Iron sulphide, oxidation

However, in addition to these layers XPS peak position analysis also suggested evidence of additive interaction on the ferrous surface, from the presence of calcium carbonate (CaCOs) on the surface at 347.leV BE and molybdenum disulphide (M0S2) at 162.2eV. As the samples are etched the CaCOs content is reduced, as is the sulphide layer (M0S2), and phosphates and the substrate material beneath becomes more apparent. When using the Al-Si alloy pin there is evidence of material transfer on top of the iron sulphide/oxide layer. 

It must be appreciated that at high temperatures platinum permits the flame gases to diffuse through it, and this may cause the reduction of some substances not otherwise affected. Hence if a covered crucible is heated by a gas flame there is a reducing atmosphere in the crucible in an open crucible diffusion into the air is so rapid that this effect is not appreciable. Thus if iron(III) oxide is heated in a covered crucible, it is partly reduced to metallic iron, which alloys with the platinum sodium sulphate is similarly partly reduced to the sulphide. It is, advisable, therefore, in the ignition of iron compounds or sulphates to place the crucible in a slanting position with free access of air. 

Metallic compositions serving as siccatives catalyze the well-known oxidation and polymerization of oil in paints and other finishes. Likewise, ferrous and other metallic objects boost self-heating in piles of lignocellulosics. Among the known catalyzing substances are iron sulphides and iron oxides from combustion gases of... 

Conditions that are conducive to the accumulation and decay of plant residues, and therefore conducive to the formation of coal, are typically associated with water-saturated and reducing environments. Consequently, a large portion of the trace elements associated with the mineral fraction of coal are expected to occur in reduced forms, primarily as sulphides or carbonates. However, because of its abundance compared to sulphur, it is unlikely that the complete reduction of iron oxides to iron sulphide would ever occur. Therefore, the presence of Fe oxides, and trace... 

Another factor which determines the presence of minerals at certain depths is the tendency to combine with others or affinity of minerals and elements. Noble gases hardly react with other elements and will, because of their low densities, consequently end up in the atmosphere. Relatively light elements (alkaline and alkaline earth metals) with a strong affinity for oxygen are found in the silicate schaal. Heavy elements which are bound to sulphur or oxgen are situated in the sulphide /oxide scale, or even deeper in the nickel/iron scale. 

A most remarkable result in the chemical study of ancient morphologies is the complex chemical composition, comprizing carbonaceous materials and different inorganic materials such as carbonates, iron sulphides and iron oxides. It can be shown that most of the inorganic constituents are biominerals produced directly or indirectly by the life activity of the involved organisms. So also these products are chemical fossils and, therefore, merit adequate consideration in our analyses. 

This metal is obtained on a commercial scale both by reducing antimony oxide with carbon and by reducing antimony sulphide by means of metallic iron. The second method possesses the advantage that antimony sulphide, a natural product, is used directly and does not need to be first converted into the oxide. The iron sulphide formed by this method is fusible and forms an immiscible layer which floats on top of the molten antimony. The addition of borax facilitates the separation of the liquid layers, and thus the globules of melted antimony are allowed to sink more easily to the bottom of the crucible and form a metallic regulus. The upper layer furthermore covers the surface of the metal and hinders its oxidation and the escape of the volatile Sb203. 

The photocatalytic processes that could have been involved in the synthesis of prebiotic molecules are not limited to iron sulphides. Photocatalytic processes occurring on the surface of other sulphide and oxide materials should be considered as well. 

The formation of iron sulphide, or any other metal sulphide, and subsequent hydrolysis to release hydrogen sulphide, represents a corrosion process. The various oxidation processes discussed ail involve the production of hydrogen sulphide, sulphur dioxide or sulphuric acid. In the absence of effective protection, any one of these is a potential corrodent, especially in association with any wear which takes place. 

