10 ABSTEACTS: MINERALOGICAL CHEMISTRY 



+ FeS 2 + 2Mx + 3S. The first three methods may be generalized as 

 the action of sulphur on ferrous sulphide: (1) in acid solutions; (2) in 

 nearly neutral solutions; and (3) in alkaline solutions, since in (1) we 

 may assume that ferrous sulphide first forms by the action of hydrogen 

 sulphide on the ferrous salt, and in (3) we know that polysulphides first 

 precipitate a mixture of ferrous sulphide and sulphur. Marcasite was 

 obtained with certainty only by method (1); low temperatures and free 

 acid favor its formation. A solution containing about 1 per cent of 

 free sulphuric acid at 100° gives pure marcasite. Less acid solutions 

 at higher temperatures give mixtures of marcasite and pyrite. The other 

 methods give pyrite, which, under certain conditions, may be mixed 

 with amorphous disulphide. It is possible that some marcasite may be 

 formed by method (4). 



Marcasite and pyrite were identified in the above products: (1) by 

 microscopic examination and crystallographic measurement. Marca- 

 site crystals with the proper axial ratio were prepared for the first time ; 

 (2) by Stokes's oxidation method. This method serves also for the 

 analysis of mixtures of the two minerals. 



Marcasite changes to pyrite with evolution of heat. The change 

 proceeds very slowly at 450° and is not accelerated by pressures even of 

 10,000 atmospheres. Marcasite is monotropic toward pyrite. This 

 is in accord with the greater inclination of marcasite to oxidize, its 

 assumed greater solubility, and the fact that its formation is conditioned 

 by the composition of the solution from which it crystallizes. 



The fact that marcasite never occurs as a primary constituent of 

 magmas, while pyrite sometimes does, is explained by the fact that 

 marcasite can not exist above 450°. The formation of pyrite in deep 

 veins and hot springs is explained by the fact that the waters from which 

 it came contained no strong acid. The marcasite of surface veins was 

 probably formed from cold acid solutions, while mixtures of marcasite 

 with pyrite were probably conditioned by higher temperature (up to 

 300° or 400°), or the presence of less acid, or both. Micro-organisms 

 may have been active in the formation of pyrite and marcasite by giving 

 rise to hydrogen sulphide. 



Pyrrhotite was formed by the decomposition of pyrite or by heating 

 marcasite, or by heating iron with excess of sulphur. The dissociation 

 of pyrite into pyrrhotite and sulphur is readily reversible. At 565° 

 (about) pyrite and pyrrhotite are in equilibrium with the partial pres- 

 sure of sulphur in H 2 S, which here amounts to about 5 mm. (data of 

 Preunner and Schupp); at 550° in hydrogen sulphide, the pyrrhotite 



