Darrell G. Schulze

Iron oxide mineralogy of some soils of two river terrace sequences in Spain

The iron oxides of soils of two river terrace sequences in Spain which show an increasing degree of redness with age were studied. Clay fractions contained only small amounts of oxalate-extractable Fe. Goethite and hematite, the only crystalline Fe-oxides identified, were determined quantitatively by X-ray diffraction (XRD) after concentrating the Fe-oxides by boiling in 5N NaOH and subtracting the step-counted diffractogram of the deferrated clay from that of the non-deferrated clay, obtaining thus a “pure” Fe-oxide diffractogram. EDTA extracted hematite preferentially to goethite as is seen by loss of red colour and by XRD. A good correlation was found between the content of hematite in the fine earth and a redness rating based on Munsell notations. In the Guadalquivir River sequence, Fed and Fed/Fet increased with age. The amount of both goethite and hematite formed from silicate-Fe increased with soil age but hematite increased more than goethite, possibly due to the xeric soil environment. Also, goethite increased in crystallinity as indicated by a decrease in XRD line broadening and Feo/Fed ratios. No such trends were found in the Esla River sequence, possibly because the initial alluvium was already highly weathered as shown by high Fed/Fet values (0.8) irrespective of terrace level. Al substitution in goethite calculated from XRD increased with soil age, reflecting the increasing acidity of the soils. Al substitution in hematite was markedly lower.

SPECTROSCOPIC SENSING OF SOIL ORGANIC MATTER CONTENT

ABSTRACT A prototype real-time soil organic matter sensor has been developed. The sensor consists of compact transmitter and receiver modules which utilize light reflectance to measure percentage organic matter. Laboratory tests were conducted to determine optimal light wavelengths for use with typical landscapes in the midwestem United States. Preliminary field tests have proven successful for soils ranging from below 1% to over 6% organic matter. The sensor promises to be useful for prescription applications of soil applied chemicals.

Is there holocene reddening (hematite formation) in soils of axeric temperate areas

Alfisols on calcareous glacial gravels in the northern forelands of the Alps, between the Upper Rhine Valley in the west and Eastern Bavaria in the east, show rubified Bt horizons. Their redness decreases from west (2.5 YR) to east (10 YR), paralleling decreasing temperature and increasing rainfall. Detailed mineralogical studies by Mossbauer spectroscopy and differential X-ray diffraction demonstrated the presence of hematite in association with goethite. Concentrations of hematite decrease from west to east with decreasing temperature. No hematite was detected in the non-calcareous residues of calcareous C horizons. Alfisols on silty glacial material in the immediate vicinity of the reddish soils have yellow-brown (10 YR) Bt horizons and are hematite-free. We conclude that rubification (hematite formation) is possible under an axeric climate provided a suitable pedoclimate exists.

Major element geochemistry and geomorphic relationships in Brazilian Cerrado soils

Abstract A thorough understanding of major and trace element geochemistry is important in assessing the impacts of rapid agriculturalization and increased human occupation of the Cerrado region of central Brazil, a region with some of the oldest soils on Earth. The objectives of this paper are to summarize the major element composition of Cerrado soils in the context of their morphology and geomorphology, to describe impacts of biogeochemical cycling on the chemical properties of Cerrado soils, and to use elemental data to provide insights on the possible origin of soil parent materials on the South American erosion surface and on the Serra Geral Basalts. We sampled three major subregions selected to be representative of the main areas of the Cerrado, namely: (i) eastern Goias, (ii) northwestern Minas Gerais, and (iii) the Triângulo Mineiro area of western Minas Gerais. Five soils were sampled in each subregion, each represented by three pedons sampled at 0–0.2 and 0.8–1.0 m depths. Geomorphologically, the soils were on the South American, Velhas I, and Velhas II erosion surfaces. All sites were carefully selected to minimize potential anthropogenic contamination. General chemical characterization was by standard procedures and total major and trace elemental contents were quantified using wavelength-dispersive X-ray fluorescence spectroscopy. The soils ranged from loamy sands to clays, with Si (142–947 g kg −1 SiO 2 ), Al (14–439 g kg −1 Al 2 O 3 ), Fe (10–331 g kg −1 Fe 2 O 3 ), and Ti (3–101 g kg −1 TiO 2 ) contents closely following the clay contents. Organic C averaged 17 g kg −1 for the surface, and 7 g kg −1 for the subsurface horizons. The Na, Ca, Mg, and K contents were very low, tended to be higher in surface than in subsurface horizons because of recycling by vegetation and addition due to atmospheric deposition, and were higher in soils containing hard plinthite nodules. The Th/Zr ratios suggest that the parent materials of the clayey soils on the South American erosion surface have a common origin in sediments from rocks in the Central Plateau of Brazil that were transported eastwards to cover the underlying sandstone. Trace element signatures indicate that basalt-derived soils occurring at different elevations in the Triângulo Mineiro area formed from distinctly different magmas that may have been deposited in different basalt flow events.

