Transport and deposition of silica

Examination of the physicochemical and geochemical data that have been given leads to the conclusion that the well known genetic scheme, in which silica is carried in ionic or colloidal form from subaerially weathered rocks and then deposited chemically in the sea basins, is not very realistic. 

To sum up, the main mode of extraction of dissolved silica coming into the seas and oceans from the weathered layer is biogenic precipitation—a process widespread both in recent and in ancient sedimentary basins. Biogenic precipitation of silica from undersaturated solutions could also have been of definite importance in the formation of the Precambrian iron cherts, but no direct evidence has been obtained as yet. 

If a volcanogenic source is assumed, one can avoid many contradictions and present a-fairly sound model of chemogenic sedimentation, including migration of silica in thermal and then in cold solutions and subsequent deposition in sea basins. 

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An aging treatment results in a partial coalescence of particles and a strengthening of the network occurs. At the neck joining the particles there is a negative radius of curvature. Thus, local solubility at the neck is less than near the particle surface. Therefore, transport and deposition of silica occur preferentially to the neck region and neck thickening results. This results in a strengthening of the particulate network, Fig.7.5 (Zarzycki etal, 1982). 

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-... 

Physicochemical data on the forms of transport and conditions of deposition of iron and silica... 

In examining the particulars of the distribution, migration, and deposition of iron and silica in natural waters it is advisable to distinguish normal sedimentary processes and volcanic sedimentary processes. By normal sedimentary processes we mean the cycle weathered layer-transport-deposition in a sedimentary basin. The main distinguishing feature of volcanogenicsedimentary processes is the endogenetic source of the material and ex-ogenetic method of its deposition. 

For a correct idea of the physicochemical conditions of accumulation of iron-ore sediments, an analysis of the forms of transport and conditions of deposition of iron and silica in recent active volcanic regions is necessary. Such an analysis should include establishing possible sources of the ore material (vents of active volcanoes, fumaroles, hydrothermal volcanic waters), the character and intensity of the process of migration and forms of transport of the ore components, and the conditions of deposition of the ore components in the course of their migration to the sea basin and when the river waters mingle with sea. waters. 

Comparison of the physicochemical and geochemical data on the forms of transport and conditions of deposition of iron and silica with modern ideas concerning the particulars of sedimentation in the Precambrian makes it possible to evaluate critically and to some extent place constraints on the numerous versions of existing genetic hypotheses. After such reexamination, the scheme of formation for the Precambrian iron-formations that is best substantiated from the physicochemical standpoint can be presented. 

C.L. Lin et al. [71] reported deposition of silica layers (plugs) with a thickness of about 1.5 pm within the pores of commercial, mesoporous y-alumina films (pore diameter 4 nm, thickness 1-3 pm) on a-alumina supports (US filter). The deposits were obtained by reaction of TEOS-oxygen (10-20%) mixtures in He as carrier gas applied in the OSG mode to the mesoporous layer. No further details (e.g., temperature or pressure) were given. Depending on these unknown conditions, dense as well as microporous silica membranes with pores down to estimated values of 0.4-0.6 nm were obtained. These membranes have interesting combinations of permselectivity and flux values for several gas combinations (see Chapter 9 on gas transport properties). 

Silica is by far the major component of the earth s crust, yet much remains to be learned of its chemistry and, in particular, its solubility behavior in water. The manner of its deposition to form such curiosities as quartz crystals containing inclusions of mineral oil, mercury, or liquid carbon dioxide remains a mystery (1). Flint, which our remote ancestors recognized as the strongest and toughest stone available, was apparently formed in some instances from the siliceous skeletons of ancient sponges by a mysterious process of solution transport. Within some plants and marine organisms, soluble silica is transported and deposited in characteristic intricate patterns. Only recently has it been recognized that soluble silica, even in trace amounts, plays a role in the development of mammals. 

Also, basic factors such as the transport of materials, residual hardness, ion leakage, soluble iron, colloidal silica and clays, and other contaminants, which can produce scales and deposits in the FW lines and other parts of the pre-boiler section, may also produce similar detrimental effects in the boiler section. In the boiler itself, however, the buildup rate may be quicker and the results may be more devastating. 

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

Approximately 40% of synthetic amorphous silica production is in Europe, followed by North America at 30%, and Japan at 12%. Although deposits of naturally occurring amorphous silicas are found in all areas of the wodd, the most significant commercial exploitation is of diatomaceous earth in industrialized countries (see Diatomite). This is because of the high cost of transportation relative to the cost of the material. Woddwide manufacturers of amorphous silica products are listed in Table 2. 

Let us examine present ideas on the reasons for the accumulation of iron in the Precambrian, on the sources and forms of transport of the iron and silica, and on the method of deposition and mechanism of formation of the typical banded structure. 

In some works a tendency toward convergence of the volcanogenicsedimentary and clastic-sedimentary hypotheses is noted. Belevtsev et al. (1966), who considered mainly the clastic-sedimentary hypothesis, postulate the extensive occurrence of acid waters in the Precambrian hydrosphere as the result of intensive volcanic activity. Tyapkin and Fomenko (1969) believe that the main source of iron and silica in the Precambrian was the basic rocks which were the chief constituent of the Earth s crust at that time, but that some was also derived from basaltic rocks erupted along abyssal faults and other products of basic volcanism. In this case it is impossible to deny the possibility that part of the iron and silica was supplied to the sea basins along with products of volcanic activity. In this scheme the role of volcanic activity in the formation of the BIF comes down chiefly to the creation of acid environments which promoted the leaching of iron compounds from basic rocks and its transport and subsequent accumulation. The primary banding is explained by periodic revival and extinction of volcanic activity, as a result of which the pH of the water basin varied, which ultimately led to deposition of iron or cherty sediments in turn. The periodicity of those cycles might have been of the order of several hundred years. 

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