe oxides in soil, and the visible range can be used to derive tristimuli soil color information. The aims of this paper are (1) to measure the abundance of hematite and goethite in Australian soils from their vis–NIR spectra, (2) to compare these results to measurements of soil color, and (3) to describe the spatial variability of hematite, goethite, and soil color and map their distribution across Australia. We measured the spectra of 4606 surface soil samples from across Australia using a vis–NIR spectrometer with a wavelength range of 350–2500 nm. We determined the Fe oxide abundance for each sample using the diagnostic absorption features of hematite (near 880 nm) and goethite (near 920 nm) and derived a normalized iron oxide difference index (NIODI) to better discriminate between them. The NIODI was generalized across Australia with its spatial uncertainty using sequential indicator simulation, which resulted in a map of the probability of the occurrence of hematite and goethite. We also derived soil RGB color from the spectra and mapped its distribution and uncertainty across the country using sequential Gaussian simulations. The simulated RGB color values were made into a composite true color image and were also converted to Munsell hue, value, and chroma. These color maps were compared to the map of the NIODI, and both were used to interpret our results. The work presented here was validated by randomly splitting the data into training and test data sets, as well as by comparing our results to existing studies on the distribution of Fe oxides in Australian soils.

Magnetic imprints of pedogenesis in Planosols and Stagnic Alisol from Bulgaria

Iron in its diverse forms in soils is an important indicator of a number of past and/or ongoing environmental processes. Iron oxide mineralogy pertinent to a particular soil type could be obtained with high resolution using magnetic methods. Planosols are a particular soil type characterized by surface water-logging leading to development of great textural differentiation with depth. This study presents magnetic investigation on three Planosols and a Stagnic Alisol from Bulgaria combined with conventional chemical extraction methods (oxalate, citrate–dithionite) and soil property characterization (pH, grain size fractions, and organic carbon content). Extensive magnetic analyses on high-resolution sampled depth profiles have been done for determination of iron (oxyhydr)oxide minerals, their stability and variations of effective magnetic grain size with depth. Zones of intensive iron oxides dissolution – the topmost soil horizons – are magnetically depleted in the content of strongly magnetic minerals, but the paramagnetic content detected through the ratio χ 77/χ 293 is enhanced accordingly. Bleached eluvial horizons, however, contain stable crystalline magnetite/maghemite fraction, albeit in a trace amounts. We ascribe this to the processes of lessivage and translocation of magnetite/maghemite from the topmost humus horizon. The main results on iron oxide mineralogy suggest that thin humus horizons are enriched with pedogenic maghemite. Thermal behavior of magnetic susceptibility suggests that ferrihydrite is present in humus and eluvial horizons of all profiles. In illuvial horizons of Planosols the common mineral is lepidocrocite, while in Alisol — Al-substituted hematite. Planosols and Stagnic Alisol contain both poorly crystalline and well crystalline iron oxides. Increasing precipitation and weathering leads to higher proportion of well crystalline pedogenic phases in the Alisol profile.

East Asian summer monsoon intensity inferred from iron oxide mineralogy in the Xiashu Loess in southern China

The Xiashu Loess, in comparison to the well-studied loess sequences in the Chinese Loess Plateau (CLP), provides a good opportunity for studying East Asian monsoon variations from a southern China perspective. Here we present a study of the iron oxide mineralogy of the Xiashu Loess using integrated geochemical and diffuse reflectance spectroscopy (DRS) measurements as well as magnetic data. Our results show that the free iron oxide (Fe d) to total iron (Fe t) ratio (Fe d/Fe t), hematite (Hm) to goethite (Gt) ratio (Hm/Gt) and saturation isothermal remanent magnetization (SIRM) to magnetic susceptibility ( χ) ratio (SIRM/ χ) all indicate particularly strong summer monsoons during the formation of paleosols PS5 and PS4 (equivalent to Marine Isotope Stage 13 and 11, respectively). However, magnetic susceptibility and Fe d/Fe t are not consistently reliable indicators of summer monsoon intensity for the whole section. Our results indicate that a multi-proxy approach can give a more reliable summer monsoon intensity reconstruction. The summer monsoon shows a cooling trend and a declining of precipitation from 0.5 to ∼0.3 Ma, after which it becomes warmer and wetter towards the top of paleosol PS1 (equivalent to MIS 5). However, PS1 was formed under a relatively cooler temperature and wetter soil conditions in comparison to PS5 and PS4. Such supra-orbital variations in the East Asian summer monsoon superimposed on the effects of glacial–interglacial cycles in southern China are also reflected in the 0.4–0.5 Ma cycle of marine carbon isotopes in the global ocean, possibly indicating a strong link between terrestrial weathering and the global carbon cycle.

