214 A TREATISE ON METAMORPHISM. 



ordinarily called a reduction, is reduction so far as the iron sulphate is 

 concerned, but is oxidation so far as the carbon compound is concerned, 

 and hence the explanation of the liberation of heat. 



Pyrite, marcasite, and pyrrhotite are also doubtless produced by the 

 action of soluble sulphides upon the iron oxides or iron salts. In the 

 change from crystallized Fe 2 3 (hematite) to FeS 2 (in the form of pyrite), 

 the volume increases 56.14 per cent. 



Alterations. — The first alteration to be considered is that of marcasite into 

 pyrite. In this alteration there is recrystallization, an increase of symmetry, 

 a decrease of 2.98 per cent in volume, but no change in chemical compo- 

 sition. The heat effect is undetermined, but probably heat is liberated. 



The mineral pyrrhotite by recrystallization passes into pyrite. This 

 change may occur in volcanic districts by the action of hydrogen sulphide 

 upon the pyrrhotite, the reaction perhaps being: 



Fe n Si 2 +10H 2 S=llFeS 2 +10H 2 . 



In this change the volume is increased 21.13 per cent. 



The minerals pyrite and marcasite may by oxidation pass directly 

 into (1) hydrated sesquioxide of iron, of which, ordinarily, limonite (not 

 crystallized; sp. gr. 3.80) is the most common kind; (2) magnetite (isomet- 

 ric; sp. gr. 5.174); (3) ferrous sulphate, which may be removed in solution, 

 or (4) may be decomposed by further oxidation, either at the place of 

 formation or elsewhere, after a longer or shorter time, into hydrated sesqui- 

 oxide of iron, ordinarily limonite. The reactions for marcasite and pyrite 

 may be as follows, assuming in each case that the sulphur, or a part of it, 

 is also oxidized: 



(1 ) 4FeS 2 +220+3H 2 0=2Fe 2 3 .3H 2 0+8S0 2 +k. 



(2) 3FeS 2 +160=Fe30 4 +6S0 2 ^k, or 

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



(3) FeS 2 +60=FeS0 4 +S0. 2 +k, or 

 FeS 2 +30+H 2 0=FeS0 4 +H 2 S+k. 



(4) 4FeS0 4 +20+7H 2 0=2Fe 2 3 .'3H 2 0+4H 2 S0 4 +k. 



As shown in Chapter XI, on "Ore deposits," pyrite and marcasite also 

 alter to hematite without oxidation by the reaction of an alkaline carbonate. 

 The alteration of common pyrrhotite into magnetite and limonite may 

 be written as follows: 



(5) 3Fe 11 S 12 +1160=llFe 3 4 +36S0 2 -|-k, or 

 3Fe„S 12 +36H 2 0+80=llFe 3 4 -|-36H 2 S-|-k. 



(6) 4Fe n S 12 +33H 2 0+1620=ll(2Fe 2 3 .3H 2 0)+48S0 2 +k.^ 



