Gibbs free energies for phases in the system Al2O3-SiO2-H2O have been calculated from reversed experiments in order to correct earlier values and to calculate a phase diagram consistent with more recent experiments. An internally consistent diagram could not be calculated that agreed with all published experiments, and choices of preferred data were made. The following Gibbs free energies, relative to the elements at STP (298.15 K, 1 bar), have been derived

The above values were calculated assuming literature values for corundum, quartz, and H2O (v).

Examination of available thermodynamic, experimental, and observational data on the aluminum hydroxides gibbsite, boehmite, bayerite, and nordstrandite suggests that these minerals are metastable with respect to diaspore + water at STP and at higher temperatures. Similarly, halloysite and dickite are metastable with respect to kaolinite at these conditions. The occurrence of these minerals in soils must therefore be ascribed to nonequilibrium processes, and the use of equilibrium phase diagrams to explain their occurrence is inappropriate.

The basal spacing of a set of smectites, with layer charges between 0.74 and 1.14 electrons per unit cell, have been measured while the smectites were in equilibrium with NaCl solutions having concentrations of 0.25 to 2.5 molal. Except for the most highly charged smectites, an expansion from ~15.5 to ~18.5 Å occurred as the NaCl concentrations were reduced. This expansion (or “crystalline swelling”) corresponds to the transition from two to three sheets of water between the silicate layers. A random interstratification of 15.5 and 18.5 Å structure units was present during the change and gave rise to broad diffraction peaks for the 001 reflections. A balance between cation hydration forces, interlamellar electrostatic forces, and van der Waals forces was apparently the basis of the relationship between surface charge and swelling. The results can be expressed in terms of the relative vapor pressures at which the transitions were half complete; these pressures increased with surface density of charge, and over the range of surface charge studied, the P/P0 values ranged from 0.943 to 0.974.

Mordenite occurs in hydrothermally altered rhyolitic volcanic rocks at the margin of a Late Miocene lake basin on the island of Samos, Greece. The mordenite was identified by its X-ray powder diffraction pattern and appearance in scanning electron micrographs. Electron microprobe analyses show high Si, Ca, and K, low Na, and high balance errors. The b cell dimension is consistent with a high Si/(Al + Si) ratio, and the balance errors are apparently due to Na deficiency. The missing Na was estimated from the amount required to give (Si + Al) = 48 and yielded analyses comparable with X-ray fluorescence analyses of samples predominantly of mordenite. Even after this correction, the mordenite had a low Na:K ratio compared with most analyses reported in the literature. The peculiar chemistry of the mordenite may have resulted from a high-temperature metasomatic origin as a result of basaltic volcanism at the basin margin and characterized by hydrothermal circulation of alkaline lake water rich in K.

Riet veld X-ray powder diffraction (XRD) analysis has been evaluated as a procedure for characterizing Al-substituted goethite according to the Rietveld scale factor, unit-cell parameters, and atom positional parameters. The study was conducted with three synthetic goethite samples for which the degree of Al substitution for Fe determined by chemical analysis was 8.0 ± 0.4, 12.0 ± 0.4, and 20.1 ± 0.4 mole %. The weight fractions of crystalline material (WFCM) in the specimens, determined from the Rietveld scale factors after correcting for adsorbed water and impurities, were 0.878 (esd = 0.014), 0.919 (0.014), and 0.965 (0.015), respectively. The Al mole % substitutions, inferred from the Rietveld cell parameters according to the method of Schulze (1984), were 10.4 ± 2.5, 16.5 ± 2.6, and 17.1 ± 2.6, respectively. The cause of the significant difference between the second value and the chemical analysis result is not known. The atom positional parameters did not differ significantly within the sample suite and agreed satisfactorily with literature values. The results have demonstrated the value of using Rietveld XRD analysis to determine simultaneously the WFCM and Al mole % substitutions, as well as to confirm the non-hydrogen atom positions.

Unheated natural mixtures of manganite and secondary pyrolusite, from the same lateritic manganiferous sequence, were studied in different orientations by high-resolution transmission electron microscopy (HRTEM), electron diffraction, and energy-dispersive X-ray analysis (EDX) to determine the fine structure of these phases, their possible crystallographic relations, and the genetic processes that led to the formation of the pyrolusite. Typical palisadic texture was observed for both minerals. Characteristic cracks parallel t. (010) of the pyrolusite structure and in particular <210> microfissures in manganite were noted as signs of structural accommodation accompanying the transformation phenomenon between these two minerals. A previously unreported manganese oxide of the spinel-type (γ-Mn2O3 or Mn3O4) was also identified in the original mixture. This oxide gave pure microdomains as intergrowths with pyrolusite adjacent to manganite. This is the first report of a natural occurrence of γ-Mn2O3. The manganite-pyrolusite transformation process and an unsuspected γ-Mn2O3 (Mn3O4)-pyrolusite transition were directly illustrated in detail for the first time. Interfaces between the concerned phases were not sharp or smooth, but exhibited strong strain contrasts and interferential periodicities. Lattice images and microdiffraction patterns proved that both transformations were oriented, suggestive of topotactic relations. In addition, the principal minerals in the matrix (illite, kaolinite, and goethite) were examined for a better understanding of their role in Mn-oxyhydroxides transformations.