Occurrence of magnetite in the sand fraction of an Oxisol in the Brazilian savanna ecosystem, developed from a magnetite-free lithology

It is relatively well established from many pedogenetic studies that, in deeply weathered Oxisols from Central Brazil, magnetite or maghemite are either inherited or transformed from magnetite of the mafic parent material. However, no similar pedogenetic pathways have been reported in the literature for other lithologies, such as limestone and pelitic rocks (shales and slates) of the Bambuí Group in Brazil. In these sedimentary, non-mafic lithologies, magnetic minerals are not likely to occur. Despite that, magnetic nodules were identified in a representative Oxisol pedon developed on this pedodomain, under savanna (Cerrado). Magnetic and non-magnetic fractions of nodules were separated with a hand magnet. Chemical and mineralogical compositions of these nodules were determined by conventional chemical methods, powder X-ray diffractometry (XRD), and 298 K Mössbauer spectroscopy. For the magnetic fraction, containing up to 84 dag/kg of Fe2O3 but also relatively rich in Al, Ti, Cr, and Si, Mössbauer measurements were also made at 4.2 K, without and with an externally applied magnetic field of 8 Tesla, and at 100 K. Mössbauer results and structural Rietveld refinement of the XRD data consistently suggest that the iron oxide mineralogy corresponds to approximately equivalent proportions of hematite and a partially oxidised magnetite, containing 3 dag/kg of iron as FeO. Laboratory tests were conducted in an attempt to produce magnetic material by heating this non-magnetic fraction. The sample was wrapped in filter paper and heated at 300°C for 30 min, and the results were compared with the naturally occurring magnetic nodules. The saturation magnetisation value of the thermally treated sample was found to be σ = 7 J/T.kg, well below σ = 16 J/T.kg of the magnetic soil nodules. Mössbauer and XRD results indicate that the iron oxide mineralogy of this laboratory-produced magnetic sample also corresponds to a mixture of partially oxidised magnetite and hematite. Two other parts of the same non-magnetic, naturally hematite-rich precursor were mixed with charcoal, to act as reducing agent, and oven-heated at 450°C and 600°C, respectively, for 1 h, producing increasing reduction of the hematite to magnetite. These laboratory simulations support the model in which magnetite in these hematite-rich nodules was formed under the influence of seasonal burning regimes of the covering vegetation, followed by partial re-conversion of the magnetite particles to hematite under long-term atmospheric exposure to air. This model is consistent with a long-term, seasonal fire history, assumed for the genesis of the Cerrado ecosystem in the Central Brazil, although a wider validity for the entire Bambuí Group area remains open.

Characterization of iron oxides in Fe-rich concretions from an imperfectly-drained Greek soil: a study by selective-dissolution techniques and X-ray diffraction

