244 A TREATISE ON METAMORPHISM. 



ism, the carbon- dioxide being liberated and the magnesia being furnished 

 for the formation of various dense magnesian minerals, such as enstatite, 

 tremolite, olivine, pyrope, etc. These changes would involve a diminution 

 of volume and an absorption of heat. 



occurrence. — The chief source of siderite is believed to be the reduction, 

 dehydration, and carbonation of limonite. (See pp. 233-234.) This change 

 is one occurring with the liberation of heat if the reaction upon the organic 

 compound be taken into account, and increase of volume. A subordinate 

 amount of siderite is also derived from magnetite. This change takes 

 place with liberation of heat and increase of volume. Siderite also forms 

 from ankerite and parankerite, arfvedsonite, garnet, hematite, hornblende, 

 hydrous ferrous silicate, limonite, magnetite, and olivine, and replaces 

 calcite and dolomite. 



Alterations. — The important alterations of siderite are into limonite 

 (amorphous; sp. gr. 3.6-4.0), hematite (hexagonal-rhombohedral; sp. gr. 

 5.225), magnetite (isometric; sp. gr. 5.168-5.18), and griinerite (monoclinic; 

 sp. gr. 3.713). The reactions are as follows: 



(1) 4FeC0 3 +20+3H 2 0=2Fe 2 3 .3H 2 0+4C0 2 +k. 



The decrease in volume is 18.22 per cent. 



(2) 2FeC0 3 +0=Fe 2 3 +2C0 2 4-k. 



The decrease in volume is 49.11 per cent. 



(3) 3FeC0 3 +0=Fe 3 4 +3C0 2 +k. 



Very often iron sulphide, as pyrite (isometric; sp. gr. 5.025) or 

 marcasite (orthorhombic ; sp. gr. 4.875), unites with the siderite to form 

 magnetite. This reaction is probably of great consequence in forming the 

 heavy beds of magnetite. (See p. 845.) It may be written: 



(4) 2FeC0 3 +FeS 2 +2H 2 0=FeA+2H 2 S+2C0 2 -k. 



The decrease in volume for the siderite alone to the magnetite, equation 

 (3), is 50.32 per cent; for siderite and pyrite, 46.67 per cent; for siderite 

 and marcasite, 47.135 per cent. 



(5) FeC0 3 +Si0 2 +nH 2 0=FeSi0 3 +C0 2 +nH 2 0-k. 



The decrease in volume, regarding the silica as a solid, is 32 53 per cent. 

 The alteration to limonite occurs in the zone of katamorphism, especially 