Relationship among derivative spectroscopy, color, crystallite dimensions, and Al substitution of synthetic goethites and hematites

Nine hematites and 22 goethites were synthesized by a variety of methods to obtain monomineralic samples having a range of Al substitutions and particle sizes. The second derivative of absorbance and Munsell color designations were calculated from visible reflectance spectra obtained from the dry powders. Unit-cell dimensions, Al substitution, infrared band positions, mean crystallite dimensions (MCD) from X-ray powder diffraction, and particle size from fiber-optic Doppler anemometry (FODA) were determined. Previously reported correlations between Al substitution, goethite unit-cell dimensions, and OH-stretching and -bending band positions were confirmed. For hematite, the position of the second derivative peak at ≈600 nm was negatively correlated with Al substitution (r = −.86). Munsell value and chroma were positively correlated with Al substitution (r =.94 for both), but hue was not related to Al substitution. Hue appeared to become redder, however, as particle size measured either by FODA or MCD increased. For goethite, the position of the second derivative minimum at ≈485 nm was negatively correlated with Al substitution (r = −.99). Munsell hue appeared to be related to both Al substitution and MCD perpendicular to (110), MCD110, with hues becoming redder with increasing Al substitution and yellower with increasing MCD110. Correlations between Munsell value and chroma and parameters such as Al substitution, particle size, and OH-stretching and -bending band positions were poor, but goethites synthesized by oxidation of Fe2+ solutions at room temperature had higher chromas than goethites synthesized hydrothermally from an Fe3+ system. Visually determined colors agreed well with calculated ones. Second-derivative spectra and color designations calculated from visible spectra appear to be potentially useful for quickly estimating other properties of goethite and hematite, such as Al substitution and particle size.

Synchrotron X-Ray Techniques in Soil, Plant, and Environmental Research

Publisher Summary New generations of synchrotrons designed exclusively as X-ray sources have followed, and these powerful sources of X-rays have become important to a wide variety of scientific disciplines. The past five years have seen a growing number of applications of synchrotron-based techniques to problems in the soil and environmental sciences. Synchtron-based techniques have applications in many other areas of agricultural research as well. The chapter highlights some of the major applications in soil, plant, and environmental research. Many of these applications represent the first use of synchrotron-based techniques in particular agricultural disciplines. The chapter describes the ways in which synchrotrons work, describe the properties of synchrotron radiation, and explain the terminology associated with synchrotron-based research. Synchrotrons vary in their capabilities, so a general understanding of their differences will allow, in a general sense, to assess the suitability of a particular synchrotron for a specific experiment. Synchrotron light is extremely intense is emitted over a wide range of energies, is highly collimated and highly polarized, and has a pulsed time structure. The chapter reviews the applications of synchrotron-based techniques to soil, plant, and environmental research and suggests possible future applications. Some techniques are well established and widely used, e.g., X-ray absorption spectroscopy, X-ray diffraction, and the X-ray microprobe. Other techniques, such as Mossbauer and infrared spectroscopies, are still being developed and there is little or no literature on direct applications to soil, plant, or environmental research.

Análise de componentes principais de atributos físicos, químicos e mineralógicos de solos do bioma cerrado

Behavior differences between soils of the South-American and Velhas geomorphic surfaces of the Cerrado region, all of them under native vegetation cover, were evaluated by physical, chemical, and mineralogical characterizations, and principal component analyses. According to their sub-region and geomorphic surfaces, the soils were separated in three groups: 1. clayey to very clayey texture, low- and mesoferric; 2. medium to sandy texture, lowferric; 3. clayey to very clayey texture, ferric. Water retention in the surface horizons of the studied soils was positively correlated with the clay and organic carbon contents. Soils of Group 3 showed the greatest amount of exchangeable bases in the surface horizons, which is mainly a consequence of the greater recycling efficiency imposed by the local forest formation when compared to the Cerrado formation (verified in 31 of the 33 soil profiles of Groups 1 and 2). Soils of Group 3, formed from mafic rocks, were the most homogeneous in mineralogical terms, since all profiles were hematitic. In averages, Groups 1 and 2 presented an overlapping mineralogical composition, though Group 1 is more gibbsitic than Group 2. In comparison to the US Soil Taxonomy, the efficacy of the Brazilian Soil Classification System at discriminating the studied soils is higher owing to the use of the ferric character in conjunction with the other attributes both systems have in common. The principal component analysis supported the understanding of the pedologic environment differences and similarities identified in the field.

