Materials representing common interstratified clay minerals are shown to be composed of aggregates of fundamental particles. Transmission electron microscopy and x-ray diffraction demonstrate that the x-ray diffraction characteristics of a wide range of interstratification can be modeled experimentally by utilizing materials containing only three types of particles. The data have been incorporated into a new model that regards interstratified clay minerals as populations of fundamental particles whose x-ray diffraction patterns result from interparticle diffraction.

The metals Cd, Pb, and Zn occurring in Haplorthods and Haplumbrepts of both sandy and loamy textures in the vicinity of Tarnowskie Gory in southwest Poland, an area that has been subjected to severe metal contamination, were studied according to a procedure based on sequential extraction analysis. Concentrations of the metals ranged from background levels to levels well in excess of the maximum tolerable limits in agricultural soils. Thus, Cd, Pb, and Zn ranged from 0.2 to 103, 14 to 7100 and 20 to 10 000 mg kg−1 soil, respectively. Soil type, texture, organic matter, and cation exchange capacity (CEC) did not influence the relative proportions of heavy metal forms in any consistent manner. However, the influence of soil pH was clear. Soil samples of pH < 5.6 contained relatively more of all metals in the exchangeable form than in samples where pH was >5.6. The most consistent distribution patterns were found when the soil samples were grouped according to their total metal contents. The inferred relative mobility of the metals and their redistribution among the different fractions are generally consistent with the findings of previous studies of contaminated soils and support the view that metals from anthropogenic sources are more mobile than those from soil parent materials.

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

Transmission electron microscopic (TEM) examination of the <0.1-/zm fraction of montmorillonite and regularly interstratified illite-smectite (I-S) shows that these clays, when dried from suspension, consist primarily of particles 10 A and 20 A thick respectively. However, X-ray diffraction (XRD) examination of sedimented aggregates of montmorillonite indicate that the effective number of unit cells that are diffracting coherently is ~9. This discrepancy can be reconciled by postulating an interparficle diffraction effect from the sedimented aggregates of oriented particles. The interfaces of these particles are capable of adsorbing water, ethylene glycol etc. so that on this basis smectite is composed of elementary silicate particles 10 A thick, and regularly interstratified I-S is primarily composed of elementary 'illite' particles 20 A thick, values which are in agreement with the TEM observations. This concept is confirmed experimentally by XRD examination of sedimented aggregates from mixed suspensions of both materials; the resulting patterns are identical to those of randomly interstratified illite and smectite layers, which indicates that the layer sequence examined by XRD has been entirely rearranged. It is concluded that the use of XRD peak breadth to determine mean crystal thickness cannot be reliably applied to these systems. Standard XRD data from sedimented aggregates may not be able to distinguish between true interstratification and interparticle diffraction effects of intimate physical mixtures.