Iron silicates, solid solutions with

Corrosion is further accelerated by the presence of impurities such as oxides, sulphides, carbides, phosphides, and silicates, since these are invariably at a lower potential than the ferrite.3 The influence of alloying elements 4 is particularly interesting. With carbon, for example, cementite or iron carbide, Fe3C, is formed, and as this is electro-negative to ferrite, the latter corrodes at the points of contact. Addition of carbon, therefore, to iron tends to enhance its corrodibility. If a third element is added to the system, its influence upon corrosion is determined largely by the manner in which it distributes itself.5 If it dissolves in the ferrite, reducing its. solution pressure, it reduces the potential difference between the ferrite and cementite, and thus enhances the resistance of the whole to corrosion. Nickel behaves in this manner, the whole of the metal passing into solid solution with the ferrite until the steel contains more than 8 per cent, of nickel. Such steels, therefore, do not readily corrode. 

Because of their sizes, neither K+ nor Si + can enter into solid solution with the magnetite and so if some silica is present in the iron oxide used, small occlusions of alkali silicates are present as separate phases in the fused catalyst. Microscopic investigations of the milled catalyst showed that whereas the larger particles still contain alkali silicate occlusions, the finer particles consist of a mixture of separate alkali silicate and magnetite particles (20). Hence, the distribution of alkali in the milled, fused catalyst is heterogeneous. During reduction and FT synthesis, however, the alkali does to some extent spread over the catalyst surface ((7), chapter 3). 

Zinc, Cu and Ni have similar ionic radii and electron configurations (Table 5.6). Due to the similarity of the ionic radii of these three metals with Fe and Mg, Zn, Cu and Ni are capable of isomorphous substitution of Fe2+ and Mg2+ in the layer silicates. Due to differences in the electronegativity, however, isomorphous substitution of Cu2+ in silicates may be limited by the greater Pauling electronegativity of Cu2+ (2.0), whereas Zn2+ (1.6) and Ni2+ (1.8) are relatively more readily substituted for Fe2+ (1.8) or Mg2+ (1.3) (McBride, 1981). The three metals also readily coprecipitate with and form solid solutions in iron oxides (Lindsay, 1979 Table 5.7). 

The common lithophile elements, magnesium and silicon, condense as magnesium silicates with chromium and lithium in solid solution. Together with metallic iron, magnesium... 

Olivine A magnesium iron silicate mineral with the common solid solution endmembers forsterite and fayalite. 

The large specific surface areas of the Fe solid phases (Fe(II,III)(hydr)oxides, FeS2, FeS, Fe-silicates) and their surface chemical reactivities facilitate specific adsorption of various solutes. This is one of the causes for the interdependence of the iron cycle with that of many other elements, above all with heavy metals, some metalloids, and oxyanions such as phosphate. 

The impurities most frequently met with in solution of and solid hydrate of potassa are carbonate of lime oxide of iron, silica, alumina, carbonate and sulphate of potassa, and chloride of potassium. > Carbonic acid is indicated by effervescence on the addition of an acid. Carbonate of lime and oxide of iron remain insoluble when the salt is treated with water. Silicic acid and... 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

In laboratory studies, silica was adsorbed and precipitated by hydroxides of aluminum, iron, manganese, and magnesium, according to Harder (42). After precipitating 15 ppm Al(OH), from a 3 ppm SiO in solution, the residual silica was 0.8 ppm, whereas with 30 ppm aluminum hydroxide, no measurable silica remained. Willey has given an excellent review of the extensive literature on the interaction of silica and alumina in dilute solutions. The low levels of soluble silica reached (39) depended on the particular solid aluminum silicate phase that was formed or present. She studied the interaction at very low concentrations, generally less than 10 ppm, and found that only I ppm SiO is required to initiate the precipitation of 1 ppm of AljOs from solution, and if more than about I ppm AljO, is in solution the silica concentration becomes exceedingly low. 

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