As Fe + ions oxidise HgS, HS and the precipitation of FcgSg free from sulphur is impossible, Monohydrated iron(III) oxide reacts with dry H2S to give primarily FcgSg some of which disproportionates into FeSg and FeS (Pearson and Robinson, 1928). A cobalt(III) sulphide cannot be precipitated from aqueous solution and there is no unequivocal evidence for its formation or that of NigSg in dry reactions. However, disulphides are known, of which the most important is FeS.2. In its commonest form, pyrites, Fe atoms and Sg pairs are arranged in a sodium chloride-type structure with the axes of the sulphur pairs parallel to the four trigonal axes (Fig. 261). 

It is possible that Gottschalk and Buehler (1912) carried out the oxidation with an excess of oxygen which would not occur in nature at the oxidation interface of a mineral deposit. Granger and Warren (1969) conducted experiments to investigate the oxidation of iron sulphides by an aerobic aqueous phase in a sterile system. The objective was to study the formation of unstable intermediate ions during the oxidation process. The intermediate sulphur species detected were sulphite and thiosulphate. In general these products are more easily oxidised than the metallic sulphides and are stable only if removed from the oxidising environment. 

In simple adsorption from aqueous solution, Hg has features in contrast and in common with the base metals. The hydroxy-cation is the active species in the model for heavy-metal adsorption and this also appears to be true for Hg. However, in contrast with Cu, Pb and Zn, the adsorption is less efficient and is strongly inhibited by the formation of halide complexes, as has been shown by Forbes et al. (1974) (Fig. 12-1). These authors also demonstrate that the adsorption of Hg to goethite is effective at pH as low as 4, allowing it to be trapped subsequent to sulphide oxidation. Whilst many minerals in weathered rocks and soils may each adsorb Hg, the relative efficiency of the hydrous iron oxides (Andersson, 1979) implies that these phases will be the dominant host in most exploration samples. However, the soil organic matter is also of importance and, although the association with Hg has been described as adsorption, it seems more... 

Bloomfield, C., 1972. The oxidation of iron sulphides in soils in relation to the formation of acid sulfate soils, and of ochre deposits in field drains. J. Soil Sci., 23 1—16. 

We should note that the common sulphide of iron (pyrite—FeS2) often contains significant amounts of the toxic semimetal arsenic, as impurities. As a result, when iron sulphides are oxidized (eqn. 5.15) arsenic is released along with the dissolved iron and sulphate. In very rare circumstances this arsenic release can result in groundwater contamination (see also Section 5.7.2). 

In a reducing atmosphere the deposit material may contain iron sulphide (FeS) formed on dissociation of coal pyrite mineral. This is likely to occur on the combustion chamber wall tubes near the burners where the reaction time is short, below one second, for oxidation of FeS residue to the oxide. It has been suggested that calcium sulphide (CaS) may also be present in the ash material deposited from a reducing atmosphere gas stream as a result of sulphidation of calcium oxide (9). 

At the surface of some marine sediments, organic sulphur can comprise as much as 50% of the total sulphur present (Francois, 1987) due to biosynthesis which incorporates sulphur of all oxidation states, but also, because of the reactivity of sulphides and polysulphides, by chemical addition. There is usually an increasing S/C ratio with depth in sediments, partly associated with humic substances, and most of this increase occurs in the oxic and suboxic zones. This organic repository may be the source of the sulphur required to convert metastable iron sulphides, formed in the lower part of the sulphur reduction zone, to framboidal pyrite, which is often found closely associated with organic matter. 

Oxides, Hydroxides, and Sulphides.—Chemical vapour deposition at 300 °C from Fe(tfac)3 (tfac = trifluoroacetylacetonate) gives p-FCjOj of the same structure as that formed by the hydrolysis of FeClg.bHjO. 2Fe203,S03,mH20 (m > 6) has been found to be amorphous and it is suggested that as there are no OH groups present, the compound should be considered as a hydrated iron(iii) oxide sulphite. A comprehensive study of ordered and disordered Scheelite-related Bi3(Fe04)(Mo 04)2 has been made. ... 

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