226 A TREATISE ON METAMORPHISM. 



A fifth source is by oxidation of ferrous iron solutions, which may result in 

 the precipitation of hematite. The first reaction occurs most extensively 

 in the zone of anamorphism; the other four occur in the zone of katamor- 

 phism, and to these positions the heat and volume reactions correspond. 

 Finally, as shown in Chapter XII, on "Ore deposits," hematite maybe 

 formed from pyrite by the action of alkaline carbonate solutions. 



In summary, hematite is derived from actinolite, ankerite, anthophyl- 

 lite, biotite, bronzite, garnet, greenalite, griinerite, hornblende, hypers- 

 thene, ilmenite, limonite, magnetite, olivine, parankerite, pyrite, serpentine, 

 and siderite. 



Alterations. — The most frequent alteration of hematite is into limonite 

 (amorphous; sp. gr. 3.6-4). The reaction is as follows: 



2Fe 2 3 +3H 2 0=2Fe 2 3 .3H 2 0+k. 



In the change the volume is increased 60.72 per cent. A second altera- 

 tion of hematite is into magnetite (isometric; sp. gr. 5.168-5.18). This 

 may be accomplished by any of the reducing agents furnished by organic 

 compounds. Supposing the reducing agent to be the partially oxidized 

 carbon compound CO, the reaction is: 



3Fe 2 3 +CO=2Fe 3 4 +C0 2 +k. 



While a reduction of the oxide of iron occurs a simultaneous oxidation of 

 the organic compound occurs, and the end result is the liberation of heat. 

 In the change the volume is decreased 2.38 per cent. A third alteration 

 of hematite is to pyrite (isometric; sp. gr. 5.025) or marcasite (orthorhom- 

 bic; sp. gr. 4.875). In the best-known instances siderite (rhombohedral ; 

 sp. gr. 3.855) or some other iron-bearing carbonate is simultaneously 

 produced. The reaction may be: 



Fe 2 3 +2H 2 S+C0 2 =FeS 2 +FeC0 3 +2H 2 0+k. 



In the change to pyrite and siderite the volume is increased 76.12 per cent, 

 and to marcasite and siderite 78.73 per cent. 



The alterations of hematite to limonite occur in the zone of katamor- 

 phism, and especially in the belt of weathering. Corresponding' with this 

 position the reaction is with liberation of heat and expansion of volume. 

 The alteration of hematite into magnetite occurs in the belt of cementation 

 and the zone of anamorphism. This agrees with the fact that the reaction 



