Illite and natural sediments

The kinetics and reversibility of radiocesium sorption on illite and natural sediments have been reviewed and interpreted in terms of a mechanistic framework. This framework is based on the premise that radiocesium is almost exclusively and highly-selectively bound to the frayed particle edges of illitic clay minerals. It is shown that in-situ Ko of radiocesium in sediments are consistent with this ion-exchange process on illite. 

Figure 4 shows the linearization applied to the Cs sorption data on illite (14) and Ketelmeer and Hollands Diep sediments. Both the illite and the sediment data show an apparently instantaneous reaction and two other distinct processes. As noted by Jannasch and co-workers (17), the sampling schedule affects the number of processes revealed by the linearization procedure. In particular, the initial reaction may, with the appropriate experimental technique and sampling schedule, be subdivided into processes operating on minute or sub-minute time scales. The purpose here, however, is to model Cs sorption kinetics on the longer time scales which are more relevant for natural systems. 

Initially in this study, it was planned to critically evaluate AG data for complex clays, including chlorite, illite, and the smectites. However, there is much evidence that these clays dissolve Incongruently so that the apparent equilibria in solution are determined by secondary phases, such as gibbsite, boehmite, amorphous silica, and ferric oxyhydroxldes. The smectites are frequently the dominant clays in the colloidal size fraction in natural sediments. They have very large exchange capacities, and exhibit wide chemical variations. Usually, one or more of these factors have not been considered in the experimental solubility work. Even if appropriate corrections could be made, it is uncertain whether a AG value so obtained would have applicability to natural systems. 

Figure 28. Depth-temperature plot of natural mineral assemblages for the fully expandable phases (Mo), random and ordered 30-80% mixed layered (ML) and superstructured, ordered 30-20% mixed layered (All) minerals. Data from Steiner (1968), S Muffler and White (1969), M Perry and Hower (1970, 1972), P Iijima (1970), I Browne and Ellis (1970), B Dunoyer de Segon-zac (1969), D and Weaver and Beck (1971), W. I-C illite, chlorite paragenesis. Tertiary or younger sediments are represented in these studies.

In nature the Fe-rich illites (glauconite and celadonite) appear to progress from the lMd to the 1M polytype. The Al-rich illites are predominantly the lMd and 2M varieties. If the 1M polytype is an intermediate phase, it is surprising that it is not more abundant in sediments. Recent studies of unmetamorphosed Precambrian sediments (Reynolds,1963 Maxwell and Hower,1967) have shown that the lMd polytype is relatively abundant in ancient sediments, particularly in the extremely fine fraction. The senior author has noted the relative abundance of the lMd polytype in the fine fraction of most Paleozoic rocks but has considered most of it to be mixedlayered illite-montmorillonite rather than illite. Weaver (1963a), Reynolds (1965), and Maxwell and Hower (1967) have shown that during low-grade metamorphism water is squeezed from the expanded layers and the lMd polytype is transformed into the stable 2M polytype. 

The formation and survival of unstable or metastable micas and clays in sediments and soils at low temperatures reflects kinetic as well as thermodynamic factors. First, the rates of reactions involving solid-aqueous and especially solid-solid transformations in dilute solutions are very slow at low temperatures (most natural waters are dilute )- The slow kinetics of clay transformations reflects small differences in free energy between stable and metastable clays. Also, the occurrence of specific clays is related to the chemistry and crystal structure of source minerals. Thus, illite often results from the weathering of muscovite, and vermiculite results from the weathering of biotite (cf. Drever 1988), consistent with the similar chemistries and structures of these pairs of T 0 T minerals. 

In the central part of the Illizi Basin and over the Ghadames Depression (WT-i, HD-i, RYB-i, AKF-i) we observe sediments characteristic of deeper marine environments, i.e. fine-grained sandstones with intercalations of clays and silts. The characteristic feature of these Siegenian sandstones is the chloritic composition of the clay fraction of their cements and the chloritic-illitic nature of the argillaceous intercalations. Because of this situation, these deposits could have been derived from a hard substrate (effusive or metamorphic rocks) inundated by the Siegenian sea and open into the direction of the present Libyan coast. 

The major metal ions (M" ) competing with radiocesium on the frayed edge sites on illitic clays in natural freshwater environments, are generally potassium (K in oxic environments, e.g. in unsaturated soils and surface waters) and ammonium under anoxic conditions in sediments). These ions, like Cs, can easily dehydrate and enter the edge-interlayer structure of illite. Divalent ions such as calcium, which are surrounded by a large and stable hydration shell, do not fit easily in this structure and are much less competitive (7). 

Tissot B, Durand B, Espitalie J, Combaz A (1974) Influence of nature and diagenesis of organic matter in formation of petroleum. Am Assoc Pet Geol Bull 58 499-506 Vandenbroucke M, Pelet R, Debyser V (1985) Geochemistry of humic substances in marine sediments. In Aiken G R, McKnight D M, Wershaw R L, MacCarthy P (eds) Humic substances in soils, sediments, and water. Wiley, New York, pp 249-273 Whitney G (1990) Role of water in the smectite-to-illite reaction. Clays Clay Minerals 38 343-350... 

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