Altered basaltic tephra from Haleakala, Maui, are characterized using multiple techniques in order to identify the minerals formed under a variety of conditions and to understand the soil formation processes here. We collected samples that are representative of typical bulk weathered material in the crater, as well as solfataric alteration in a hydrothermal environment. For this study X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy are coupled with spectroscopic techniques including Mössbauer, visible-infrared reflectance, transmission infrared, and thermal infrared emission spectroscopies to analyze these samples. The unaltered tephra are composed of feldspar, glass, pyroxene and olivine. Observed alteration products include Fe oxides, phyllosilicates and sulfates, as well as SAED amorphous Al-Si-bearing material. These samples are potential analogs for altered volcanic material on Mars as the pedogenic influences and contact with plants and animals are minimal. Results from this study may help to determine spectral signatures of these samples that could be used for identification on Mars of the minerals observed here.

A field-mapping and crystal-chemical study of two alunite- and halloysite-rich deposits in the Turplu area, situated northwest of Balıkesir on the Biga Peninsula of northwest Turkey reveals a mineralogically diverse and a potentially economic clay deposit. The mineral assemblage along fault zones is dominated by halloysite and sometimes alunite. The alunite is nearly end-member in composition (a = 6.995 Å, c = 17.195 Å) often occurring with a minor Ca phosphate phase. Of the two deposits studied, the more northerly mine contains more alunite relative to halloysite. Geochemical alteration indices suggest that the northern mine has experienced a slightly greater degree of hydrothermal modification. Halloysite is found in both hydrated and dehydrated states and assumes a tubular morphology. Observations by transmission and scanning electron microscopy are consistent with a model of halloysite dehydration, where the shapes transform from an open-hole tubular morphology to a closed-hole unfurled morphology.

Mineral paragenesis includes the effects of initial deposition of volcanic tuffs and andesite on top of karstic terrain. The contact between altered volcanics and underlying limestones is irregular and appears to have provided a mechanism to flush both hydrothermal and meteoric waters through the volcanics. Periods of hydrothermal alteration (hypogene) contemporaneous with extensional and strike-slip faulting have resulted in alunite and halloysite deposits. Hydrothermal alteration is concentrated near the fault zones. Because of subsequent weathering (supergene) away from the fault zones, much of the andesitic volcanic rocks have been altered to a more smectite-rich and kaolinite-bearing assemblage. The deposits continue to be both plastically deformed in the alunite/halloysite regions and to undergo brittle deformation in the saprolitized volcanics. Tectonic deformation has mixed the contacts, such that limestone blocks are entrained into parts of the alteration zones. Gibbsite and gypsum are common weathering products associated with limestone block inclusions. Genetic models for the origins of alunite-halloysite deposits in NW Turkey should consider as possible influencing factors the underlying lithologies, the extent of hydrothermal alteration, and recent weathering by meteoric fluids. In the case of the Turplu deposits, karstic limestones, hydrothermal circulation of sulfate-rich waters, and a post-alteration history of meteoric weathering were all important factors in their formation.

Mixed-layer illite-smectite with high degree of ordering (rectorite-like clay) and with three types of interlayer cations — K+, Na+ and ${\text{NH}}_4^{+}$, was determined in the high-sulfide epithermal mineralization of the Western Carpathian Mountains. The tobelitic portion of the clay comprises 5–10%. Ammonium was detected both by chemical analysis and by Fourier transform infrared analysis. Some of the ammonium can be assigned to a poorly bound form, not fully fixed in the illitic interlayer. The finest size fraction separated from the samples behaves differently from the rest of the sample. It contains two layers of water molecules in the expandable interlayers as determined by X-ray diffraction, whereas coarser fractions have only one water layer in the interlayers.

Thermogravimetric analysis combined with mass spectrometry was used to study H2O bound to samples of illite-1M, illite-2M2 and leucophyllite-1M. Samples were heated in a helium atmosphere at different temperatures and after heating at each given temperature were cooled to 35°C. Each cycle in the mass 18 spectrum of each illite sample contains a low-temperature peak at 60–80°C, a medium-temperature peak at 340–360°C, and a high-temperature peak at a temperature that is very close to the maximum temperature of sample heating of a given cycle. Within each heating-cooling cycle, the sample weight at the beginning of cooling is lower than that at the end of the same cooling stage because of H2O resorption. However, the number of H2O molecules released during each medium-temperature heating cycle is equal to the number of H2O molecules resorbed during the corresponding cooling stages.

The weight losses, under medium-temperature heating, of the illite samples are related to dehydration when H2O molecules located in K-free sites of the illite interlayers are removed. The medium-temperature peak is reproducible for each cycle because during each cooling stage the illite interlayers resorb the same number of H2O molecules that were lost during the preceding dehydration.

