ALTERATIONS OF PYRRHOTITE, PYRITE, AND MARCASITE. 215 



If in the production of the limonite the pyrrhotite passes through the 

 stage of ferrous sulphate the reaction producing the sulphate may be : 



(7) Fe n S 12 +460=llFeS0 4 +S0 2 +k, or 



Fe u S 12 +H 2 0+430=llFeS0 4 +H 2 S+k. 



The change from the ferrous sulphate to the limonite is the same as in 

 the case of pyrite and marcasite. Where water is present the S0 2 produced 

 in the above reactions would unite with water and form H 2 S0 3 , or if further 

 oxidized H 2 S0 4 . 



As the end results of alteration are usually limonite or magnetite, the 

 volume relations for these two compounds will be given. In the change 

 of pyrite to limonite the volume is increased 2.93 per cent; to magnetite, 

 is decreased 37.48 per cent. In the change from marcasite to limonite the 

 volume is decreased 0.14 per cent; to magnetite, is decreased 39.34 per 

 per cent. In the change of pyrrhotite to magnetite the volume is decreased 

 24.27 per cent; to limonite, is increased 24.68 per cent. 



When pyrite and marcasite pass into limonite there is a change from 

 a crystalline to an amorphous form. In the alteration of pyrite to magne- 

 tite the system does not change. In the alterations of pyrrhotite and mar- 

 casite to pyrite there are changes from lower degrees of symmetry to the 

 highest degree of symmetry, that of the isometric system. The change 

 from marcasite to pyrite occurs especially in the zone of anamorphism, 

 subject to the principle there obtaining that the changes take place with 

 decrease in volume. The change of marcasite to pyrite is an excellent 

 illustration of the principle that where the pressure is great minerals tend 

 to pass into other minerals having a higher degree of symmetry and a higher 

 specific gravity (see pp. 360-365). The abundance of marcasite as an 

 autogenic constituent in rocks not deeply buried, its absence in the rocks 

 which have been in the lower zone, and the presence of pyrite in these 

 rocks, are thus all explained. Where the pressure is small near the surface 

 marcasite with lower symmetry and lower specific gravity than pyrite may 

 abundantly form. At depth where the pressure is great pyrite of higher 

 specific gravity and higher symmetry forms. If rocks near the surface in 

 which marcasite has formed are buried to a great depth by superimposed 

 strata the marcasite previously formed changes to pyrite. 



Similar statements can not be made concerning pyrrhotite and pyrite, 

 for these miuerals have unlike compositions. Doubtless where the necessary 