To determine the parameters that control the attack of Mn minerals by acid ammonium oxalate in darkness (AAOD), rhodochrosite, pyrolusite, manganosite, hausmannite, and bixbyite were shaken with AAOD for 2 hr. These treatments were followed systematically by X-ray powder diffraction (XRD) and AAOD-extractable Mn analyses. About 5% of original hausmannite (surface area = 6 m2/g) remained in the solid residue of the AAOD treatment; however, if the hausmannite surface area was increased to 8 m2/g, by grinding, it completely dissolved in oxalate. Synthetic hausmannite of high surface area (37 m2/g) and rhodochrosite were completely dissolved by oxalate. Manganosite (1.5 m2/g) and especially pyrolusite (~ 1 m2/g) were more resistant to AAOD attack. Ground manganosite (4.2 m2/g) dissolved completely, but ground pyrolusite (7.2 m2/g) was only partially attacked by AAOD, inasmuch as about 25% of pyrolusite was found in the residue. An increase of the extraction time to 4 hr did not completely dissolve the ground pyrolusite.

As a result of the AAOD treatment, MnC2O4 · 3H2O and MnC2O4 · 2H2O precipitated from the oxalate solutions with all starting minerals, except pyrolusite (~ 1 m2/g), which only slightly dissolved. The seldom reported MnC2O4 · 3H2O phase was identified in residues of freshly extracted samples by its strong characteristic peak at 6.5.-6.6 Å, the intensity of which gradually decreased and disappeared over several days when the sample was exposed to ambient conditions (22°C and 70% relative humidity). The trihydrate phase also collapsed after heating AAOD-treated rhodochrosite at 50°C; α-MnC2O4 · 2H2O was identified as the main crystalline product. Heating the α-MnC2O4 · 2H2O product at 115°C overnight transformed most of it to MnC2O4. The color of the oxalate-treated samples ranged from pinkish-gray to black (7.5 YR); their surface area ranged from about 20 to 30 m2/g. The degree of transformation of Mn minerals by oxalate depended on the surface area and structural characteristics of the starting materials.

Dealumination of vermiculite was carried out using (NH4)2SiF6 solutions. The dealuminated products were studied by high-resolution solid state 29Si and 27Al nuclear magnetic resonance. A decrease in the cation-exchange capacity (CEC) resulted from the partial removal of Al from the tetrahedral layer, which decreased the framework negative charge, and from the partial replacement of Mg by Al in the octahedral layer, which increased its positive charge contribution. The lowest CEC was obtained by swelling the structure with butyl-ammonium prior to the reaction with (NH4)2SiF6. Thus, CECs in the range observed for beidellite were measured; however, the lowest (Al/Si)IV ratio was still more than twice as high as in beidellite. In addition, the dealumination reaction yielded noncrystalline silica as a by-product.

In contact with a solution of Al hydroxypolymer (Al13), the dealuminated vermiculite showed no 18-Å reflection characteristic of Al13-intercalated smectite; instead it showed an ill-defined interstratification. For some samples, however, a significant increase in the specific surface area (as much as 230 m2/g) was observed, suggesting that an intercalation of Al moieties did occur. The 27Al resonance spectra of the intercalated structure showed at least two components in octahedral coordination. On thermal activation, a resonance line attributable to pentacoordinated Al was observed.

To identify the far-infrared (FIR) absorption bands related to K cations in micas, spectra of muscovite, phlogopite, and biotite were compared with the spectra of pyrophyllite and talc, which have no compensating cations, and to the spectra of micas in which K is substituted by mono- and divalent cations. Dichroic experiments using single crystals of these micas showed that some of the bands or components related to K have a strong in-plane and out-of-plane dichroic character. These experimental data led to the assignment of the FIR bands at 110, 91, and 83 cm−1 for muscovite, phlogopite, and biotite, respectively, which have no in-plane and out-of-plane dichroic character, to the vibration of the double ring of oxygen atoms that constitutes the cage in which K is located (mode III).

