Strukturveranderungen von Tonmineralien unter verschiedenen thermodynarnischen Bedingungen (111)Transformative Umwandlungen von Mineralen der Kaolinit-Gruppo unter erhohteni Drnck bei Anweaeriheit von mineralisiereriden Losungen l ) I n the earlier papers 11, Frank-Kamenctzkij o t al., 1966, 1968) data regarding the structural and facial changes in kaolin group minorals under conditions of interaction wit'h hydrothermal chloride solutions of K, Na, Ca, Mg a t P H ,~ = 900 kg ern-,, T = 200-500 "C were dis-cussed. I n the prescnt paper the results of these investigations are summed up.The diffractomotrical, olectronographic arid electron microscopic investigations of initial, intermodial and final phasis mainly of layered silicate and alnminous silicates show a cristallographic inheritance of structures during the transformation of kaolinite int,o montmorillonite, micas and mixed layored facies (rectorit, tosudit etc.) This is seen in tho retention of the main features in the basal pla,ne, in the character of population of the octahedral positions, in the tendency t o retain the sequence of alternation and imposition of layers (polytype), and in the processes of doformation that the structures of the init'ial minerals undergo during such changes. The wide participation of ordered and disordered mixed-layered structures in these reactions is tho result of structural changes by trarisformatical means. At the same time, tho chemical nature of the medium exerts considorable influence on the course and nature of the resulting structures. Thus, the mechanism of transformational thormal changes of aluminium silicates, which is already known for "dry-systems", is observed t o a great extent in the case of complex polycomponcntal systems under H,O pressure and is characterised by the specific individual features discussed in this paper.
It has been shown that clay minerals transform into (Mg, Al) or (Ni,Al) serpentine-like phases if treated hydrothermally in the presence of MgCO3 or NiCO3 (Shitov et al., 1974, Frank-Kamenetskii et al., 1978, 1983a,b; Kotel'nikova et al., 1976; Varela et al., 1983; Kotov et al., 1985; Ryumin et al., 1978) and that the polytypism of the initial kaolins is inherited by the products. It has also been shown that (Mg,Al) serpentine-like phases react with KC1 to form dioctahedral micas and Mg-rich serpentine phases, but the latter do not react to trioctahedral micas (Kotel'nikova et al., 1976). Consequently, it was desirable to look for mechanisms for obtaining trioctahedral micas from serpentine-like phases.
Amorphous, or X-ray amorphous, is a common term used to describe mineral precipitates lacking long-range order. Earth materials, such as amorphous FeS, silica, and Fe(III)-hydroxide, are abundant in natural environments and play an important role in geochemical processes and the fate of contaminants. While use this term persists in the Geosciences community, it is important to resolve the short-to medium-range structure and fundamental size of these solids to deepen our understanding of their role in geochemical processes. In this study, we report on the structure and size of socalled amorphous FeS, the first Fe-S phase formed in anoxic sediments which, over time, will convert to pyrite (FeS 2). Amorphous FeS can also sequester metals and metalloids, such as cadmium and arsenic and react with contaminants, such as Cr 6+. High-energy X-ray scattering data for freshly precipitated and aged FeS (7 days at 70 C) were collected at the Advanced Photon Source and analyzed using the PDF method. Results indicate that amorphous FeS has short-to medium-range order consistent with that of crystalline mackinawite. Further, the range of structural coherence (i.e., fundamental particle size) as can be obtained from PDF [1] is initially about 2 nm, but increases to 4.5 nm for the aged sample. Hence, the initial FeS precipitate is better described as a nanocrystalline material with mackinawite structure. This technique holds great promise for further study of nanocrystalline earth materials lacking long-range order.
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