Fe-rich concretions commonly occur in Greek soils with alternating drying and waterlogging periods. This study was conducted to characterize the iron oxides in Fe-rich concretions from the upper solum of an Alfisol with seasonal perched water table by the combination of selective dissolution and X-ray diffraction (XRD) techniques. The results showed that more than 75% of the total iron (Fet) was associated with the crystalline and the amorphous Fe oxides, indicating a strong accumulation of free iron oxides (Fed) in concretions. Amorphous iron compounds (Feo) was present at high concentrations and fluctuated with profile depth. The active Fe ratio (Feo/Fed) values that ranged from 0.35 to 0.41 reflected an association of poorly crystalline goethite with some ferrihydrite. The XRD data showed that the Fe-rich concretions consisted of quartz, feldspars, illite and gypsum. The mineralogy of iron oxides in concretions was determined by comparison of XRD patterns for dithionite-citrate-bicarbonate (DCB) treated (deferrated) and untreated (non-deferrated) samples. Poorly crystalline goethite as demonstrated by broad lines in the diffraction patterns and ferrihydrite were the iron oxides detected in the concretions. This mineral assemblage appears to be related to the pedoenvironmental conditions in which the concretions were formed and indicates that the mechanisms governing the formation of crystalline Fe oxides from ferrihydrite are retarded by the presence of crystallization inhibitors.

Climatic interpretation of the Luochuan and Lingtai loess sections, China, based on changing iron oxide mineralogy and magnetic susceptibility

Hematite and goethite, estimated from diffuse reflectance spectra, and magnetic susceptibility (MS) have been determined from both the Luochuan and Lingtai loess sections of the Loess Plateau of central China. Changing concentrations of these iron oxides and MS were used to determine paleoclimatic change. Based on a model that relates pedogenic production of hematite and magnetite to precipitation, we propose two precipitation phases on the Loess Plateau during the last ∼2.6 Ma. From the Wucheng-Red Clay boundary up to paleosol S5 (MIS 13), precipitation on the Loess Plateau was moderate, probably not frequently exceeding 450 mm/yr, and highly seasonal. From S5 to the present, precipitation during interglacial times has gradually increased to the present levels of 600–650 mm/yr for the localities examined. Throughout the Pleistocene, spanning both glacial and interglacial conditions, hematite to goethite ratios display a consistent decrease. This decrease suggests that precipitation on the Loess Plateau has been getting less seasonal and/or has been occurring during times when temperature was lower. In addition to these long-term changes, it also appears that iron oxide records from the Loess Plateau exhibit 400–500-ka cycles from the Wucheng-Red Clay boundary to S5. These 400–500-ka cycles may be analogous to similar length cycles recorded in δ 13C marine records in the South China Sea and may be linked through weathering and silica production as a result of the Asian monsoon.

Quantificação de óxidos de ferro de Latossolos brasileiros por espectroscopia de refletância difusa

A cor é um dos principais atributos considerados na classificação dos solos, indicando a riqueza em matéria orgânica e a natureza mineralógica dos óxidos de ferro presentes. Este trabalho teve por objetivo avaliar a capacidade preditiva da cor do solo, determinada pela técnica de refletância difusa, na quantificação dos óxidos de Fe hematita e goethita em diferentes Latossolos brasileiros. Os espectros de refletância separaram os solos mais claros e amarelados dos mais escuros e de coloração vermelha mais intensa, indicando o aumento da refletividade decorrente da redução dos teores dos pigmentos hematita e matéria orgânica. A técnica espectroscópica permitiu a obtenção de valores de matizes mais precisos, com vantagem sobre a Caderneta de Munsell, que fornece dados a intervalos de 2,5 YR. A presença de hematita e goethita nas amostras foi facilmente identificada pela utilização da segunda derivada da função Kubelka-Munk. A similaridade dos espectros desta função, obtida para a terra fina, terra fina tratada com hipoclorito de sódio e fração argila, revela o pequeno efeito da matéria orgânica na expressão das curvas derivativas. Tais resultados indicam que esta técnica pode ser usada sem que seja necessária a separação da referida fração textural, como normalmente observado em outras técnicas de caracterização mineralógica. A segunda derivada da função Kubelka-Munk permitiu ainda a adequada estimativa dos teores de goethita e, principalmente, de hematita presentes nas diferentes amostras.

