High-resolution transmission electron microscope images of dioctahedral mixed-layer clay structures (illite/smectite) having various substitutional and polytypic schemes were modeled using computer simulation methods. Both one- and two-dimensional calculations were performed using parameters characteristic of a typical range of imaging conditions. One-dimensional images formed by imaging only 00/ diffractions show three important results: (1) The 20-Å periodicity resulting from rigorously ordered R1 illite/smectite can be imaged, but unconventional focus conditions may be necessary. (2) For crystals oriented with the electron beam perfectly parallel to the layers, the brightest fringes in the image correspond to either the octahedral sheets or the interlayer sites, depending on focus conditions. Misorientation of the crystal, however, by only 1° or 2° shifts the positions of the fringes by 1 to 3 Å. Furthermore, in tilted specimens, some defocus values produce images suggesting that smectite layers have a 11–13-Å periodicity, despite the uniform 10-Å periodicity present in the model structure. Thus, direct correlations between image and structure generally should not be made. (3) Two-layer polytypes of pure illite or pure smectite can also produce images with a 20-Å periodicity.

Two-dimensional images additionally showed that the cross fringes produced by some hkl diffractions can be imaged. The simulations showed that these cross fringes ideally might permit the determination of both layer stacking and compositional periodicity, but the fringes are lost by misorientations of a few degrees. These image simulations demonstrated, therefore, that mixed layering of illite and smectite theoretically can be directly imaged by transmission electron microscopy of chemically untreated specimens, but ambiguities may exist in the detailed intepretation of the images.

Potentiometric titration analysis was used to examine the hydrolysis behavior of Fe2+ Fe3+, and Al3+ in pure solution and in mixture, in order to evaluate the potential for coprecipitation and mixed solid-phase formation. Mixtures of Fe3+ and Al3+ did not interact during neutralization; base consumed in their respective buffer regions was equivalent to the total metal added. Fe2+-Al3+ solutions, however, showed excess base consumption in the Al3+ buffer region, indicating hydrolysis of Fe2+ at lower than normal pH. Ferric/ferrous iron analyses of systems at the Al endpoint (pH 5.5) showed amounts of oxidized Fe equivalent to the excess base consumption (∼10% of total Fe), with substantial amounts of Fe2+ sorbed to or occluded within Al polymers present. Increased electrolyte levels or the presence of SO42- inhibited oxidation and sorption of Fe2+ on Al surfaces, suggesting that Fe hydrolysis and oxidation was catalyzed at the surfaces. Increasing Al3+: Fe2+ ratios in the titrated solutions also increased the amount of Fe2+ coprecipitation, supporting a surface-mediated reaction mechanism. Ferrous iron oxidation was sensitive to O2 levels, which also affected the amount of coprecipitation. These findings suggest that surface-facilitated oxidation of Fe2+ may be important in the formation of mixed Fe-Al mineral phases in dilute soil solutions.

The weathering products of primary biotite, chlorite, magnetite, and almandine in mica gneiss and schist in the North Carolina Blue Ridge Front were determined. Sand-size grains of biotite, the most abundant, readily weathered mineral in the parent rock, have altered to interstratified biotite/vermiculite, vermiculite, kaolinite, and gibbsite in the saprolite and soil. Fe2+-chlorite in the parent rock was relatively resistant to chemical weathering, which appears to be confined to the external surfaces of particles. Magnetite grains in the saprolite are essentially unaltered, but those in the soil contain abundant crystallographically controlled etch pits and are coated with oxidation crusts. Almandine altered to goethite, hematite, and gibbsite as the rock weathered to saprolite. Extensively weathered almandine grains were found to contain etch pits and what appeared to be oxide coatings. Apparently, a rapid release of Fe during weathering produced hematite, whereas slower release of Fe favored the formation of goethite.

The mineralogy of the clay fraction was studied for soils and saprolite on two Blue Ridge Front mountain slopes. The clay fraction contained the weathering products of primary minerals in the mica gneiss and schist parent rocks. Gibbsite is most abundant in the saprolite and residual soil horizons, where only chemical weathering has been operable. In colluvial soil horizons, where both physical and chemical weathering have occurred, the clay fraction consists largely of comminuted primary phyllosilicates —muscovite, chlorite, and possibly biotite—and their weathering products: vermiculite, interstratified biotite/vermiculite (B/V), and kaolinite. The clay-size chlorite contains Fe2+ as indicated by Mössbauer spectroscopy, and is more resistant to weathering than biotite. The vermiculite and B/V, both weathering products of biotite, contain Fe3+. Vermiculite in colluvial soils and, especially, surface horizons is weakly hydroxy-interlayered. The kaolinite in the clay fraction resulted at least partly from the comminution of kaolinized biotite in coarser fractions.

