ALLEN: ORE DEPOSITION 175 



sufficiently low. The quantity of free sulphuric acid must be 

 about 1 per cent at 100° to insure the formation of marcasite free 

 from pyrite, while at 300° it must be considerably more, if indeed it 

 is possible to get a product free from pyrite at that temperature. 

 Pyrite and marcasite are very commonly found together in 

 nature, often in the same hand specimen and sometimes inter- 

 grown. Altho in some cases the two minerals probably belong 

 to different periods of deposition, it seems entirely probable, in 

 view of the laboratory results, that in other cases they are con- 

 temporaneous. 



The Relation of Marcasite to Pyrite. — When marcasite is heated 

 to a temperature of 450° C, under conditions which preclude 

 oxidation, it changes to pyrite. Not only does its color become 

 yellower and its density higher (the density of marcasite is 4.889, 

 that of pyrite 5.02), but its behavior with a ferric sulphate solu- 

 tion proves conclusively that it has become pure pyrite; for while 

 at the start the ferric sulphate oxidized only 14 per cent of the 

 sulphur of all the marcasite it decomposed, now it oxidizes 56 

 per cent. In a comparatively simple way, it can be shown that 

 heat is set free during this change, which proves, of course, that 

 marcasite possesses more energy than pyrite. Pyrite cannot 

 be changed directly into marcasite, i.e., the change is irreversible 

 and marcasite is what is called a monotropic form. The question 

 of the chemical constitution of the two forms will not be discussed 

 here, except that it may be remarked that they appear to be poly- 

 morphic forms in which the difference is one of crystalline structure 

 rather than of a chemical nature. More important for the prob- 

 lems of ore deposition is the significance of the temperature at 

 which the change occurs. It goes on at 450° very slowly and could 

 not be detected at 400° in a period of four hours. It will be ob- 

 served, therefore, that marcasite cannot have formed in nature 

 above 450° and any minerals which can be shown to have been 

 precipitated at the same time with marcasite are subject of course 

 to the same limitation. Pressure can not be invoked to qualify 

 this statement, for a pressure of several thousand atmospheres 

 was found not to accelerate the change. We may also conclude 

 that a paramorph of pyrite after marcasite would show that the 



