A preliminary survey of electronic absorption spectra of clay minerals reveals the utility of u.v.-visible spectroscopy in the elucidation of structural, physical, and chemical properties of such systems. Spectra, which were obtained in the suspension, film, and single crystal states (where applicable), are interpreted in terms of iron-associated transitions. Microcrystalline clay minerals typically show Fe(lll) in octahedral oxo-ligand geometry whereas mica-type minerals may show a range of iron species, including octahedral Fe(III), tetrahedral Fe(III), and octahedral Fe(II). Iron affects the local site geometry and in “high iron” minerals may dictate layer geometry and subsequently the crystalline form.

A series of mixed iron and titanium oxide coprecipitates ranging in composition between 0 < Ti/Ti + Fe < 1 was synthesized and aged under varying conditions of pH, temperature and time in order to establish a working model for pedogenic titanium and titano-ferric oxides. X-ray powder diffraction (XRD), selective chemical dissolution, magnetic susceptibility, charge distribution and electron optical data indicate that the freshly prepared Fe-Ti oxides consist of an Fe-rich (Ti-ferrihydrite) phase (Ti/Ti + Fe ⩽ 0.70) having pH-dependent positive charge and a Ti-rich phase (Ti/Ti + Fe ⩾ 0.7) with permanent and pH dependent negative charge.

Synthetic Ti-ferrihydrite and amorphous TiO2 were completely soluble in acid ammonium oxalate (2 hr extraction in the dark) whereas poorly crystalline anatase (width at half height, WHH > 2.0°2θ) was partly oxalate soluble. NH4-oxalate soluble Ti was particularly high in soils developed under a cool montane climate (afro-alpine) and lower in soils of warmer subtropical climate, which contain anatase and rutile.

Several mixed Fe-Ti crystalline phases were identified after aging NH3 coprecipitates of Fe and Ti nitrate at 70°C and pH 5.5 for 70 days:

1. (1) goethite and hematite in the composition range 0 < Ti/Ti + Fe ⩽ 0.20; at low Ti concentrations (<5 mole %) goethite was favored and/or hematite inhibited

2. (2) microcrystalline pseudorutile in the composition range 0.20 ⩽ Ti/Ti + Fe ⩽ 0.70

3. (3) anatase and ferriferous anatase in the range 0.70 ⩽ Ti/Ti + Fe < 1.0; with decreasing proportion of Ti the crystal-Unity of anatase decreased.

The results suggest that secondary or pedogenic Ti-Fe oxides can form by coprecipitation and crystallization in the weathering solution, and emphasize the essential role of water (as opposed to dry oxidation) in the alteration of primary titaniferous minerals.

The pH, Eh, electrical conductivity (EC), and the amounts and valency of replaceable iron were measured periodically on Fe2+- and Fe3+-saturated montmorillonite and cation-exchange resin at three temperatures. Differences in the pattern of change of pH, Eh, and EC with time appear to be related more to the histories and modes of preparation of the systems than to intrinsic differences in the hydrolysis of the iron in them. Electron transfer reactions involving crystal components of the clay can cause oxidation of adsorbed Fe2+ ions; the activation energy (Ea) for oxidation on the clay's surface was 6 kcal/mole, less than a third of the activation energy reported for Fe2+ oxidation in solution. In the Fe2+-resin, where Ea = 10.7 kcal/mole, perturbed surface-water molecules may act as electron acceptors enhancing Fe2+ oxidation.

Polymerization and precipitation of the adsorbed iron is affected by the necessity to maintain electroneutrality, the ability of the iron-hydroxy ions and small polymers to move about in the voids of the ion exchanger, and the steric hindrance posed by the matrix of the ion exchanger to the formation of large polymers. In resin, little or no iron precipitates, probably due both to steric hindrance and the inability of the resin to release ionic components to maintain electroneutrality. In clays, steric hindrance is small, and Al and Mg are released from the crystal to maintain electroneutrality, thus the precipitation of iron is abundant and is controlled by the rate of release of Al and Mg from the crystal.

Previous studies of the line profiles of the basal reflections of microcrystalline muscovites were refined by an adaptation of the method developed by Maire and Méring. In order to evaluate the variation of interlayer spacings, the method required only relative values of Fourier coefficients, without the correction for instrumental broadening, which was the source of one of the most critical problems previously. Instead of Kα radiation, Kβ radiation was used to record line profiles since difficulties associated with the separation of Kα1 and Kα2 radiations could not be overcome satisfactorily.

The data reconfirmed that the line broadening of 00l reflections was due not only to a small particle-size effect, but also to structural disorders involving the variation of the interlayer spacings. For the four specimens investigated here, the mean squares of the variation of interlayer spacings ranged from 0 to 0·0358, the square roots of which were inversely proportional to the total number of interlayer cations. It is considered that the observed distortions were mainly attributed to non-uniform interlayer spaces between silicate layers arising from an irregular distribution of interlayer cations. The data also indicated that the crystallites of all four specimens consisted of a similar number of layers. The method showed promise for the study of the nature and extent of structural disorders in micas or other silicate minerals.

Neutron inelastic scattering spectra for kaolinite, dickite, pyrophyllite, and muscovite show a characteristic peak between 850 and 910 cm−1, while those for chrysotile, antigorite, talc, phlogopite, and amphibole minerals show characteristic peaks at 620–650 cm−1 and 460–510 cm−1. These peaks correspond to localized torsional oscillations of (OH)− groups. Lower-frequency peaks are also observed and are associated with optical and acoustic modes involving hindered translations. Within a series, the similarity in the shapes and the positions of the peaks indicates that the motions of the (OH)− groups are determined primarily by nearest-neighbor cation coordination. Differences between the two series can be attributed to the different environments when the octahedral layer of the lattice is populated either by two or by three cations.

