846 A TKEATISE ON METAMORPHISM. 



This diminution of volume furnishes an entirely adequate cause for the 

 transformation. 



Also magnetite may be produced under conditions of partial oxidation 

 of iron sulphide, such as pyrite and pyrrhotite, according- to the following 

 reactions : 



3FeS 2 +4H 2 0+40=Fe 3 4 +4H 2 S+2S0 2 . 

 3Fe u Si 2 +36H 2 0+80=llFe 3 4 +36H 2 S. 



Le Conte describes the formation of magnetite by the imperfect 

 oxidation of iron sulphide at Sulphur Bank, California." 



Some magnetites are undoubtedly formed by the alterations of limonite 

 or hematite or intermediate compounds. The change of these oxides of iron 

 into magnetite is likely to take place in the deep-seated zone where organic 

 or other material is present which can take away the oxygen. At the same 

 time dehydration occurs. The reaction may be written as follows : 



3(Fe 2 3 ).nH,0-0=2FeA+3nH 2 0. 



The oxygen abstracted unites with the carbon to form carbon dioxide, 

 or with hydrogen to form water. Where the limonite or hematite is asso- 

 ciated with iron sulphide it is not necessary that organic matter be present 

 in order that the change shall take place; the sulphide may serve as the 

 reducing agent. In this case the reaction may be written : 



22(Fe 2 3 .nH 2 0)+FeS 2 +2H 2 0=15Fe 3 4 +2H 2 S0 4 +22nH 2 0. 



Supposing the original oxide were hematite, the volume of the 

 magnetite is 3.58 per cent less than that of the hematite and pyrite. So far 

 as water was present dehydration took place and the percentage of decrease 

 in volume is greater than given. Where the amount of the reducing agent 

 is not sufficient to change all of the limonite or hydrated hematite to 

 magnetite, dehydration onty takes place for the remainder of the material, 

 and consequently with the magnetite a variable amount of hematite is often 

 associated. The segregation of iron oxide into rich deposits is further 

 considered in Chapter XII, on "Ore deposits." 



"Le Conte, Joseph, and Rising, W. B., The phenomena of metalliferous vein formation now in 

 progress at Sulphur Bank, California: Am. Jour. Sei., 3d ser., vol. 24, 1882, p. 33. 



