1126 A TREATISE ON METAMORPHISM. 



compounds in the zone of anamorphism by the processes of dehydration, 

 decarbonation and deoxidation. 



Magnetite. — The formation of magnetite in the zone of anamorphism is 

 fully discussed on pages 845-846, and will not again be described here. 

 The reactions producing magnetite are believed to be as follows: 



3FeC0 3 +0=FeA+3C0 2 



2FeC0 3 +FeS 2 4-2H,0=FeA+2H 2 S+2C0 2 



3(Fe 2 3 . nH 2 0)-0=2Fe 3 4 +3nH 2 



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



zincite. — In order to produce zincite in the deep-seated zone from smith- 

 sonite, it is only necessary that the water and the carbon dioxide be driven 

 off, thus: 



ZnC0 3 . H,0=ZnO+H 2 04-C0 2 



If smithsonite were formed in the belt of weathering, as explained on 

 pages 1144-1145, 1147-1148, and later by burial transferred to the zone 

 of anamorphism, the above reaction would almost certainly take place. 



FrankHnite. — -In the case of frauklinite it is necessary only to suppose 

 that with the smithsonite there was a certain amount of hydrous oxide of 

 iron and manganese. It is well known that the smithsonite produced in 

 the belt of weathering almost universally contains hydrous oxide of iron. 

 Not infrequently it is also associated with hydrous manganese oxide. If 

 smithsonite, hydrous oxide of iron, and hydrous oxide of manganese be 

 taken in the proper proportions, and subjected to dehydration, decarbona- 

 tion, and deoxidation, franklinite may be produced. For simplicity suppose 

 that the iron, zinc, and manganese in the resultant franklinite are equally 

 abundant as ferrous oxide, and iron and mang-anese equally abundant as 

 ferric oxide. The reaction may then be written as follows: 



2(ZnC0 3 . H 2 0)+4(Fe,0 3 . HH,0)+4(Mn 2 3 . H 2 0)-40= 

 6[(FeZnMn)0. (FeMn) 2 3 ]+12H 2 0+2CO, 



Hematite.— It is well known that hematite occurs as a gangue mineral in 

 connection with the sulphides in veins, although so far as I know there is 

 no case in which such hematite occurs in sufficient amount to be called an 

 ore. In this connection the recent work of Stokes is interesting." He has 

 shown that FeS 2 as pyrite and marcasite is a ferric iron; and further, that 



"Stokes, II. N., On pyrite and marcasite: Bull. U. S. Geol. Survey No. 186, 1901, pp. 50. 