Two distinct features are characteristic of leucophyllite during heating-cooling treatments. First, the number of H2O molecules resorbed during cooling is significantly greater than the number of H2O molecules lost during dehydration. Second, the medium-temperature peaks in the spectrum appear only in the last five cycles and the maximum-peak temperature is 450–460°C. These data indicate that the heating-cooling treatments are accompanied by partial rehydroxylation. This rehydroxylation occurs during each coolingstage when a small number of resorbed H2O molecules are trapped in the interlayers, although most migrate into the octahedral sheet of the 2:1 layers and reform as OH groups. The crystal chemical factors responsible for the dehydration and rehydration as well as for the rehydroxylation reactions are discussed and speculation about the origin of the low- and medium-temperature H2O losses is presented.

The formation of hard hematite in steam generators with relatively high levels (5–10 µg/L) of dissolved oxygen at temperatures around 280–290°C and pressures around 6–8 MPa can serve as an analog for the formation of hard hematite in sedimentary processes. Furthermore, in steam generators, as well as in nature, hematite is an effective cementing agent, capable of incorporating as much as twice its own weight of other solids to form a hard composite material. Laboratory simulations showed ferrihydrite to be the likely starting material for the formation of hard, dense hematite at temperatures much lower than those required for sintering of anhydrous hematite. These laboratory simulations, performed at temperatures around 260°C and pressures of ∼500 MPa, resulted in the formation of hard hematite or hematite-based composite solids over periods of 3–5 h, compared with several months in steam generators and many years in nature. The amount of water present during the synthesis (10–15% of the weight of dry ferrihydrite) and the gradual removal of water proved to be key parameters in the formation of hard, dense hematite. The mechanism, studied by means of X-ray diffractometry, Mössbauer spectroscopy and infrared spectroscopy, appeared to involve build-up, then gradual condensation of OH bridges, leading to the conversion of ferrihydrite to hydrohematite with approximately 4–5% of residual water. The presence of other solids, such as copper and its oxides, alumina and silica, in large quantities, resulted in smaller grain size of the hydrohematite product but did not affect its mechanical properties. On the other hand, the use of hydrazine to provide a reducing environment produced goethite during the precursor synthesis stage and soft magnetite during the pressing stage. However, whenever hematite was produced, it could not be subsequently reduced to magnetite by hydrazine under the reaction conditions specified above. The mechanical properties as well as the spectroscopic characteristics of the product of pressing agreed with observations on sedimentary hematite-cemented rocks.

The citrate-induced desorption kinetics of pre-adsorbed Se from montmorillonite (Mt) and its complexes with hydroxyaluminum (HyA-Mt) and hydroxyaluminosilicate (HAS-Mt) were studied. The mole fraction of pre-adsorbed Se released in 24 h from different clays followed the trends of Mt>> HAS-Mt> HyA-Mt with a significant increase with elevated citrate concentration in the desorbing solution. In contrast, the amount of Se adsorbed per unit mass of different clays followed a clear-cut opposite trend of HyA-Mt>HAS-Mt>>Mt. The Se desorption kinetics in different systems indicated multiple rate characteristics; where an initial fast reaction (0.25–2 h) was followed by a slow reaction (2–16 h). Of the six different kinetic models tested (zero-, first-, and second-order; power function, Elovich, and parabolic diffusion), the second-order rate equation showed the overall best fit to the fast and slow desorption kinetic data from the clays. Based on second-order rate constants, the rates of the mole fraction of Se desorption by citrate from different clay systems at 298 K followed the order Mt > HAS-Mt >> HyA-Mt. For both fast and slow reaction, the rates of desorption increased proportionally with the level of citrate. Replicate experiments conducted across a range of temperature (288–318 K) yielded Arrhenius parameters that followed the order HyA-Mt > HAS-Mt >> Mt. Considering that a lower mole fraction of Se desorption after a particular reaction period, slower desorption kinetics, and a greater activation energy of desorption are the indices of increased adsorption bond strength, the results clearly indicate that HyA- and HAS-interlayering and coatings on Mt not only augmented its Se adsorption affinity and capacity, but also increased the adsorption bond strength. Silication in HyA not only reduced the Se adsorption capacity, but also weakened the adsorption bond strength. This establishes a significant role of HyA/HAS-interlayering and coating on Mt in influencing the rate of citrate-induced release of Se. Reduction in surface-positive potential following citrate adsorption on the clay surface, a direct ligand exchange between Se and citrate, and structural dissolution are possible mechanisms responsible for citrate-induced Se desorption in the present study.

