Iron sediments structure

The find is associated with other specimens of the same kind and together with non-structured carbonaceous debris of similar chemical composition. The assemblages are arranged along bedding planes or other primary patterns of the sediment which is a chert, stromatolite, banded iron formation, shale or related rock. 

Models for the formation of Precambrian sediments suggest that the chemical sediments (such as cherts) of the Isua supracrustal belt have formed as shallow water deposits. This is in agreement with structures locally preserved in the metacherts of the sequence. After deposition, the supracrustals were folded and metamorphosed. Finally, the metamorphism reached lower amphibolite facies and in consequence, most of the primary minerals became recrystallized. As a result all chert now appears as quartzite. But apparently metacherts, magnetite iron formation and quartz carbonate rocks have retained their major element chemistry largely unaltered during metamorphism (Nutman et al., 1984) 119). 

At room temperature colloidal solutions of iron hydroxide can be obtained only by way of prolonged dialysis (Glazman et al., 1958). And finally, experiments are known in which sols were obtained by peptization, by treating freshly precipitated, washed Fe(OH)3 sediment with ferric chloride while heating. A dilute solution with a certain amount of HCl acts on freshly precipitated Fe(OH)3 as ferric chloride does. The sols of Fe(OH)3 obtained by peptization are no different in structure from the sols obtained by hydrolysis. 

Special experimental investigations of the properties, particulars of structure, and variations as a function of time, temperature and pressure have not been made so far for the iron cherts. However, the main components of such sediments—iron hydroxide and silica—have been rather well studied, and some data which are of interest to understanding the diagenetic processes are considered in this section. For the other components—magnetite, siderite, and sulfides—the very limited experimental data were examined in our previous work (Mel nik, 1972b). 

Structure of sediments. The properties of iron hydroxide sediments—color, density and degree of dispersion, texture, character of heating curves, dif-fractograms, and paramagnetic resonance spectra—depend on many factors, the most important of which are not only the aging time, but also the pH of the medium, character of the original solutions (ionic or colloidal), and presence of electrolytes. 

Our experiments also established that the process of aging of freshly precipitated iron hydroxides does not proceed in the same way in sediments obtained from ionic and colloidal solutions of Fe. Whereas colloidal sediments remain X-ray-amorphous for a long time, sediments from ionic solutions relatively rapidly acquire a crystal structure which is most clearly manifested in alkaline, is less ordered in acid, and almost X-ray-amorphous in neutral environments. 

Differences in the iron hydroxide sediments obtained from ionic and colloidal solutions and aged at different pH are also recorded by the methods of proton magnetic resonance (PMR). Typical PMR spectra are determined by protons of the OH groups. No lines were found that might be related to other types of water. A relationship of the second moments of the PMR lines to the increase in pH was established, especially in precipitates obtained from ionic solutions (Fig. 53a). Interpreting the relationship found, it can be presumed that as pH increases, the magnetic structure of the hydroxides is enhanced. It is typical that fresh precipitates obtained from colloidal solutions have a more ordered structure than fresh precipitates from ionic solutions. 

From these data it follows that when iron is precipitated in acid and neutral environments the first products should be X-ray-amorphous highly dispersed iron hydroxides, which in the course of time acquire the crystal structure of goethite or hematite. The mechanism of this process depends on kinetic factors (rate of oxidation of Fe " ), form of migration of the iron (ionic or colloidal), and acidity of the parent solution. In neutral environments ferrihydrite possibly is formed as an intermediate metastable phase, especially if the iron migrates in colloidal form or in the form of the Fe ion. The products of diagenesis of such a sediment may be both goethite (in the case of low Eh values typical of the Precambrian iron-ore process) and dispersed hematite (in the case of deposition of the oxide facies of BIF). 

Analysis of determinations of the solubility and particulars of the structure of iron hydroxides makes it possible to divide the sediments into two groups. 

Solid-phase speciation has been measured both by wet chemical extraction and, for arsenic, by instrumental methods principally X-ray absorption near edge structure spectroscopy (XANES) (Brown et al., 1999). La Force et al. (2000) used XANES and selective extractions to determine the likely speciation of arsenic in a wetland affected by mine wastes. They identified seasonal effects with As(El) and As(V) thought to be associated with carbonates in the summer, iron oxides in the autumn and winter, and silicates in the spring. Extended X-ray absorption fine stmcture spectroscopy (EXAES) has been used to determine the oxidation state of arsenic in arsenic-rich Californian mine wastes (Eoster et al., 1998b). Typical concentrations of arsenic in sods and sediments (arsenic <20 mg kg ) are often too low for EXAFS measurements, but as more powerful photon beams become available, the use of such techniques should increase. 

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