OXIDATION OF INORGANIC COMPOUNDS. 467 



the needs of man, the metals in subordinate quantity, such as manganese, 

 copper, zinc, lead, etc., have great importance. 



iron. — Since ore deposits are treated in another place, iron is the only 

 metal which will here be considered. The iron oxidized occurs in the 

 minerals mainly in the form of monoxide. Considerable quantities of it 

 occur as sulphides, and unimportant amounts as arsenides, etc. 



Ferrous oxide occurs in the following classes of minerals: Oxides, 

 carbonates, and silicates. Of the oxides, magnetite is the most important. 

 This may be oxidized without hydration into hematite. This change is 

 very well illustrated by the martite ores of the Lake Superior region 

 and by the pseudomorphs of hematite after magnetite in the martite- 

 bearing schists. Simultaneously with the oxidation of magnetite hydration 

 may take place and thus produce hydrated hematite, limonite, or the other 

 hydrated oxides of iron. The change of magnetite to hematite involves an 

 increase in volume of only 2.5 per cent, but where magnetite is changed to 

 limonite the increase in volume is large — 64 per cent. Iron in the form of 

 carbonate may occur as pure iron carbonate or as iron carbonate in combi- 

 nation with various proportions of magnesium and calcium. The oxidation 

 of the ferrous iron of the iron carbonate may be accomplished by oxygen 

 and moisture. However, in the soil the process usually takes place with 

 the assistance of bacteria, A certain group of bacteria requires various 

 carbonates for its development. 11 The oxidation of the iron carbonate, 

 whether with or without bacteria, changes it to ferric oxide. Where the 

 oxidation takes place without hydration hematite is formed; where with 

 hydration, limonite, or some of the other oxides of iron. Where bacteria 

 are an agent the common product is limonite. The oxidation of the carbon- 

 ates involves decarbonation, and on account of this fact there is decrease 

 in volume — in the case of hematite 49 per cent, and in the case of limonite 

 18 per cent. The transformation of the ferrous oxide of the silicates to 

 feme oxide, either . anhydrous or hydrous, occurs simultaneously with 

 processes of . carbonation and hydration of the other bases. So far as 

 there is oxidation and hydration of the iron, this would tend to increase the 

 volume; but the amount of increase can not be calculated independent! v 

 of the conjoint processes. 



"Lafar, Franz, Technical Mycology, translated by C. T. C. Salter, London, Lippincott, 1898, pp. 

 360-362. 