The effect of FeCl3 in acetonic medium on the structure of Na-bentonite was studied using X-ray diffraction (XRD), 57Fe Mössbauer spectroscopy, X-ray fluorescence spectroscopy and infrared spectroscopy to describe the structure of the bentonite before and after treatment. In the samples treated with FeCl3, an increase in the basal spacing was found by XRD, while a new magnetically split component assigned to Fe3+ incorporated within the interlayer regions of montmorillonite showed up in the low-temperature Mössbauer spectra. The Mössbauer parameters observed were close to those of Fe oxyhydroxides, suggesting the presence of some kind of nanoparticles. These results show that the treatment with acetonic FeCl3 solution is an effective method for introducing Fe into montmorillonite in the form of Fe3+ accommodated in the interlayer region. The treated samples proved to be efficient Lewis catalysts in the acylation of aldehydes (benzaldehyde and 4-OH-benzaldehyde) by acetic acid anhydride.

Clay particle aggregation affects a number of environmental processes, such as contaminant sorption/desorption, particle movement/deposition, and sediment structure and stability, yet factors that control clay aggregation are not well understood. This study was designed to investigate how microbial reduction of Fe(III) in clay structure, a common process in soils and sediments, affects clay-particle aggregation. Microbial Fe(III) reduction experiments were conducted with Shewanella putrefaciens CN32 in bicarbonate buffer with structural Fe (III) in nontronite as the sole electron acceptor, lactate as the sole electron donor, and AQDS as an electron shuttle. Four size fractions of nontronite (D5–D95 of 0.12–0.22 µm, 0.41–0.69 µm, 0.73–0.96 µm and 1.42–1.78 µm) were used to evaluate size-dependent aggregation kinetics. The extent of Fe(III) bioreduction and the amount of exopolysaccharide (EPS), a major biopolymer secreted by CN32 cells during Fe(III) bioreduction, were measured with chemical methods. Nontronite particle aggregation was determined by photon correlation spectroscopy and scanning electron microscopy. The maximum extent of Fe(III) bioreduction reached 36% and 24% for the smallest and the largest size fractions, respectively. Within the same time duration, the effective diameter, measured at 95% percentile (D95), increased by a factor of 43.7 and 7.7 for these two fractions, respectively. Because there was production of EPS by CN32 cells during Fe(III) reduction, it was difficult to assess the relative role of Fe(III) bioreduction and EPS bridging in particle aggregation. Thus, additional experiments were performed. Reduction of Fe(III) by dithionite was designed to examine the effect of Fe(III) reduction, and pure EPS isolated from CN32 cells was used to examine the effect of EPS. The data showed that both Fe(III) reduction and EPS were important in promoting clay mineral aggregation. In natural environments, the relative importance of these two factors may be dependent on local conditions. These results have important implications for understanding factors in controlling clay particle aggregation in natural environments.

Adsorption by synthetic 2-line ferrihydrite and hematite of glyphosate and phosphate, separately and together, was compared with adsorption results for goethite, gibbsite and two kaolinites in order to determine adsorption differences and similarities, in particular competition and phosphate preference, of these variable-charge minerals. Hematite rapidly adsorbed both compounds, while adsorption by ferrihydrite was slow, in particular of glyphosate, probably because of very slow diffusion of the bulky glyphosate molecules into interior sites in ferrihydrite particles. Accordingly, the Langmuir adsorption capacity of glyphosate (GAC) was considerably smaller (1.85 µmol m−2)than GAC for hematite (2.61 µmol m−2). The phosphate adsorption capacities (PAC) for ferrihydrite and hematite were more alike, 2.91 µmol m−2 and 2.85 µmol m−2, respectively. Differences between surface coordination (mono- or bidentate) may also contribute to the observed differences but conflicting information about the nature of the surface complexes makes this a difficult contributary factor to assess. The minerals were found to exhibit great variation in extent of competition and phosphate preference. Little competition and phosphate preference characterized hematite adsorption, while phosphate almost completely outcompeted glyphosate on goethite; ferrihydrite adsorption fell between these extremes. These differences may be attributed to different numbers of common (competitive) and specific (selective) adsorption sites on the three Fe oxides with a decreasing number of common sites in the order: goethite>>ferrihydrite>hematite, i.e. almost all goethite sites are common but with strong phosphate preference, while most hematite sites are specific for either glyphosate or phosphate. Alternatively, the result may be explained by adsorption in more planes, e.g. glyphosate adsorption onto the inner-Helmholtz-plane-adsorbed phosphate. For all six minerals compared, desorption of glyphosate following phosphate addition was found to be significantly correlated with the difference between the amounts of phosphate and glyphosate adsorbed indicating that this difference may be used as a competition index for predicting the influence of phosphate on glyphosate adsorption.