Diffuse reflectance spectra of Al substituted goethite; a ligand field approach

Previous investigations of goethite revealed a substantial variation of color and diffuse reflectance spectra (DRS) in the extended visible range (350–2200 nm). To better understand the causes of this variability and to assess the potential of DRS as a mineralogical tool, we investigated the DRS of pure and Al-substituted goethite, α-Fe1−xAlxOOH with x from 0 to 0.33, and mean crystal lengths (MCL) from 170 to 1800 nm. The strongly overlapping ligand field bands were extracted by fitting the single-electron transitions 6A1 → 4T1, 6A1 → 4T2, 6A1 → (4E; 4A1), and 6A1 → 4E(4D) as functions of the ligand field splitting energy, 10 Dq, and the interelectronic repulsion parameters, Racah-B and -C. With x increasing from 0 to 0.33, 6A1 → 4T1 decreased from 10,590 to 10,150 cm−1 (944 to 958 nm), and 6A1 → 4T2 decreased from 15,310 to 14,880 cm−1 (653 to 672 nm), while 10 Dq increased from 15,770 to 16,220 cm−1. From the change of 10 Dq we calculated a decrease of the Fe-(O,OH) distances from 202.0 to 200.9 pm (−0.5%). This decrease is smaller than the average decrease of all (Al,Fe)-(O,OH) distances (−1.8%) calculated from the change of the unit-cell lengths (UCL). That is, there remains a substantial difference in size between the larger Fe- and the smaller Al-occupied octahedra in the solid solution which may indicate the existence of diaspore clusters within the goethite structure. The increasing strain in the crystal structure due to the size mismatch and limited contractibility of the oxygen cage around Fe may be the primary reason for Al substitution being restricted to x < 0.33. The bands 6A1 → (4E; 4A1) and 6A1 → 4E(4D) did not shift, indicating a constant covalency of the Fe-(O,OH) bonds with B = 628 cm−1 and C = 5.5B. Whereas variation of band energies could be explained in terms of the Fe-(O,OH) ligand field, the variation of color and band intensities was mainly determined by crystal size. Although our study confirmed the potential of DRS for mineralogical investigations, there is still a gap between the fundamental theory and the explanation of some spectral features.

Structural environment and oxidation state of Mn in goethite-groutite solid-solutions

Abstract Both X-ray absorption and diffraction techniques were used to study the structural environment and oxidation state of Mn in goethite-groutite solid solutions, α-MnxFe1-xOOH, with xMn ≤ 0.47. Rietveld refinement of X-ray diffraction (XRD) data was employed to investigate the statistical long-range structure. The results suggest that increasing xMn leads to a gradual elongation of Fe and Mn occupied octahedra which, in turn, causes a gradual increase of the lattice parameter a and a gradual decrease of b and c in line with Vegard’s law. X-ray absorption fine structure (XAFS) spectra at the MnKα and FeKα edges revealed, however, that the local structure around Fe remains goethite-like for xMn ≤ 0.47, while the local structure around Mn is goethite-like for xMn ≤ 0.13, but groutite-like for higher xMn. The spectral observations were confirmed by XAFS-derived metal distances showing smaller changes around Fe and larger changes around Mn as compared with those determined by XRD. Therefore, the XAFS results indicate formation of groutite-like clusters in the goethite host structure for xMn > 0.13, which remain undetected by XRD. The first prominent resonance peak in the X-ray absorption near-edge spectra (XANES) of the Mn goethites was 17.2 to 17.8 eV above the Fermi level of Mn (6539 eV), in line with that of Mn3+ reference compounds, and well separated from that of Mn2+ and Mn4+ compounds. Therefore, Mn in goethite is dominantly trivalent regardless of whether the samples were derived from Mn2+ or Mn3+ solutions. This may indicate a catalytic oxidation of Mn2+ during goethite crystal growth similar to that found at the surface of Mn oxides.

THE EFFECT OF Al ON Fe OXIDES. XIX. FORMATION OF Al-SUBSTITUTED HEMATITE FROM FERRIHYDRITE AT 25°C AND pH 4 TO 7

Iron oxides in surface environments generally form at temperatures of 25 ± 10°C, but synthesis experiments are usually done at higher temperatures to increase the rate of crystallization. To more closely simulate natural environments, the transformation of 2-line ferrihydrite to hematite and goethite at 25°C in the presence of different Al concentrations and at pH values from 4 to 7 was studied in a long-term (16–20 y) experiment. Aluminum affects the hydrolysis and charging behavior of 2-line ferrihydrite and retards crystallization. Al also promotes the formation of hematite over goethite and leads to multidomainic discoidal and framboidal crystals instead of rhombohedral crystals. The strong hematite-promoting effect of Al appears to be the result of a lower solubility of the Al-containing ferrihydrite precursor relative to pure ferrihydrite. Hematite incorporates Al into its structure, as is shown by a decrease in the a and c-cell lengths and a decrease in magnetic hyperfine fields (Mössbauer spectroscopy). With hematite formed at low-temperature, these decreases were, however, smaller for the cell length and greater for the magnetic field than for hematite produced at higher temperatures. Both phenomena are removed by heating the hematite at 200°C. They are attributed to structural OH and/or structural defects. The relative content of Al in the structure is lower for hematite formed at 25°C than for hematites synthesized at higher temperatures (80 and 500°C). The maximum possible substitution of one sixth of the Fe positions was not achieved, similar to soil hematites. These results show that properties of widely distributed soil Al-containing hematites can reflect formation environment.