The hematite content ranged from 0 to 8% of the clay fraction and strongly correlates (r =.95) with dry clay redness, as measured by the redness rating: RR = (10 - YR hue) × (chroma) ÷ (value). The hematite is largely a product of the weathering of almandine; thus, the soil redness is dependent on the amount of almandine in the parent materials and its degree of weathering in the soils. Goethite (13–22% of the clay fraction) imparts a yellow-brown hue to soils derived from almandine-free parent rocks. The release of Fe in relatively low concentrations during the weathering of Fe-bearing primary minerals, particularly biotite, appears to have promoted the formation of goethite.

The layer charge of five smectites, one vermiculitic material, and five reduced-charge clays was determined by the alkylammonium and structural formula methods. The two sets of results were found to be linearly correlated (r =.961); however, the values that were determined by the alkylammonium method were 20 to 30% lower than those determined by the structural formula method, and the regression slope for their linear relationship was 1.67. The fact that the structural formula method includes the effects of cations on the lateral edges of the clay particles probably accounted for some of the differences in the magnitude of the results but should not have caused the regression slope to deviate substantially from 1.00. Therefore, inaccurate estimates of the packing density of alkylammonium cations in the interlayer space of 2:1 phyllosilicates were deemed responsible for the systematic divergence of the results of the two methods. To satisfy the need for a relationship between the two methods of determining layer charge, an empirical means of adjusting the alkylammonium values has been proposed and shown to yield values of layer charge that are comparable to those determined by the structural formula method.

Perlite, which occurs at the contacts between dacite and lacustrine tuffs of Miocene age in the Guryongpo area, Korea, has undergone more sluggish and incomplete diagenetic alteration than the surrounding zeolitic tuffs. Alkali-clinoptilolite, mordenite, smectite, K-rich gel-like glass, low-cristobalite, and K-feldspar are characteristic diagenetic phases in the altered perlite.

Hydration of the glass to form perlite resulted in the expulsion of cations, mainly Na, from the glass into the pore fluid and in the relative enrichment of K in the perlite. Hydration of the glass also resulted in increased alkalinity of the pore fluid, which, in turn, affected the nature and behavior of subsequent glass dissolution. Textural observations and chemical data on the early diagenetic phases indicate a sequence of incongruent dissolution reactions, which depended on silica activity and alkalinity of the ambient pore fluid: Reaction (1) 12.5 perlitic glass + 3.88 K+ + 0.65 H+ + 15.4 H20 = smectite + 9.5 gellike glass + 4.03 Na+ + 0.25 Ca2+ + 10.55 H4SiO4 released Ca and silica which were consumed by reaction (2) perlitic glass + 0.1 Ca2+ + 0.1 H4SiO4 + 0.1 H+ + H20 = clinoptilolite + 0.1 K+ + 0.2 Na+. The paragenesis from glass via smectite to alkali zeolites in most glass-bearing rocks may be explained by a sequence of such dissolution reactions. Still later reactions involved the transition from less-silicic clinoptilolite to an assemblage of silicic Na-clinoptilolite + mordenite and the crystallization of the gel-like glass to K-feldspar.

Glauconite from the oxidized and reduced zones of soil-geologic columns at two Coastal Plain sites, one in Maryland and one in New Jersey, was examined by Mössbauer spectroscopy. The data indicate that glauconite in the reduced zones had a higher proportion of its structural iron in the ferrous, as opposed to the ferric state. The Fe2+/Fe3+ ratio ranged from 0 to 0.2 for the glauconite from the oxidized zone and was about 0.35 for the glauconite in the reduced zones. Despite the presence of pyrite in the reduced zone, which might be expected to make ferric iron unstable because of the presence of sulfide S, about 75% of the Fe in the glauconite in the reduced zone was in the ferric state. Thin section analysis showed some glauconite in the reduced zones to be intimately associated with pyrite and some aggregates of fine pyrite crystals were locally present in cracks in glauconite pellets. In the oxidized zones, pyrite was absent and the glauconite was more yellow under plane-polarized light, as opposed to more green for the glauconite in the reduced zones. These data indicate that reports of studies of glauconite should stipulate whether samples are from the oxidized or reduced zone of soil-geologic columns.

