Nonexchangeable polymers in interlayers of expansible phyllosilicates influence thermal dehydration in ways not completely understood. Thermal dehydration of hydroxy-interlayered vermiculite (HIV) from Florida soils, for example, results in irreversible d001 shifts. This study was conducted to characterize HIV dehydration as a function of time (t) and temperature (T), and to determine how reversibility of dehydration is affected by elevated T. Clay-sized HIV from 3 soils was heated incrementally and d-spacing shifts (Δd) were monitored by X-ray diffraction (XRD) at low relative humidity (RH). Samples were then mounted on a metal heating strip in the XRD focal plane and scanned repeatedly at constant T levels to monitor Δd with t. Finally, Δd in response to RH shifts from <5% to >85% was determined at 25°C and at elevated temperatures. Incremental heating revealed a Δd plateau roughly corresponding to the z dimension of hexameric octahedrally coordinated Al. The initial slope of Δd-vs-t curves increased with T. The same maximum Δd reached at 200°C was reached at 160°C, but more slowly. All samples exhibited reversible and irreversible dehydration, the former being attributable to sites in equilibrium with external vapor and the latter to sites requiring heat for desorption. Reversible sites were not perturbed by moderate heating, but were apparently eliminated by polymer dehydroxytation. The dehydration behavior of HIV could be explained by steric resistance of water vapor diffusion within a tortuous interlayer polymeric network. Alternatively, new polymer/oxygen-surface bonds exceeding the hydration energy of interlayer components could form via heat-induced re-articulation of polymer/oxygen-surface bonds at smaller basal spacings.

Color variations from brown to yellow of synthesized goethite have been studied colorimetrically and spectroscopically. Goethite with various colors was synthesized at pH 13 and 40°C by varying the incubation time. Colorimetry revealed that the b* value (yellowish chroma) in L*a*b* color space was a quantitative indicator of color variations of the diluted samples. From UV-VIS-NIR spectra, the increase in the b* value was found to be caused by the increase in crystal field absorptions due to goethite formation around 500 nm. The b* value was a good indicator of the relative proportion of goethite in the precipitates including ferrihydrite. X-ray diffraction patterns and infrared spectra revealed that crystallization of goethite was associated with loss of water from the proto-ferrihydrite.

Six well-characterized soil kaolins from widely separated sites in south-western Australia and four reference kaolins were studied by 57Fe Mössbauer spectroscopy at room temperature after removal of non-structurally-bound iron with dithionite-citrate-bicarbonate solution. The soil kaolins and one of the reference kaolins were also studied at temperatures near 16 K. The soil kaolins were remarkably similar in crystal size, crystallinity, dehydroxylation temperature, cation exchange capacity, surface area and iron content. Müssbauer spectra of the soil kaolins at room temperature were also essentially identical consisting of a quadrupole-split doublet superimposed on a broad component which indicated that all of the iron was present as Fe(III) and that slow paramagnetic relaxation effects were present. Mean values for the chemical isomer shift and quadrupole splitting of the doublet for the soil kaolins were 0.33 and 0.55 mm/s respectively which indicates that the iron is in the octahedral sites of the kaolin lattice. The spectra of the soil kaolin samples at temperatures near 16 K showed a further slowing down of the paramagnetic relaxation and confirmed that no discrete iron oxide minerals were present.

Mössbauer spectra of the four reference kaolins at room temperature showed a doublet component similar to those for the soil kaolins. Three of them showed evidence for other spectral components including, in two cases, a component due to the presence of Fe(II).

Evidence of chlorite weathering to halloysite appears to be limited. Trioctahedral ferruginous chlorite occurring in granitic rocks and in situ altered phases isolated from the horizons of two soils in southern Korea were studied by scanning and transmission electron microscope (SEM, TEM), chemical, and X-ray diffraction (XRD) techniques to determine the mechanism of alteration and nature of alteration products. Chlorites were entirely converted to halloysite, kaolinite, and Fe-oxyhydroxides in the thick Bt horizon. In the E and BC horizons, in addition to 1:1 silicate clays and Fe-oxyhydroxides, chlorite-like and intergradient vermiculite-kaolin minerals were also found. Total chemical analysis of chlorite flakes revealed losses of substantial amounts of Fe and Mg. Large parallel sets of galleries suggested extensive exfoliation and expansion of chlorite flakes. Tubular halloysite formed bridges between the walls of galleries. The SEM and TEM analyses showed very distinct coatings (0.2–0.3 μm thick) of Fe-oxyhydroxides above and below the surface of galleries that consisted of rounded, sub-rounded, elongated, ultramicrocrystalline particles (goethite and hematite). The 1:1 mineral species found in the thick Bt horizon had a tubular and crumpled lamellar morphology. The presence of Fe likely creates a misfit between tetrahedral and octahedral sheets and results in the morphology of the 1:1 clays observed under SEM and TEM. The presence of intergradient minerals between chlorite and 1:1-type clays in the surface and transitional BC horizon suggests that, in addition to losses of OH sheet -Fe and -Mg, the chlorite with mostly Al-octahedra is partly transformed to double 1:1 silicate clays and intergradient minerals.

Synthetic hematites with Al substitutions between 0 and 18 mol % were synthesized at different temperatures and water activities. The cell-edge lengths a for different synthesis conditions decreased linearly with increasing Al substitution. The regression lines, however, had different slopes and intercepts: the series with the highest synthesis temperature (1270 K) had the most negative slope. With increasing Al substitution, the hematites contained increasing amounts of non-surface water. Significant correlations were found between these chemically determined water contents and the deviations of the unit-cell parameters a, c, and V relative to the corresponding 1270 K regression lines. To explain the measured X-ray peak intensities, structural OH had to be included into the theoretical calculations. From intensity ratios normalized to I113, it is possible to determine the structural OH separately from the Al substitution, which can be assessed by the shift of the cell-edge lengths relative to the 1270 K regression lines. The incorporation of Al and OH into the hematite structure induces strain, which was quantified by X-ray diffraction.