In order to investigate the state of iron in naturally occurring allophanic clays, Mossbauer resonance absorption spectra were measured for allophanic clays and allophanes together with reference samples of imogolite and hisingerite (amorphous iron silicate).
The Mossbauer spectra of allophanic clays gave a sharp ferric quadrupole doublet, which strongly suggests iron in a relatively uniform octahedral coordination.
The Mossbauer parameters (quadrupole splitting and isomer shift) of Fe³⁺ in allophanic clays and allophanes were significantly different from those of hisingerite which had been reported by Kohyama and Sudo (1975) as a very poorly crystalline form of nontronite.
The appearance of poor but apparently superimposed spectra consisting of ferric and ferrous quadrupole doublets in allophane strongly suggested that iron still remained in part in the above-mentioned octahedral coordination, although dithionite-citrate treatment caused the change in the coordinated state of iron involved. However, iron in a uniform coordination could not be detected for imogolite.
Some discussion on the form of iron (Fe³⁺) in allophane and allophanic clays was made by comparing Mossbauer parameters of iron in these clays with those of iron in some other naturally occurring iron-containing minerals.

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Ten dredge samples and three piston cores collected from the head and the central part of the Persian Gulf were analyzed for clay minerals. Illite plus a random mixed-layer of illite-montmorillonite with approximately 10-20% expandable layer and chlorite are the principal constituents in the clay mineral assemblage. Montmorillonite is distributed widely but less in the amount, and kaolinite is present in the least amount. These clay minerals seem to be of terrigenous origin. There are two possible sources of these clay minerals: the Tigris, Euphrates, and Shatt-Al-Arab Rivers of Iraq which enter into the head of the gulf, and numerous streams flowing the Zagros Mountain area in the coast of Iran.
The constancy of the clay mineral composition with depth in each core taken from the head of the gulf suggests that the source areas for the clay mineral have remained unchanged for a long time, probably since the Late-Pleistocene, and changes in the environment have not brought about any diagenetic change in the sediment.

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The viscosities of dilute allophane and imogolite clay suspensions were determined in order to infer the shape and size of these minerals in the suspension. The viscosity determination for montmorillonite and halloysite suspensions was also carried out for comparison.
The determined viscosity of clay suspension was different from one clay mineral to another and decreased in the order imogolite>>montmorillonite>allophane>halloysite, The Einstein equation was applied to the viscosity of clay suspension with some modifications, and the particle shape of each clay mineral was inferred by assuming an appropriate amount of hydration water for each mineral. The inferred particle shape of montmorillonite and halloysite accords with their platy and spherical shape, respectively. The particles of allophane and imogolite were inferred as discs and threads, respectively. It was suggested that these minerals are present as aggregates of their structural units even in dilute suspensions.

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The state of iron in allophanic clays was compared with that of iron in hydrous iron silicate hisingerite on the basis of their Fe L-emission band spectra. The result suggested that the form of iron in allophanic clays might differ from that of iron in hisingerite samples, which had been reported as being very poorly-crystallized variants of nontronite and of iron saponite.

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Between fully ordered and fully disordered (amorphous) states of clay minerals, many stages of partial order and disorder occur. Disorder of various kinds can be classified under the following headings:(i) Disorder at the atomic level, (ii) Disorder in layer stacking, and (iii) Disorder associated with mixed layering. This review is concerned principally with layer stacking disorders, particularly in kaolinite group minerals. The type and frequency of the disorder is discussed. Also considered are variations of layer spacings associated with lateral layer displacements particularly in kerolite, a talc-like mineral. The evidence that allophane may be a disordered defect form of kaolinite mineral is summarized.

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The interior structure of spherical halloysite particles in two weathered pumice samples was examined by electron microscopy in combination with potassium acetate treatment for a gentle breaking up of the spherical particles and by dissolution analysis with oxalate-oxalic acid-difference infra-red spectroscopy. The results indicated concentric and discontinuous stackings of kaolin unit layers and a cavity in the core. Allophane was not detected in any substantial amount, but some defect structure intermediate between allophane and halloysite was suspected in the spherical particles. An outward, layer by layer growth of the spherical halloysite particles was suggested.

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