234 A TREATISE ON METAMORPHISM. 



decomposing' to serve as reducing agents and to furnish abundant C0 2 to 

 unite with the iron. The reduction may be by the passage of CO into 

 C0 2 , of C into C0 2 , or of C into CO, as follows: 



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



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



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



So far as the iron is concerned, its reduction and dehydration absorb heat, 

 but the oxidation of the C or CO and the union of the C0 2 and FeO both 

 liberate heat, the amount of which is greater than that absorbed, so that in 

 each of these reactions heat is liberated. In all of the reactions the volume 

 is increased 22.27 per cent. 



The reduction of the iron of limonite so as to produce protoxide for 

 the formation of iron carbonate may of course be accomplished by carbu- 

 reted hydrogen, especially methane (CH 4 ), rather than by the compounds 

 suggested; but the carbureted hydrogen compounds are so numerous and 

 the resultant compounds so uncertain that no attempt will be made to 

 formulate equations for possible changes with these substances as reducing 

 agents. 



The change of limonite to siderite is one which occurs extensively in 

 rocks bearing organic compounds in the zone of katamorphism. The 

 formation of the abundant siderites which are used as iron ores of Carbon- 

 iferous and later age are believed for the most part to be thus derived from 

 limonite in the upper zone. The reactions correspond perfectly to this 

 position, being those which occur with liberation of heat and very consid- 

 erable expansion of volume. The siderite thus formed may later be 

 decomposed into various other compounds, or even reproduce limonite, but 

 the consideration of such changes belongs under "Siderite." 



BEUGITE GROUP. 

 BRITITE A>D GIBBSITE. 



Bruoite: 



Mg(OH) 2 . 



Rhombohedral. 



Sp. gr. .2.38-2.40. 

 Gibbsite (hydrargillite) : 



Al(OH) 3 . 



Monoclinic. 



Sp. gr. 2.28-2.42. 



