Experimental data on the properties and transformations of cherty iron-formations

Experimental data on the properties and transformations of cherty iron-formations 

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). 

The most important characteristics which make it possible to compare sediments of the same composition but different degrees of dispersion and crystallinity are their structure, thermochemical properties, and solubility. 

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. 

A large number of published works deal with the iron hydroxides (Levina and Yermolenko, 1955 Meleshko, 1959 Feitknecht and Michaelis, 1962 Krause and Lezuchowska, 1962 Feitknecht and Schindler, 1963 Lezuchowska, 1963 Chalyy and Lukachina, 1970), but previously it was not clear which of the crystalline phases—goethite, lepidocrocite, or hematite—is thermodynamically stable in aqueous solutions at low temperatures. Therefore additional experimental and theoretical investigations were needed to determine the equilibrium parameters more precisely (Yepatko and Mel nik, 1965 Mel nik, 1972b Vorob yeva and Mel nik, 1977), from which it was concluded that a-FeOOH is in equilibrium with pure water below 100°C. 

---