NEWMOD®, developed by R.C. Reynolds, Jr., has been an important tool for evaluating quantitatively X-ray diffraction (XRD) patterns from interstratified clay minerals for more than 20 years. However, a significant drawback to the NEWMOD® approach is that analyses are done by forward simulation, making results sensitive to user input and starting-model assumptions. In the present study, a reverse-fitting procedure was implemented in a new program, FITMOD, which automatically minimizes the differences between experimental and simulated XRD patterns. The differences are minimized by varying model parameters (such as Reichweite, crystal-size distribution, cation content, type of disorder, etc.) using the downhill simplex method. The downhill simplex method is a non-linear optimization technique for determining minima of functions. This method does not require calculation of the derivatives of the functions being minimized, an important consideration with many of the parameters in NEWMOD- type simulations. Instead, the downhill simplex method calculates pseudo-derivatives by evaluating sufficient points to define a derivative for each independent variable. The performance of FITMOD was evaluated by fitting a series of synthetic XRD patterns generated by NEWMOD+, yielding agreement factors, Rwp, of <0.3%. As long as the correct interstratified system was specified (e.g. illite-smectite), excellent fits were obtained irrespective of the starting parameters for the simulations. FITMOD was also tested using experimental XRD patterns, which gave very good fits, in agreement with previously published results. The optimization routine yields good fits for both synthetic and experimental XRD profiles in a reasonable time, with the possibility of varying all important structural parameters. FITMOD automatically provides optimum fits to experimental XRD data without operator bias, and fitting efficiency and accuracy were, therefore, significantly improved.

The incorporation of boron (B) and nitrogen (N) into illite is the key demand-side process responsible for the diagenetic budget of these elements in sedimentary basins, with important implications for pore-water chemistry, natural-gas composition, and borehole geophysics. The purpose of the present study was to take advantage of recent advances in quantitative mineral analysis of sedimentary rocks which have opened new possibilities for investigating this particular process. In order to avoid complications with recycled (detrital) N and B, clays from pyroclastic horizons of sedimentary rocks (bentonites) were used. The B and N contents in illite-smectite were measured in samples from different sedimentary basins, representing a complete range of diagenetic alteration. The bulk-rock chemical measurements, performed on raw rock samples in order to avoid any loss of exchangeable B and N, were referred to the contents of illite-smectite clays and to the content of illite alone, both measured by a combination of XRD and chemistry-based techniques.

Both B and N (as NH4) are present in illite, so their contents in illite-smectite clay increase in a more or less linear manner with progressing illitization. Thus, during diagenesis, the illite-smectite clay is a net consumer of B and N from the pore water. The amount of N in individual illite layers decreases during diagenesis and the amount of B either decreases or remains stable. Bentonitic illite must acquire both B and N from outside of the bentonite bed. In one diagenetic cycle, bentonitic illite fixes up to 800–1000 ppm B and up to >1% N expressed as (NH4)2O, corresponding to >20% of the fixed cation sites.

Guanidine and imidazole are important functional molecules that constitute the side chain of basic amino acids (arginine and histidine); these molecules are capable of interacting with mineral surfaces. However, little information is available about the effect of these molecules on mineral dissolution, including amorphous silica. In this study, to evaluate the effect of these organic molecules on the dissolution rates of amorphous silica, dissolution experiments were performed in solutions containing these molecules and other related heterocyclic compounds. The dissolution experiments were conducted by the batch method using 0.1 g of amorphous silica and 100 mL of 0.1 mM NaCl solution with 0.0, 0.1, 1.0, and 10.0 mM of guanidine, imidazole, pyrazole, or pyrrole at pH values of 4, 5, and 6. The results demonstrated that these compounds can enhance the dissolution rate of amorphous silica, depending on their ionic speciation in the following order: guanidine = imidazole > pyrazole > pyrrole. When 10.0 mM solutions were used, both guanidine and imidazole greatly increased the dissolution rate with an enhancement factor of 5.5—6.5, pyrazole exhibited a smaller change in the dissolution rate with an enhancement factor of 1.5—2.4, and pyrrole exhibited no significant enhancement. ChemEQL calculations confirmed that guanidine (pK = 13.6) and imidazole (pK = 6.99) are fully protonated and mostly present as cationic species in a pH range of 4—6; therefore, these compounds are capable of interacting with the >SiO- sites of amorphous silica. Pyrazole (pK= 2.61) and pyrrole (pK = 0.4), however, existed mostly as neutral forms. The concentrations of cationic species of pyrazole and pyrrole were at least one and three orders of magnitude lower than those of fully protonated compounds, respectively; therefore, pyrazole and pyrrole were less reactive than the fully protonated compounds on the surfaces of amorphous silica.