Geology and geochemistry of the Águas Claras and Pico Iron Mines, Quadrilátero Ferrífero, Minas Gerais, Brazil

The Aguas Claras and Pico Mines are two world-class iron-ore mines hosted within the Lower- Proterozoic banded iron-formations (locally known as itabirites) of the Minas Supergroup located in the Quadrilatero Ferrifero district, Minas Gerais, Brazil. The Aguas Claras orebody consists of a 2,500-m-long roughly tabular-shaped lens hosted within the dolomitic itabirite of the Caue Formation. Dolomitic itabirite is the protore of the soft high-grade iron ore, which is the main ore type of the Aguas Claras orebody, representing about 85% of the 284 Mt mined since 1973, with the remaining 15% comprising hard high-grade ore. Hematite is the main constituent of the iron ores. It occurs as martite, granular hematite and locally as specularite. Magnetite appears subordinately as relicts within martite and hematite crystals. Gangue minerals are very rare. These consist of dolomite, chlorite, talc, and apatite, and are especially common in contact with the protore. This virtual absence of gangue minerals is reflected in the chemistry of ores that are characterized by very high Fe contents (an average of 68.2% Fe). The Pico orebody is a continuous ~3,000-m-long body of a lenticular shape hosted within siliceous itabirite, which is the protore of the soft high- and low-grade ores at the Pico Mine. The soft high-grade ores, together with the low-grade ores, called iron-rich itabirite, are the main types of ore, and respectively represent approximately 51 and 29% of the reserves. The remaining 20% consists of hard high-grade ore. The iron oxide mineralogy is the same as that of the Aguas Claras Mine, but in different proportions. Gangue minerals are very rare in the high-grade ores, but are slightly more common in the iron-rich itabirite. Quartz is the dominant gangue mineral, and is found with minor quantities of chlorite. The chemistry of the high-grade ores is characterized by high Fe contents (an average of 67.0%) and low P, Al2O3, and SiO2, which are concentrated in the fines. Iron-rich itabirites average 58.6% Fe and 13.5% SiO2. The genesis of the soft high-grade ores and iron-rich itabirites is related to supergene processes. Leaching of the gangue minerals by groundwater promoted the residual iron enrichment of the itabirites. This process was favored by the tropical climate and topographic situation. The original composition of the itabirites and the presence of structures controlling the circulation of the groundwater have influenced the degree of iron enrichment. The hard high-grade ores are of a hypogene origin. Their genesis is attributed to hydrothermal solutions that leached the gangue minerals and filled the spaces with hematite. This process remains a source of debate and is not yet fully understood.

Óxidos de ferro das frações areia e silte de um Nitossolo desenvolvido de basalto

Foi estudada a composição mineralógica dos óxidos de ferro das frações areia, silte e argila de cinco amostras coletadas de um perfil de um Nitossolo Vermelho desenvolvido de basalto toleítico, localizado próximo a Tupaciguara (18 º 35 ' 33 '' S; 48 º 42 ' 18 '' O), na região do Triângulo Mineiro, Minas Gerais. Porções de areia e de silte foram submetidas à separação magnética e a ataques químicos seletivos com NaOH 5 mol L-1 e com uma mistura ditionito-citrato-bicarbonato, com o objetivo de identificar os principais óxidos de ferro magnéticos e suas associações mineralógicas. Maghemita (fórmula ideal, gFe2O3) foi o único mineral magnético identificado nas frações areia e silte; não foi encontrada evidência de ocorrência de magnetita nessas frações. Os resultados Mössbauer apenas sugerem a co-existência de mais de um tipo cristaloquímico de maghemita, na fração areia, embora não sejam claramente separáveis dos espectros Mössbauer obtidos sem campo magnético aplicado. A fase magnética mais rica em Al tem fórmula média, deduzida de resultados de microssondagem eletrônica, Fe2,36(2)3+ Al0,24(2)3+ Ti0,06(3)4+ Ä0,341(1) O4 (Ä = vacância catiônica). A ilmenita férrica detectada de dados Mössbauer na porção magnética da fração silte corresponde a uma solução sólida xFe2+Ti4+O3(1-x)Fe2(3+)O3, com x » 0,83. São discutidas algumas dificuldades e alternativas de interpretação dos resultados Mössbauer, como os do presente caso, em que ocorrem contribuições espectrais simultâneas de hematita e maghemita.

