OCCURRENCE OF PYRRHOTITE, PYRITE, AND MARCASITE. 213 



the genesis of ore deposits. These, however, will be considered only in the 

 chapter on that subject. 



PYRRHOTITE, PYRITE, AND JVIARCASITE. 



Pyrrhotite: 



Fe 5 S 6 to Fe 15 S 16 ; chiefly Fe n S 12 . 



Sp. gr. 4.58^.64. 

 Pyrite : 



FeS 2 . 



Isometric. 



Sp. gr. 4.95-5.10. 

 Marcasite ■• 



FeS 2 . 



Orthorhombic. 



Sp. gr. 4.85-4.90. 



occurrence. — Pyrrhotite, pyrite, and marcasite are very widespread acces- 

 sory minerals, occurring in rocks of all ages and all kinds. So far as known, 

 these minerals aro not abundant original pyrogenic constituents, although 

 they frequently are found along the contact between intrusive and other 

 rocks, occurring in both the intrusive and the intruded rocks. Pyrrhotite 

 is an original mineral in meteorites. These minerals extensively form in 

 rocks in volcanic districts through the action of solutions of hydrogen 

 sulphide and other sulphide solutions upon iron salts. As secondary minerals 

 in the sedimentary rocks, and to a less extent in the igneous rocks, the 

 sulphides are extensively formed through the reducing action of organic 

 compounds upon the sulphites and sulphates, especially the latter, and par- 

 ticularly iron sulphate. Such reduction is characteristic of the belt of 

 cementation and the zone of anamorphism; but in the latter zone pyrrhotite 

 or pyrite, rather than marcasite, probably forms. 



The reducing agent of the sulphites and sulphates may be either a 

 solid organic compound or one of its gaseous products of decomposition, 

 such as carbon monoxide (CO) and carburetted hydrogen (CH 4 ). If the 

 reducing agent be taken as CO, the reaction for pyrite and marcasite 

 may be: 



2FeS0 4 +7CO=FeS 2 +FeC0 3 +6C0 2 +ka 

 and for pyrrhotite: 



12FeS0 4 +45CO=Fe 11 S, 2 +FeC0 3 +45C0 2 +k. 



If the reducing agent were taken as carbon, similar results would be 

 obtained, except that the amount of C0 2 would be less. This action, while 



"See page 210. 



