G. Norman White

ANALYSIS AND IMPLICATIONS OF THE EDGE STRUCTURE OF DIOCTAHEDRAL PHYLLOSILICATES

Crystal growth theory was applied to describe edge sites of phyllosilicates. Three face configurations were found to exist. One face has one tetrahedral site per tetrahedral sheet and two octahedral one-coordinated sites per crystallographic area ac sin β, where a and c are layer dimensions and β is the angle between them. The other two faces are similar except that they have one less octahedral site which is replaced by one SiIV-O-AlVI site in this same ac sin β area. A transfer of bonding energy from the remaining octahedral site to the SiIV-O-AlVI site is believed to neutralize all edge charge on faces containing these latter sites at normally encountered pHs (pH 3–9). A similar charge rearrangement along the edges results in an apparent decrease in the permanent charge of the mineral with an increase in edge area.On the basis of such an analysis, lath-shaped illite can be described as a very fine grained dioctahedral mica in which the apparent deficient occupancy of the octahedral sheet, presence of excess water, and measurable cation-exchange capacity may in part be the result of a large ratio of edge area to total volume, with no other chemical or structural change in the mica layers. The increasing importance of edge charge relative to layer charge produces erroneous formulae for 2:1 phyllosilicates in very fine grained samples containing fewer than 2 of 3 octahedral sites occupied by cations, on the basis of a 22-charge half cell.

Intercalation and surface modification of smectite by two non-ionic surfactants

Non-ionic surfactants Brij 56 and Igepal CO 720, containing hydrophilic poly(ethylene oxide) (PEO) segments, expanded smectite from 1.5 nm to 1.7 nm at room temperature. The surfactant-smectite composites had larger layer spacings than Ca-smectite after heat treatment. The surfactant-smectite composites were solvated and expanded to 1.8–1.9 nm by polar solvents, glycerol and water, but were not affected by the non-polar or weakly polar solvents, toluene, hexane or octanol. The hydrophilic PEO segments of non-ionic surfactants would logically access the interlayer spaces of smectite whereas the hydrophobic segments extend away from the mineral. The molecular structure and solvation properties suggest that the surfactant molecules are probably concentrated in the margin area of the interlayer galleries forming an annular ring structure between two neighboring silicate sheets. Only two layers or less of the surfactants could access the interlayer galleries of smectite and layer spacings did not exceed 1.8 nm even where excess surfactant was introduced into the composites. The layer spacings of the surfactant-smectite composites were well preserved during water or electrolyte solution washings, indicating stability of most non-ionic surfactant molecules in the interlayer galleries even though ∼30% of the adsorbed Igepal CO 720 was desorbed by exhaustive washing. The non-ionic surfactant treatment preserved >80% of the CEC of the smectite. The interlayer cations of the resulting surfactant-smectite were exchangeable as in the untreated smectite. Therefore, the non-ionic surfactant-smectite was much more efficient at removing heavy metal ions than activated carbon or cationic surfactant-treated smectite. The surfactant-smectite composites effectively removed aromatic chlorophenols from a pH 4.9 acetate buffer solution while untreated smectite did not adsorb these molecules. The enhanced adsorption of the aromatic compounds is attributed to the aliphatic segments of the two surfactants.

Genesis and morphology of iron sulfides in gray kaolins

Many of the presently oxidized Georgia kaolins probably existed originally in the reduced (gray) state. For that reason, the distribution of iron sulfides in presently gray kaolins may elucidate features observed in oxidized kaolins. An understanding of the nature of gray kaolins may also aid in the development of processing strategies for the exploitation of these abundant resources. The size, morphology, and degree of crystallite bonding of iron sulfides (pyrite and marcasite) in three gray kaolin cores from the Macon, Georgia kaolin district were examined by X-ray powder diffraction, X-ray fluorescence, scanning electron microscopy, and other physical and chemical methods. Pyrite and marcasite were identified as dominantly gravel- and sand-size equant crystals and crystal aggregates, rather than as framboids. Pyrite crystals commonly showed rough octahedral faces, which extended over minor cube faces. Locally, spiral growth dislocations were also observed. Marcasite was found in radiating, prismatic, and tabular crystals. In general, the marcasite crystal aggregates were much more fragile than those of pyrite. The dominance of octahedral crystal shapes and textures of the pyrite suggest inorganic precipitation from solutions supersaturated with respect to octahedral crystal faces.