Local geoelectrical models of the Martian subsurface for shallow groundwater detection using sounding radars

Low‐frequency sounding radars should be able to probe the Martian subsurface layers down to varying depths, depending on the geoelectrical properties of the sounded sites. We present in this work four frequency‐dependent geoelectrical models of the Martian subsurface in the 1–20 MHz frequency band, based on laboratory electromagnetic characterization of Martian soil analogues. Those models correspond to local Martian sites that we considered to be of particular interest in the search for water using mainly the Ground‐Penetrating Radar (GPR) instrument of the Netlander mission. Results and discussion are also valid for both sounding experiments MARSIS and SHARAD. The four models of the Martian subsurface are designed to represent terrains where recent fluvial‐like features suggest the presence of near‐subsurface ground ice and probably liquid water. We performed measurements on volcanic and sedimentary materials that may be present on these sites under the appropriate geophysical conditions that may exist in those terrains. We then simulated the backscattered radar echo arising from each site in the 2 MHz frequency band, using the Finite Difference Time Domain (FDTD) algorithm, in order to evaluate the instrument performances to probe the subsurface stratigraphy of each site. Our results confirm that the near‐subsurface rich iron oxide mineralogy controls the instrument performances in terms of penetration depth and signal‐to‐noise ratio in the 2 MHz frequency band. We finally discuss the geophysical and geoelectrical sounding conditions that could lead to an ambiguous detection of shallow subsurface water on Mars for the Netlander GPR.

Iron oxide mineralogy in late Miocene red beds from La Gloria, Spain: rock-magnetic, voltammetric and Vis spectroscopy analyses

Free ferric oxides of a red bed series were analyzed by rock-magnetic techniques (IRM component analysis) and by two less traditional methods: visible spectroscopy and voltammetry. All three methods have low limits of detection, making them suited for this type of analysis. The red bed samples studied contained clay minerals, quartz, and calcium carbonate as major constituents. Free Fe oxides occurred at a concentration of 0.3–2.1%, i.e. in the majority of the samples below the detection limit of X-ray powder diffraction. The combination of the employed analytical techniques enabled to characterize the mineralogy of free ferric oxides and to estimate the ratio of goethite and to hematite. This ratio changes substantially within the section that probably indicates paleoclimatic changes.

Influence of Organic Matter on the Iron Oxide Mineralogy of Volcanic Soils

Samples of soils developing on volcanic materials that represent about 70% of the agriculture land area in Chile were studied by chemical methods, powder X-ray diffractometry (XRD) and Mossbauer spectroscopy. The soil samples were collected from native areas of Ultisols, Andisols, and Andisols presenting seasonal cycles of poor drainage, at depths between 0–15 cm and 15–30 cm from the top of the profile. The degree of crystallinity of iron oxides was evaluated from the ratio of amounts of Fe extracted with oxalate-oxalic acid (FeOX) and with bicarbonate-citrate-dithionite mixtures (FeDBC), FeOX/FeDBC. Results show that the specific saturation magnetization of soils and crystallinity of iron-rich minerals increase with decreasing organic matter contents of soils. From XRD and Mossbauer data, ferrihydrite (ideal formula, Fe5HO8⋅4H2O) could be the precursor either of the soil magnetite (Fe3O4) or hematite (αFe2O3). Older soils were found to be in an intermediate stage regarding the transformation of magnetite to maghemite (γFe2O3).

