Iron sediments silicate

In sediments, oxidized iron is commonly present as the oxides magnetite, hematite, goethite, limonite, and as silicates such as ferric chamo-site. In reduced form iron is present as siderite, several iron (II) silicates, sulfides, such as pyrite, FeS2, and as undefined compounds of the general type FeS nH20, sometimes referred to as hydrotroilite. In rare cases iron occurs in native form in wood in bogs, in petrified wood mixed with limonite and organic matter, and in carbonaceous sediments. 

Reactions between organic matter and iron (III) silicates have not yet been considered in this discussion. In the models above, all iron was assumed to be present originally as FeOOH. However, in actual sediments some probably is present as nontronite, which possibly could react with organic matter, forming carbon dioxide and a hypothetical iron (II) sheet silicate ... 

Iron oxide sediments are formed in the absence of or at low concentrations of active forms of silicic acid, carbonic acid, and sulfur in the waters. According to the data of investigations of the composition of recent ferric iron sediments, they consist mainly of amorphous hydroxides of Fe " " (Strakhov, 1947, 1960 Ostroumov, 1955 Volkov and Tikhomirova, 1966 ... 

Silicate iron sediments are formed in the case of joint deposition of iron and silica. According to the data of thermodynamic calculations, chemical deposition of pure amorphous silica (Si02(a)) occurs only in the case of fairly... 

Deposition of primary iron sulfides depends on many natural factors—pH, Eh, concentration of iron and sulfur, presence of reactive forms of carbon dioxide and silicic acid, etc. To ascertain the conditions of chemogenic formation of iron sulfides, a special examination of the effect of all these factors is needed. We will limit ourselves to determining the conditions of formation of sulfide iron sediments from waters with a constant sulfur content, and then we will examine the effect of a change in its concentration. 

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

Formation of silicate or carbonate-silicate sediments begins at a ratio of activities of Fe " " Fe " = 10 to 10 " (ferric iron is absent) which corresponds to an Eh range of —0.025 to —0.085 V. In this same range, formation of complex hydroxide-carbonate-silicate iron sediments is possible. The relative amount of hydroxide in such sediments depends on the oxygen supply, and that of siderite on the carbonate capacity of the sedimentary basin. 

Recent sediments of water basins. In recent basins iron sediments consist mainly of the iron hydroxides Fe(OH)3 or Fe203-nH20, but in very rare cases silicates and carbonates of Fe ", pyrite, and hydrotroilite enter into the composition of the sediment all together they constitute reactive (mobile) iron, which actively takes part in the diagenetic processes. A mixture of clastic minerals, which decompose negligibly and take practically no part in the processes of diagenesis, constitute another group. 

In the analysis of the deposition of iron sediments it has already been mentioned that quite likely both iron silicates and carbonates and amorphous iron hydroxide were formed, which could convert to other forms both during the formation of the sediment and in subsequent diagenesis. Reduction of hydroxide could have been controlled by external (atmospheric) or internal (organic matter, free carbon in the sediment) oxidation-reduction buffer systems. All these variants need additional consideration in the thermodynamic analysis of diagenetic processes. 

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. 

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

The processes described and their kinetics is of importance in the accumulation of trace metals by calcite in sediments and lakes (Delaney and Boyle, 1987) but also of relevance in the transport and retention of trace metals in calcareous aquifers. Fuller and Davis (1987) investigated the sorption by calcareous aquifer sand they found that after 24 hours the rate of Cd2+ sorption was constant and controlled by the rate of surface precipitation. Clean grains of primary minerals, e.g., quartz and alumino silicates, sorbed less Cd2+ than grains which had surface patches of secondary minerals, e.g., carbonates, iron and manganese oxides. Fig. 6.11 gives data (time sequence) on electron spin resonance spectra of Mn2+ on FeC03(s). 

The Fe content of sediments varies greatly with the type of rock (Wedepohl, 1969 a). Sandstones contain ca. 10 g kg Fe, claystones ca. 50 g kg and carbonatic rocks ca. 4 g kg Fe. In recent deep sea sediments Fe contents are low in carbonates (9 g kg ), but high in clays (65 g kg ). Sedimentary iron minerals belong to the groups of oxides, carbonates, clay silicates and sulphides. In addition, Fe is a common impurity in other sedimentary minerals. 

Figure 5-8 A Pb-Pb isochron that determined the age of the Earth to be about 4.55 Ga. Stony and iron meteorites as well as a sediment of the Earth are plotted on a Pb-Pb isochron. The sediment, as a "bulk sample of the silicate Earth in terms of Pb isotopes, plots on the same line as the meteorites, suggesting that the Earth and meteorites formed at the same time and are the same age. Erom Patterson (1956). Later studies reveal a more detailed evolution history of the Earth, including core formation (about 4.53 Ga), atmospheric formation (about 4.45 Ga), and crustal evolution.

A striking example of iron-enrichment of a detrital silicate material is presented by Giresse and Odin (1973). Recent, kaolinite-rich sediments containing about 8% Fe total on the West African Continental Shelf, are transformed into pellets rich in montmorillonite which contain 16-21% total iron. This material is then progressively enriched in potassium to form glauconite. 

There is no indication of a release of Zn from the sediments during the development of anoxia, unlike the release of phosphate and dissolved silicate. Zn bound to algal material may be dissolved upon mineralization of this material and Zn bound to manganese oxides upon dissolution of manganese oxides. It appears, however, that Zn is efficiently retained in the sediments, probably through bonding to other less soluble particles, such as iron oxides and silica parts of diatoms. In the presence of sulfide, Zn is probably retained in association with sulfide-containing particles. 

Iron is a common constituent of marine sediments. Magnetite is found in beach sands and iron is coituuoti in glauconitic manne silicates. Iron oxides and sulfides occur where anaerobic conditions and elevated temperatures arc found, as in the hot. salty brines found near rifts. Iron is a major constituent of the ferromanganese nodules. 

The weathering of surface rocks has had a critical role in the chemical evolution of the continental crust for most of the Earth s history. In the presence of air and water, mafic minerals tend to rapidly weather into iron (oxy)(hydr)oxides, clays, and other silicate minerals, and at least partially water-soluble salts of alkalis (sodium and potassium) and alkaline earths (calcium and magnesium). In contrast, quartz in felsic and intermediate igneous rocks is very stable in the presence of surface air and water, which explains why the mineral readily accumulates in sands and other sediments. 

The effectiveness of zerovalent iron in removing arsenic from water also greatly depends on the properties of the iron. As(III) removal is especially effective with high surface area 1-120 nm spheres of zerovalent iron (Kanel et al., 2005). Provided that interfering anions (such as, carbonate, silicate, and phosphate) are insignificant, colloidal spheres of zerovalent iron could be injected into arsenic-contaminated soils, sediments, and aquifers for possible in situ remediation (Kanel et al., 2005, 1291). 

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