Decomposition of Iron Pyrites. .'3 Of) 



XXI. —On the Variation of Decomposition in the Iron Pyrites ; 

 its cause, and its relation to density. 



BY ALEXIS A. J U LIEN. 

 Read April 26, 1886. 



The popular name, iron pyrites, comprises three distinct min- 

 eral species, pyrrhotite, marcasite, and pyrite, all consisting of 

 combinations of iron and sulphur, and differing in method of 

 crystallization, density, hardness, color, and other properties. 

 It is well known that these three minerals differ in their modes 

 and degrees of decomposition, when exposed to the weather in 

 outcrops of rock, masonry, and Ik aps of mined coal. But there 

 are equally decided differences in the methods of weathering in 

 varieties of one of these minerals, pyrite, which have not yet 

 been generally recognized, and whose cause yet remains un- 

 explained. 



Two general processes of decomposition of these minerals 

 occur at the ordinary temperatures of the atmosphere. 



1. Envelopment by a crust, and finally complete alteration 

 into compact iron oxide, generally in some hydrated form, ac- 

 companied by a trace of sulphuric acid and sometimes by free 

 sulphur. 



This process is generally slow, and probably conditioned upon 

 a limited supply of moist air, find sometimes upon precipitation 

 by basic solutions, e. g., of earthy or alkaline carbonates. 



Pyrite is commonly attacked in this way, and sometimes 

 marcasite and pyrrhotite. 



2. Conversion into copperas and other iron-sulphates, often 

 with the development of free sulphuric acid, generally accom- 

 panied by the production of more or less hydrated iron-oxide ; 

 this process is always attended by a splitting up of the mass,' 

 and its minute disintegration effected by the crystallization of 

 the copperas, with a similar rending action to that which is 

 exerted by frost. 