Bands at 146, 136, and 152 cm−1 for muscovite, phlogopite, and biotite, respectively, which have a strong out-of-plane dichroic character, were assigned to the out-of-plane vibrations of K atoms (mode IV). Some of the components at 190, 158–153, and 145–124 cm−1 for muscovite, phlogopite, and biotite, respectively, which have a strong in-plane dichroic character, are possibly related to the in-plane vibrations of K atoms (modes I and II). The 165-cm−1 band of muscovite is a lattice mode of vibration. Frequencies of modes III and IV of the compensating cations of micas in which K was substituted by mono- and divalent cations exchanged as a function of $\sqrt{Z/m}$ where Z is the charge and m the mass of the cation. Modes III and IV were well resolved and very sensitive to the crystallochemical properties of the structure (di- or trioctahedral character, Fe content, etc.). FIR spectroscopy may therefore be an important tool that uses compensating cations as probes to study the interactions between the cations and the structure of mica minerals.

Diffuse reflectance infrared (IR) spectra show that pressing kaolinite into pellets for transmission IR studies changes relative intensities of hydroxyl peaks between 3700–3600 cm-1. The magnitude of change depends on the absolute pressure, the pressing time and whether a salt matrix is used. Pressing with KBr causes larger differences than pressing neat. Diffuse reflectance IR does not require sample pressing, eliminating this variable in kaolinite studies. This improves IR's ability to distinguish kaolinites according to their hydroxyl group differences and may lead to a better understanding of kaolinite hydroxyl structures.

Samples from initially smectite-rich Tertiary continental volcaniclastic sediment from the Deer Lodge and Big Hole basins of southwestern Montana show a general decrease in illite/smectite (I/S) expandability with increasing burial depth. The mineralogical trends in cuttings from seven wells are interrupted by discontinuities in which I/S expandability abruptly decreases by 30 to 80%. These discontinuities coincide with stratigraphic unconformities in four wells in which the stratigraphy is known. Core samples show a wide range of I/S expandabilities over short stratigraphic intervals, possibly due to composition, porosity, and permeability variations. Sericite coexists with I/S in the deep core samples. A core sample from 7958 ft (2425 m) contains an R3-ordered I/S having a nearly ideal 3:1 illite: smectite ratio, similar to the mineral tarasovite. The structure of this I/S is dominated by stacks of four 2:1-layer fundamental illite particles and small proportions of thicker particles randomly interstratified among the four-layer particles.

The effect of heat treatments on the total charge and water adsorption by Ca-, Na- and Li-saturated kaolinite was studied using extraction techniques and thermal gravimetric analysis, respectively. Measurements of cation exchange capacity indicated that the total charge of Li-kaolinite was reduced by approximately 50% after heating to 110 or 130°C. In contrast, the total charge of Ca-kaolinite remained essentially constant while that of Na-kaolinite decreased slightly. Water adsorption and desorption on Ca- and Li-kaolinite following heat treatments at 150°C were consistent with the total charge of the respective kaolinites. Ion extraction of Li-kaolinite using NH4C1 revealed that only 6% of the Li remained exchangeable after heating, while Al and H were released. Thus, non-exchangeable Li ions not only reduced the total charge of the kaolinite but also displaced Al and H from the kaolinite structure. Infrared spectroscopy also indicated that Li migrated into the kaolinite structure and replaced a portion of the Al from the octahedral sheet. The results presented here indicate that Li-kaolinite represents a surface of reduced charge rather than a surface free of cation-hydration effects. Therefore, Li-kaolinite is not recommended as a reference for the study of vapor-phase adsorption, and conclusions based on such a reference material should be reevaluated.

Use of a discriminant analysis has verified and grouped three suspected varieties of kaolinite found in kaolin-rich clay strata of late Paleocene to early Eocene age across north-central Mississippi. Initial identification of each type of kaolinite was based on clay-texture characteristics observed on scanning electron micrographs and the differences in pattern configurations of X-ray diffractograms. The discriminant function used for data treatment clearly segregated and grouped each variety. The discrimination variables were found to be the Hinckley index and, to a lesser extent, the Si4+ content relative to the Al3+ content.

The oldest variety is the Blue Mountain clay, composed of preserved hexagonal plates usually clustered into booklets with a vermiform texture. The Ashland variety, stratigraphically younger than the Blue Mountain clay, appears to have been derived from the erosion of the Blue Mountain clay. The Ashland cannot be recognized by any type of diagnostic texture, as it is made up of individual plates that have been corroded and abraded to the point where a hexagonal outline can no longer be recognized. The Sardis variety is the stratigraphically youngest of the three varieties and is at least a second, or possibly a third generation detrital product. The Sardis clay can be recognized by a distinct “ribbon” or “swirl” texture commonly found in ball clays.

Data from this study are not sufficient for complete petrogenetic interpretation. However, speculation on possible differences in depositional environments and modes of deposition can be based on the data at hand. The Blue Mountain variety is considered from previous studies to be primary. The Ashland variety is probably a first generation alluvial clay. The Sardis variety appears to be a multiple generation, detrital product that accumulated as part of overbank swamp deposits.