At pH 12 Co-ferrihydrite transformed to either Co-goethite or Co-magnetite, the latter compound appearing at Co additions of > 18 mole %. Although Co was readily taken up by the magnetite structure, chemical analysis showed that no more than 7 mole % substitution of Co in goethite was achieved. Hematite formation was not strongly promoted by the presence of Co; with 9 mole % Co in the system, the amount of hematite relative to goethite in the product increased slightly. Co-substituted goethites grew as long, thin crystals. The presence of Co promoted some dendritic twinning of goethite. Cobalt retarded the transformation of ferrihydrite to more crystalline oxides mainly by stabilizing ferrihydrite against dissolution. A comparison of Co with divalent ions previously studied showed that their stabilizing ability decreases in the order Cu > Co > Mn, i.e., they follow the Irving-Williams series for the stability of metal complexes.

Gibbsite samples of various particle sizes (0.2–80 μm) were heated at 250°C in a series of straight-chain primary alcohols under the autogenous vapor pressure of the alcohol (alcohothermal treatment of gibbsite). The treatment in ethanol yielded pure boehmite, the morphology of which was similar to that of the boehmite obtained by hydrothermal treatment of gibbsite. In middle-range alcohols, the boehmite yields were low (50% if 80 μm gibbsite was used), and the products were contaminated by a poorly crystallized phase, having a χ-alumina-like structure. The products preserved the morphology of the originating gibbsite, this feature being similar to the thermal dehydration of gibbsite. Complete conversion to boehmite was also attained in mineral oil (a hydrocarbon mixture, which was used as a limit of higher alcohol. The morphology of the boehmite formed in this medium was identical to that of the product prepared by thermal dehydration of gibbsite in a sealed bomb without a medium. If fine particle-size gibbsite was used, the yield of boehmite decreased and the yield of the poorly crystallized phase increased in all the media.

The reaction mechanisms may be discussed in terms of the reported mechanisms for the thermal and hydrothermal formations of boehmite from gibbsite. Thus, in lower alcohols boehmite formed by a dissolution-recrystallization mechanism, whereas in middle-range or higher alcohols it formed by intra-particle hydrothermal reaction mechanism proposed by de Boer and coworkers for the thermal dehydration of gibbsite. The difference in behavior in middle-range and higher alcohols can be explained in terms of the solubility of water in the medium: In the middle-range alcohols, water molecules formed by partial dehydration of gibbsite were removed from the gibbsite particles into the medium so that dehydration proceeded in a manner similiar to that of thermal dehydration, whereas in the higher alcohols, the low solubility of water in the medium allowed the water molecules to remain on the surface of the particles, thereby promoting the complete hydrothermal formation of boehmite.

1-4-diazabicyclo[2,2,2]octane dihydrochloride (DABCO·2HC1) was reacted with two vermic-ulite samples to produce intercalates which, at room temperature, had relatively sharp, single crystal X-ray diffraction patterns. At higher temperatures (250°C) the stacking order decreased, and consequently the 0kl reflections with k ≠ 3n became increasingly diffuse. The stacking order of previously heated samples returned when they were cooled. A superstructure was present in which DABCO cations occupied the corners and center of a cell 3a × b, compared with the standard vermiculite cell.

DABCO-intercalated Nyasaland vermiculite had the following monoclinic subcell (symmetry C1) parameters under ambient conditions: a = 5.341(2), b = 9.249(3), c = 14.50(1) Å, and ß = 96.98(5)°. Differential Fourier analyses and least-squares refinement led to a final R value of 12.6% for 1814 reflections. The crystal structure analysis showed that individual DABCO ions were not symmetrically positioned between the silicate layers. A network of inorganic cations and water molecules was also present and governed the interlayer separation. At 250°C the d value was 13.7 Å, consistent with a dehydrated structure, in which each organic pillar has one amino group keyed into a ditrigonal cavity and the other amino group riding on the basal oxygens of an opposite tetrahedron.

The isothermal solid state reactions of Na2CO3 and other salts at 550°C with montmorillonite and with 1:1 mixtures of montmorillonite-illite and montmorillonite-chlorite were investigated. At a concentration of 20 wt. %, Na2CO3 caused the complete destruction of the X-ray powder diffraction (XRD) pattern of montmorillonite alone and in mixtures with other clays. Less deterioration of the XRD pattern was found for NaOH, NaF, K2CO3, and Li2CO3. NaCl, Na2SO4, Na3PO4, MgCO3, and CuCO3 had no effect on the XRD traces of montmorillonite. The time dependence of the Na2CO3-montmorillonite interaction at 550°C suggested that the reaction is diffusion controlled in three dimensions, i.e., diffusion was directed from the surface of the particle towards its center.