The spectra of the hydrated minerals, montmorillonite, hectorite, and halloysite, show lines characteristic of liquid water. Upon dehydration, peaks corresponding to the motions of structural (OH)− units are observed.

The dissolution of synthetic magnetite, maghemite, hematite, goethite, lepidocrocite, and akaganeite was faster in HCl than in HClO4. In the presence of H+, the Cl− ion increased the dissolution rate, but the ClO4− ion had no effect, suggesting that the formation of Fe-Cl surface complexes assists dissolution. The effect of temperature on the initial dissolution rate can be described by the Arrhenius equation, with dissolution rates in the order: lepidocrocite > magnetite > akaganeite > maghemite > hematite > goethite. Activation energies and frequency factors for these minerals are 20.0, 19.0, 16.0, 20.3, 20.9, 22.5 kcal/mole and 5.8 × 1011, 1.8 × 1010, 7.4 × 107, 5.1 × 1010, 2.1 × 1010, 3.0 × 1011 g Fe dissolved/m2/hr, respectively. The complete dissolution of magnetite, maghemite, hematite, and goethite is well described by the cube-root law, whereas that of lepidocrocite is not.

The Fremont provide an important case study to examine the resilience of ancient farmers to climatic downturns, because they lived at the far northern margin of intensive maize agriculture in the American West, where the constraints on maize production are made abundantly clear. Using a tree-ring and simulation-based reconstruction of average annual precipitation and maize growing degree days, along with cost-distance to perennial streams, we model spatial variability in Fremont site density in the eastern Great Basin. The results of our analysis have implications for defining the ecological envelope in which farming is a viable strategy across this arid region and can be used to predict where and why maize farming strategies might evolve and eventually collapse as climate changes over time.

Glauconite pellets of vermiform and lobate morphology occur together in Eocene geologic formations in Maryland. Morphologically, the vermiform pellets appear to be identical to those that have previously been called “altered biotite”. In thin sections these pellets do show a well-defined micaceous morphology with the layers running across the worm-like pellets. Some zones in these pellets appear to be “crystals” that are up to 30 × 70 μ and nearly rectangular in cross section. However, there are tiny cracks along cleavage planes within these “crystals”. Externally, the lobate pellets have many rounded lobes and are similar to one of the shapes that Burst has called free-form. In thin section under crossed nicols these pellets have a grainy appearance, indicating that the lobate pellets are composed of many small zones, each about 5–20 μ across. Within these zones the mineral glau-conite has a single orientation, but the zones are not lined up with each other to give the gross micaceous appearance that is associated with the vermiform pellets.

Random powder X-ray diffraction patterns (prepared with a large 114·59mm Norelco powder camera) of individual vermiform and lobate pellets are nearly identical. Eight vermiform and 9 lobate pellets gave the same mean 001 (10·2 Å) and 060 (1·518 Å) spacings. The patterns from both kinds of pellets are similar, except for the absence of some weak lines, to Warshaw’s (ASTM) pattern for glauconite. The patterns have lines indicating a 1 M polytype, however, hkl lines with k ≠ 3n are broad indicating some disorder. In addition to X-ray diffraction patterns, the K2O content (6·7 per cent) and CEC (29 me/100 g as Ca replaced by Mg) of the pellets indicate that interstratified expanded layers may be the main source of the disorder.

If the vermiform pellets are altered mica, the alteration has been sufficient to give a product that is definitely identified as glauconite by X-ray methods. The possibility of mica alteration is suggested by the geographic nearness of the Piedmont (a mica source area) and the occurrence of Piedmont-type quartz with the glauconite pellets. Alternatively, the vermiform pellets may form during glauconite crystallization or recrystallization processes. The probability that both kinds of pellets obtained their morphology before or during, rather than after, the time they became glauconite (mineralogically) suggests that the proper environment may form glauconite from a variety of starting materials.

The lattice collapse and potassium fixation upon heating was studied for four expanding clay minerals in the potassium form by determining the change in X-ray patterns, and the decrease of both the cation exchange capacity and the total surface area derived from water vapour adsorption isotherms.

In the literature, two factors promoting collapse have been considered: a high degree of tetrahedral substitution and a high potassium ion population in the unit layer surface. For the four minerals studied these two factors varied between wide limits, and the ease and degree of collapse were indeed found to correlate with these two factors. A third factor which has been considered previously is the degree of crystallinity of the mineral, which is determined by the origin of the clay. For the two bentonites which were investigated, one shows poor crystallinity, indicating its volcanic origin, the other shows a higher degree of crystallinity with boundary conformity in stacks of layers, indicating its formation by weathering of a micaceous mineral. The latter shows a higher degree of collapse.

Reaction with Na2S solutions at high pH led to almost complete, reversible reduction of iron in montmorillonite, whereas the structural iron of nontronite persisted in the ferric form. Concentrated Na2S solutions caused severe corrosion of nontronite and extracted appreciable amounts of iron, which was precipitated as sulphides. In contrast the morphology of montmorillonite was preserved and only very minor amounts of iron were extracted. These differences were attributed to the high concentration of Fe-OH-Fe groups in nontronite, which are unstable on reduction. The difference between Na2S and other reducing agents is discussed.