Iron sediments oxide-carbonate-silicate-sulfide

Oxide-carbonate-silicate-sulfide iron sediments can be formed under conditions of joint interaction of iron compounds with reactive forms of silicic... 

Fig. 42. Relationships between iron compounds in primary oxide-carbonate-silicate-sulfide sediment in the case of an excess of silica (graph of Efi versus log for pH = const, ap = 10...

The formation of other primary iron mineral compounds is determined by the presence in solution of active forms of silica, carbon, and sulfur. The sediments can be subdivided provisionally into four groups oxide, silicate, carbonate, and sulfide, formally corresponding to the sedimentary facies of the iron-formation, according to James (1954). For each group of sediments the relationships between iron compounds were determined by calculation, in a wide range of pH (from 0 to 14) and Eh (from -I-1.0 to —1.0 V). 

Fig. 39. Relationships between iron compounds in various primary sediments for activity of Uf g-ion/I A = oxide B = silicate C = carbonate D = sulfide (Uj = 10 g ion/1).

On the basis of thermodynamic constants obtained for hydroxide compounds of iron with different aging time and also of experimental data, the physicochemical character of the diagenetic transformations of iron sediments of various compositions (oxide, silicate, carbonate, sulfide) can be traced. The results obtained are represented graphically in the form of stability diagrams of iron compounds as a function of variations in the main parameters governing the physicochemical character of the environment of diagenesis—pH, Eh, activity of iron and dissolved forms of sulfur and carbon dioxide. 

Diagenesis of oxide-silicate-carbonate-sulfide sediments. If reactive sulfur, carbonic acid, and silicic acid are present at the same time in original iron sediments of any composition, the mineral associations formed are determined by the concentrations of these active forms. Figure 59a gives a diagram of the relationships between the crystalline iron compounds in the system Fe-Si02-C02-S-H,0 for a — 10 and = 10 g- ion/1. 

Abstract Iron being the fourth most abundant element in the earth crust, Te Mossbauer spectroscopy has become a suitable additional technique for the characterization of all kind of soil materials and minerals. However, for that purpose a good knowledge of the spectral behavior of the various minerals is indispensable. In this chapter a review of the most important soil materials and rock-forming minerals is presented. It starts with a description of the Mossbauer spectroscopic features of the iron oxides and hydroxides, which are essentially present in soils and sediments. Further, the Mossbauer spectra from sulfides, sulfates and carbonates are briefly considered. Finally, the Mossbauer features of the typical and most common silicate and phosphate minerals are reported. The chapter ends with some typical examples, iUustrating the use and power of Mossbauer spectroscopy in the characterization of minerals. 

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