Iron-rich spinel phases from sand fraction of three Chilean soils developing on volcanic materials

The magnetic separates from the sand fraction of three Chilean soils forming on volcanic materials were studied by powder XRD, RT and 80 K Mössbauer spectroscopy, saturation magnetization measurements, and scanning electron microprobe analysis. Soil samples were collected from the B-horizon of pedons from native and cultivated areas. Results, particularly from powder x-ray diffractometry, Mössbauer spectroscopy, and chemical analysis, reveal relatively complex magnetic mineral assemblages in all samples. Fe (>70 mass% Fe2O3), Ti (ranging from ~2 to ~26 mass% TiO2), Al (from 1 to ~6 mass% Al2O3) and Mg (from 1 to ~6 mass% MgO) are the main elements found in the iron oxide-spinel structures. In at least one sample, namely the magnetically separated portion from the sand fraction of the fertilized Collipulli soil, the 80 K Mössbauer spectrum indicates that magnetite undergoes the Verwey transition. This sample, with saturation magnetization being of order of 40 J T−1 kg−1, is also the most magnetic of all magnetic separates. Results on the iron oxide spinels, from electron microprobe data, reveal a large variability not only of chemical composition, but also of crystalline structure and magnetic properties of these magnetic minerals. The effect of agricultural practices, particularly the continuous chemical fertilization and soil acidity correction over several years, on the iron oxide mineralogy could not be unequivocally established, but the present data provide a relatively detailed description of the magnetic iron oxide assemblages of these Chilean volcanic soils.

Fracture wall cements and coatings from two clayey till aquitards.

Secondary minerals occurring at the faces of fractures, the only reliable visual evidence of the presence of hydraulically conductive fractures in clayey unlithified aquitards, have been characterized for two uncontaminated field sites, Dalmeny, Saskatchewan, and Laidlaw, Ontario. Preliminary identification of secondary minerals and their variations with depth was made using a Munsell Color Chart. Subsequent microscopic analyses (petrography, electron microprobe analysis, scanning electron microscopy, and X-ray diffraction) were used to identify iron-oxide mineralogy. Iron oxides were identified as goethite, ferrihydrite, and hematite at Dalmeny, where they occur to depths of 10 to 15 m, and goethite and ferrihydrite at Laidlaw, observed to depths of 7 m. In both cases, the identification of ferrihydrite was tentative due to the problems of small sample size and peak overlap in X-ray diffraction. The iron oxides do not form coatings on the surfaces of the fractures as had been previously thought; rather they form cements linking the matrix grains. Thus there is potential for decreased permeability and increased surface reactivity parallel to and inward from the fracture faces. The pattern of iron-oxide distribution suggests that the youngest deposits, and those with the greatest surface reactivity and potential for contaminant retardation, are found at greatest depths in the fractures. Manganese oxides form in isolated clusters in larger pores and indentations, although the exact manganese minerals could not be firmly identified.

Magnetic Properties and Magnetic Mineralogy of Subtropical Ferrisols

AbstractMineral magnetism parameters, including magnetic susceptibility (χ), frequency dependent susceptibility (χfd), anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM), had been measured on 55 Ferrisols formed on Quaternary red clay in subtropical regions of China. Magnetic measurement data, combining with chemical selective dissolution technique of iron oxide and X‐ray diffraction analysis, provided insight into the magnetic characteristics and magnetic mineralogy of Ferrisols. Results showed that the strongly magnetic minerals were found in most of the Ferrisols. The χ values of Ferrisols had a highly significantly exponential positive relation with free iron oxide (Fed) contents of soils (R2=0.5971), and the χfd values of Ferrisols were highly significantly linearly correlated with Fed contents of soils (R2=0.4289). The χfd and ARM susceptibility (χARM) values of Ferrisols showed that the magnetic minerals in the soils existed mainly in superparamagnetic (SP)/stable single domain (SSD) grain sizes. The χ values of Ferrisols were highly significantly positive proportional to χARM of soils (R2 = 0.9429), indicated that the magnetism of Ferrisols mainly came from the contribution of pedogenic SP and SSD magnetic grains. Mineral magnetic measurements and X‐ray diffraction patterns indicated that iron oxide mineralogy of Ferrisols was comprised of hematite (α‐Fe2O3), maghemite (γ‐Fe2O3) and goethite (α‐FeOOH), suggesting that maghemite is the main magnetic minerals in Ferrisols. The possible mechanisms of origin of maghemite in Ferrisols were also discussed.