Carbon dioxide magnetite formation

The formation of a passive film of iron oxide (magnetite, Fe304), under sulfite or hydrazine reducing conditions, is optimized at pH of 11 to 12. The downside is that the decomposition of carbonates and bicarbonates produces carbon dioxide, the primary cause of condensate system corrosion. 

Both reactions were accomplished experimentally. It actually turned out (Mel nik, 1966a, c) that in a stream of water vapor the dissociation of siderite to magnetite with liberation of carbon dioxide and hydrogen takes place at a lower temperature (by 60-80°C) than in a stream of nitrogen with liberation of CO2 and CO. The experimental works of other authors (Baykov and Tumarev, 1937 Berg and Buzdov, 1961) have shown that in a neutral atmosphere or in the presence of CO2, siderite dissociates according to reaction (4.18) with the formation of CO. 

Therefore the formation of magnetite in that way could hardly be of essential importance in the metamorphism of iron-formations, and martitiza-tion is still less hkely. However, in deposits of other genetic types, for instance skam deposits, oxidation of iron silicates to magnetite at the contact with large masses of carbonate rocks (dolomite, magnesite) can be considered an ore-forming process. The last conclusion is still feasible because the carbon dioxide released in the dissociation of carbonates probably had an undisturbed CO O2 ratio. 

When olivine and pyroxene are oxidized the same regularities are observed as in the oxidation of carbonates. In particular, it was established in a study of the rich and unique olivine-magnetite iron ores of the Volodarsk deposit (Ukrainian shield) that the magnesium content of the silicates increases as the magnetite content of the ore increases (Mel nik and Yaroshchuk, 1966). In this case the most likely oxidants may be water and carbon dioxide, the main components of the fluids causing metasomatic reworking of the olivine-and pyroxene-bearing iron formations. 

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