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Abstraet--M6ssbauer and i.r. spectra of a series of nontronites show that Fe 3 + and AI 3 § are distributed between tetrahedral and octahedral sites. The M6ssbauer results have reaffirmed the occupation by Fe 3+ of octahedral sites at which these ions are coordinated to pairs of OH groups in both cis and trans configurations. The distribution of Fe 3+ between these two sites varies considerably but in all of the nontronites some Fe 3 § occurs in the trans site in contrast to the all cis occupancy of the centro-symmetric structure proposed by Mering and Oberlin (1967). In one of the nontronites the distribution of Fe 3 + between these two sites approaches that in the ideal non-centrosymmetric structure proposed for montmorillonite.

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A B S T R A C T: Replacement of kaolinite by dickite has been observed to occur with increasing depth of burial in sandstones from three different basins on the Norwegian continental shelf. In the Garn Formation (Middle Jurassic) of Haltenbanken, samples from 1.4-2-7 km below the sea floor (I10~ contain kaolinite, whereas deeper than 3.2 km (130~ mainly dickite is present. In the Statfjord Formation (Late Triassic-Early Jurassic) from Gullfaks and Gullfaks Sew Fields, transformation of kaolinite to dickite occurs at ~3.1 km below the sea floor (120~ From the St~ and Nordmela Formations (Lower to Middle Jurassic) to the Troms Area, kaolin polytypes have been identified in only two shallow and two deep samples, but the results are consistent with the transformation depth determined in two other areas studied. These occurrences are significant because they allow the temperature of the kaolinite/dickite transformation to be established with greater confidence than had been possible previously. Also the observation of this transformation in all three areas so far examined indicates that it may be a general and predictable feature of kaolinbearing sandstones worldwide and therefore a potentially reliable paleogeothermometer. In most cases, the kaolinite occurs as relatively large vermicular crystals, whereas dickite forms more euhedral, blockier crystals. This morphological difference, together with the nature of the structural difference in octahedral occupancy between the kaolinite and dickite, suggests that the transformation occurs by dissolution and reprecipitation, rather then in the solid state. KAOLIN POLYTYPESKaolinite and dickite are two polytypes of the kaolinite sub-group of clay minerals (Bailey, 1980a) which also includes the polytype nacrite. The AIPEA nomenclature thus recommended the name "kaolinite" for both the polytype and the sub-group, so that when one says that the clay present in a rock is "kaolinite", it remains unclear whether the polytype present is in fact dickite, kaolinite, both, or has not actually been determined. The alternative sub-group name "kandite" has been specifically disallowed (Bailey, 1980a), so probably the best solution is informal use of the term "kaolin" for the sub-group, as used by

A B S T R A C T :The unique properties of imogolite are closely related to its structure, which is a tube of 23-27 /~ outer diameter and ~ 10 A inner diameter, with an AIOH outer surface and SiOH inner surface. Acid dispersions contain the long, positively-charged tubes as isolated units or small bundles, which form bulky gels in alkali, and flocculate with negatively-charged colloids, polyvalent anions, and long-chain anionic detergents. Sorption properties are associated with the 10 A intra-tube pores and with inter-tube channels of variable dimensions. Surface acidity is less than that of layer-silicate clays. The chemical and mechanical stability, biological activity, filmand fibre-forming characteristics, and conditions of synthesis are reviewed, on the basis of both new and published findings. Areas of potential application are indicated.Imogolite is a hydrous aluminium silicate with a unique tubular structure and novel properties; as such, it has no immediate application in any commercial process where layer-silicate clays now play a role. The properties of the isolated product and of its dispersions can be rationally related to its structure, and this relationship is reviewed here. Emphasis is placed on those features which might find practical applications. Problems of synthesis and isolation of imogolite are also discussed.