Studies of the paragenesis of authigenic illite in arkosic sandstones of various regions and ages have revealed that the illitization of kaolinite is an important reaction accounting for the formation of authigenic illite in sandstones during burial diagenesis. The illitization of kaolinite takes place at an intermediate burial depth of 3–4 km, where pressure can reach values of 100 MPa (≈ 1000 bars). The purpose of the present study was to analyze the effect of pressure on the rate of kaolinite illitization in alkaline conditions. Hydrothermal reactions were conducted on KGa-1b kaolinite in KOH solution at 300°C and under pressures of 500, 1000, and 3000 bars for 1 to 24 h. The visual examination of the X-ray diffraction (XRD) patterns indicated a notable influence of pressure on the reaction rate. Molar percentages of muscovite/illite formed at each time interval were calculated from the analysis of two diagnostic XRD peaks, representing the 060 reflections of kaolinite and muscovite/illite. The data were modeled to obtain the initial rate of conversion at each pressure. The results indicated that the initial rate of kaolinite to muscovite/illite conversion is one order of magnitude greater at 3000 bars than at 500 or 1000 bars. Comparison of these data with those in the literature show a faster conversion rate (several orders of magnitude) in an initially high-alkaline solution than in a near-neutral solution.

The characterization of poorly crystalline minerals formed by weathering is difficult using conventional techniques. The objective of this study was to use cutting-edge spectroscopic techniques to characterize secondary Fe mineralogy in young soils formed in basaltic cinders in a cool, arid environment. The mineralogy of a chronosequence of soils formed on 2, 6, and 15 thousand year old basaltic cinders at Craters of the Moon National Monument (COM) was examined using synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy in combination with selective extractions. Fe K-edge XAFS is useful for determining speciation in poorly crystalline materials such as young weathering products. Over 86% of Fe in the soil clay fractions was contained in poorly crystalline materials, mostly in the form of ferrihydrite, with the remainder in a poorly crystalline Fe-bearing smectite. The XAFS spectra suggest that ferrihydrite in the 15 ka soil clay is more resistant to ammonium oxalate (AOD) extraction than is ferrihydrite in the younger materials. Fe in the poorly crystalline smectite is subject to dissolution during citrate-bicarbonate- dithionite (CBD) extraction. The results indicate that relatively few mineralogical changes occur in these soils within the millennial time frame and under the environmental conditions associated with this study. Although the secondary mineral suite remains similar in the soils of different ages, ferrihydrite crystallinity appears to increase with increasing soil age.

Differentiating clay minerals that formed in a supergene environment during deep chemical weathering from those that formed during hydrothermal alteration at higher temperatures associated with a mineralizing event is important in the exploration for epithermal Au deposits. The purpose of this study was to further elucidate this topic by comparing morphological and chemical properties of clay minerals in saprolite overlying epithermally altered bedrock at the Vera Au deposit, Queensland, Australia, with those of clay minerals in saprolite overlying bedrock adjacent to the epithermal alteration zone. X-ray diffraction (XRD) and analytical transmission electron microscopy (ATEM) investigations identified kaolinite, illite, and interstratified illite-smectite, together with quartz, Fe and Ti oxide minerals, and the sulfate minerals jarosite, gypsum, alunite, and natroalunite. Kaolinite crystals within the weathered argillic alteration zone proximal to the epithermal quartz vein are generally larger (up to 3 μm in diameter) and better formed (subhedral to euhedral) than crystals in saprolite distal to the hydrothermal alteration zone, in which smaller (mostly <1 μm), subhedral to anhedral crystals dominate. Energy-dispersive spectrometry (EDS) analysis of single crystals indicated that kaolinite within the alteration zone has an Al/Si ratio indistinguishable from reference kaolinite and has small Fe concentrations, whereas distal saprolitic kaolinite has smaller Al/Si and greater Fe/Si ratios, consistent with the formation of low-Fe kaolinite during hydrothermal alteration and higher-Fe kaolinite during weathering. Illite and interstratified illitesmectite (I-S) were distinguished from kaolinite by their morphology and greater K/Si and smaller Al/Si ratios. The Illite and I-S morphology ranged from thin irregular masses through lath-like crystals in hydrothermal samples to larger, irregularly shaped crystals. The Ca/Si and K/Si ratios of single crystals in Ca-saturated clay minerals were consistent with the I-S interstratification parameters determined from XRD patterns.

The effects of surfactants on bentonites have been of great scientific interest for many years. Even though quaternary alkylammonium salts (QAS) have been studied, very few data are available on the comparative performance of different chain-length QAS for the modification of the surface properties and adsorption properties of bentonites. The objective of this study was to investigate the effect of chain length on the adsorption of cationic surfactants onto bentonite. The surface and adsorption properties of different chain-length QAS, i.e. hexadecyltrimethylammonium bromide (HTAB, C16), tetradecyltrimethylammonium bromide (TTAB, C14), and dodecyltrimethylammonium bromide (DTAB, C12), to produce organo- bentonites (OB) were studied. The concentrations of QAS were selected based on the cation exchange capacity (CEC) of the clay mineral. Zeta potential, swelling, and viscosity measurements and scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) analyses were used to explain the changes in surface properties. The results indicated that the best modification of bentonite was obtained using a 16-carbon chain length QAS (HTAB) in a 1:1 ratio of QAS to CEC. The basal spacing at this concentration was measured to be 22.19 Å, which also corresponded to the maximum adsorption density. The OB produced at this concentration showed the best hydrophobic character based on the swelling tests in toluene. The extent of hydrophobicity and adsorption density was correlated with the CEC and alkyl chain of the QAS. All these properties were used to elucidate the mechanism of modification governing the bentonite/QAS system.

