Iron sediments

In the latter case, electron uptake occurs after decarboxylation of the ketoacid via Fe3+, which is able to take up electrons in strongly oxidized regions of the black-band iron sediments. Fe-ions act catalytically on the process of thioester formation, which can then occur without the help of enzymes. Thus, it was solar UV irradiation which carried the prebiotic thioesters across the energy threshold. 

Some of these species can carry out photosynthesis, but they may do so by engulfing symbiotic algae. It has been suggested that Grypania found in iron sediments could be the earliest eukaryotes. 

Note that plots, such as Figure 5.1, provide information only on the net outcome of chemical reactions. In the case of iron, a small addition does take place in estuaries as a result of desorption of Fe from the surfaces of riverine particles. As these solids move through the estuarine salinity gradient, the major cation concentrations increase and effectively displace the iron ions from the particle surfeces. Since this release of iron is much smaller than the removal processes, the net effect is a chemical removal of iron. Sedimentation of these iron-enriched particles serves to trap within estuaries most of the riverine transport of reactive iron, thereby preventing its entry into the oceans. 

N. Pueschel, C. (1985) Scanning electron microscopy of iron sediments formed by Thiobadllus ferrooxidans. Geomicrobiol. J. 4 231-268... 

Pryor, M.J. Evans, U.R. (1950) The reductive dissolution of ferric oxide in acid. I. The reductive dissolution of oxide films present on iron. J. Chem. Soc., 1259-1266 Puchelt, H. (1973) Recent iron sediment formation at the Kameni Islands, Santorini (Greece). In Amstutz, G.C. Bernard, A.J. (eds.) Ores in sediments. Springer Berlin, 227-245... 

The cherty iron-keratophyre formation (CIK) is characterized by the para-genetic association of acid dacite-rhyolite lavas, tuff-keratophyres, and cherty iron deposits. The formation is more uniform in facies. It is believed that the cherty iron sediments were formed near volcanic centers at times when volcanism was dying out. Semenenko assigns the deposits of the Upper suite of the Upper Konka group in the Konka syncline, and also the rocks of the Upper Bazavluk group in the Teplovka belt in the Verkhovtsevo syncline, to the CIK. 

These two types of hypotheses characterize the extreme points of view. However, detailed study has shown that there often are transitions between cherty iron-formation related to volcanogenic complexes and cherty iron-formation which at first glance has no such relationship. The existence of transitions has made it possible to postulate a remote indirect relationship to volcanism and to presume a volcanogenic source of the material even for those iron-chert complexes in which there are no volcanics at all. This point of view borders on the first group of hypotheses and requires an answer to a number of controversial partial questions of the problem how is the formation of the iron cherts related to volcanism in space does this relationship change in time, does the character of volcanism itself change, and how does this affect the accumulation of cherty iron sediments ... 

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 ... 

Thermodynamic analysis of the relationships between the primary iron sediments confirms the geologic data. On the /i-pH diagram (Fig. 39) it is seen that in oxidizing environments iron is deposited in the form of amorphous Fe " " hydroxides, and in reducing environments, finely divided magnetite is formed. Hydroxides of divalent iron are deposited only in a narrow field of highly reducing environments near the limit of stability of water at atmospheric pressure. 

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... 

Carbonate iron sediments are formed when iron is precipitated in the presence of dissolved carbonic acid or as a result of interaction of the primary sediments with organic matter in the course of diagenesis. The determination of the stability of primary iron carbonates with respect to iron oxides and hydroxides was made on the basis of using the functional dependence of and on pH, calculated by the method of Garrels and Christ (1968) for the system carbonate-water. According to this dependence, in conditions of FeCOj in equilibrium with water the value of partial pressure of carbon dioxide decreases in proportion to increasing pH. 

Sulfide iron sediments can be formed both by direct chemogenic deposition from solution and in the course of diagenesis of the original sediments (Curtis and Spears, 1968 Skripchenko, 1969). 

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... 

Analysis of the relationships between iron compounds in the system Fe-Si02-C02-H20-S was made for aqueous environments and iron and sulfur contents of 10 and 10 , respectively. In such environments iron sediments of all the groups examined earlier are formed. 

At pH = 6, which presumably occurred in the ancient Precambrian sedimentary basins, migration of iron could have been accomplished only in the form of Fe " at an Eh not lower than —0.10 and not higher than 0.0 to + 0.10 V. A lowering of Eh would have led to massive deposition-of pyrite, and an increase in deposition of Fe hydroxides. The actual conditions of chemogenic formation of cherty iron sediments consisting of siderite, iron... 

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. 

Thus from the thermodynamic data it follows that if acid or neutral silica-bearing solutions arrived in the depositional basin, the action of the presumed geochemical barriers (gradients of pH, Eh, concentration) could not operate in isothermal conditions of chemogenic deposition of silica, to form cherty or cherty-iron sediments. Only a change in concentration due to evaporation of substantial volumes of water in closed basins could have led to deposition of silica. An easy calculation shows that to deposit chert bands 0.3-0.5 cm thick in that way, a 100-m water layer has to be evaporated. Thus the formation of thick piles of Precambrian iron formations would have required the evaporation of a fantastic amount of water from restricted... 

Deposition of iron in the form of suspensions when hot springs mix with sea waters first occurs in the upper layers of the sea, in which case almost pure iron hydroxide is formed. Gradual dilution leads to expansion of the region where suspended iron sediment is deposited in the water. With depth, the iron hydroxides are replaced by clastic silica. 

Analysis of the energy of processes of diagenesis of cherty iron sediments... 

Let us examine the particulars of diagenesis of natural iron sediments. 

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. 

Equilibration may proceed differentially in iron sediments, depending on the amount and reactivity of organic matter. 

Precambrian cherty iron sediments. Data on diagenetic processes that went on in ancient rocks which have not retained their original form can be obtained only indirectly, on the basis of a number of assumptions. In any version of the chemogenic hypothesis, the original cherty iron sediment must have been amorphous-dispersed, and the processes of diagenesis were an important step in the formation of the crystalline rock. However, it is quite unclear whether essential mineral transformations occurred, or whether diagenesis was limited to recrystallization of the original sedimentary miner-... 

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. 

The question of the possibility of formation of diagenetic magnetite should be partieularly considered. As is seen, there is no magnetite field on the diagrams characterizing the conditions of formation of complex iron sediments. Judging from calculations using the new consistent constants. 

The data obtained correspond in general features to geologic ideas concerning the directionahty and character of diagenetic transformations in iron sediments in an open system, where the activity of iron and sulfur, the... 

Physicochemical model of the formation of cherty iron sediments in the Precambrian... 

Now it remains to put together the partial conclusions obtained and attempt to present a generalized physicochemical model of the formation of banded cherty iron sediments. It should be kept in mind that the transport and deposition of substantial amounts of iron and silica from low-... 

From the data given it also follows that the formation of iron sediments was limited to shallow depths where Fe(OH)3 and FeCOj could not be dissolved when they passed through reducing zones. Preservation of sediments already deposited was aided by the conservation of amorphous silica, which was gradually being deposited in these same parts of the water bodies. 

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