The effect of plant species on soil-clay mineralogy has not been studied widely. In the present study, the clay mineralogy of top soils under Sequoia gigantia and grass regimes, maintained side by side for up to 150 y in the parks of several French châteaux, were investigated using X-ray diffraction (XRD) methods and chemical analyses of the clay fractions. The seven paired samples that were studied originated from soils developed on calcareous, granitic, and loess substrates. The XRD data indicated the presence of a trioctahedral chlorite with a trioctahedral hydroxy-Mg sheet in sequoia soils observed in four of seven of the sites whereas it was absent from the adjacent prairie-soil samples. Parent materials influenced the formation of magnesian chlorite as it was observed in all soils developed on granite and in none of the soils developed on limestone. The exchangeability of the interlayer hydroxy-Mg sheet replaced by K+ from newly formed chlorite in a 14 y old sequoia-influenced soil suggests that the mineral was initially a hydroxy-interlayered mineral. Increased stability was observed in the older (100 and 150 y) soil chlorites, indicating a progression of polymerization of the Mg hydroxy-interlayered material. The small amount of chlorite in the whole clay assemblage impeded the observation of changes in Mg content by direct chemical measurements of the clay fractions but the systematically greater amount of exchanged Mg2+ ion measured under sequoia compared with adjacent prairie supports the formation of Mg magnesian chlorite.

The results presented indicate, on the one hand, the importance of plant regimes in controlling the soil chemistry and hence the clay mineralogy of surface soil horizons (magnesian chlorites were observed only under sequoia), and, on the other hand, that parent material modulates this plant influence (chlorite formation was observed on granite-derived soils).

The coefficient of friction of clay minerals at the micro-scale has generally not been studied due to difficulties in obtaining measurements in a bulk-soil volume undergoing shear at such small scales. Information on friction at the micro-scale may provide insight into grain-scale processes that operate in bulk samples or in natural faults. The objective of this study was to develop a method to measure the microscale friction coefficient of smectites. The experiments described show that the axial atomic force microscopy method can be adapted to easily obtain accurate coefficient of friction (μ) measurements for smectites from force curves involving colloidal probes. The method allows for the measurements to be performed over spatial scales of a few μm, can be carried out under dry conditions or a wide range of aqueous solutions, and requires no calibration beyond making a few microscopic measurements of the probe. This method provides measurements of micro-scale normal and shear forces between minerals, which can be used for a variety of applications such as the study of shear deformation, consolidation, and fault dynamics. Control tests of silica on mica (μ = 0.29±0.02) agree with literature values where limits indicate one standard deviation. Coefficient of friction values for wet and dry Na-montmorillonite were determined to be 0.20±0.03 and 0.72±0.03, respectively.

Clay minerals are abundant in soils and sediments and often contain Fe. Some varieties, such as nontronites, contain as much as 40 wt.% Fe2O3 within their molecular structure. Several studies have shown that various Fe-reducing micro-organisms can use ferric iron in Fe-bearing clay minerals as their terminal electron acceptor, thereby reducing it to ferrous iron. Laboratory experiments have also demonstrated that chemically or bacterially reduced clays can promote the reductive degradation of various organics, including chlorinated pesticides and nitroaromatics. Therefore, Fe-bearing clays may play a crucial role in the natural attenuation of various redox-sensitive contaminants in soils and sediments. Although the organochlorinated pesticide p,p′-DDT is one of the most abundant and recalcitrant sources of contamination in many parts of the world, the impact of reduced Fe-bearing clays on its degradation has never been documented. The purpose of the present study was to evaluate the extent of degradation of p,p′-DDT during the bacterial reduction of Fe(III) in an Fe-rich clay. Microcosm experiments were conducted under anaerobic conditions using nontronite (sample NAu-2) spiked with p,p′-DDT and the metal-reducing bacteria Shewanella oneidensis MR-1. Similar experiments were conducted using a sand sample to better ascertain the true impact of the clay vs. the bacteria on the degradation of DDT. Samples were analyzed for DDT and degradation products after 0, 3, and 6 weeks of incubation at 30°C. Results revealed a progressive decrease in p,p′-DDT and increase in p,p′-DDD concentrations in the clay experiments compared to sand and abiotic controls, indicating that Fe-bearing clays may substantially contribute toward the reductive degradation of DDT in soils and sediments. These new findings further demonstrate the impact that clay materials can have on the natural attenuation of pollutants in natural and artificial systems and open new avenues for the passive treatment of contaminated land.