1S94.] ' -^-1^ , [Brown. 



A Comparative Study of the Chemical Behavior of Pyrite and Marcasite. 



By Amos Peaslee Broicn. 



(Read before the American, Philosophical Society, May 18, IS94.) 



While much has been done in the way of investigating the chemical 

 properties and constitution of the various artificial chemical compounds, 

 comparatively little attention has been paid to the constitution of the 

 naturally occurring chemical compounds. The carbon compounds, for 

 instance, have in an immense number of cases been investigated with 

 sufficient exactness to allow of our expressing their constitution by means 

 of structural formulae, but to how- many minerals, aside from the simplest 

 compounds, can we assign structural formulae that are based on any 

 knowledge that we possess of their reactions? It is true that mucii has 

 been done in the way of the artificial production of minerals, and some 

 knowledge of the constitution of certain minerals has been arrived at by 

 a study of their decomposition products, but very little in comparison to 

 the extent of the field. There are probably several reasons for this neglect 

 of the study of the chemical properties and constitution of minerals, as 

 want of homogeneity in the minerals themselves, difficulty of procuring 

 or separating pure material for investigation, and similar difficulties 

 which do not so frequently occur with artificial compounds. It thus hap- 

 pens that mineral chemistry is not as much studied as it deserves tp be. 

 Certain groups of minerals have, however, received some attention ; for 

 instance. Prof. F. W. Clarke has been carrying on a very interesting series 

 of investigations on t,he constitution of certain silicates which have been 

 productive of most valuable results. The natural sulphides, sulpharsen- 

 itles and sulpho-salls present some very interesting problems in regard to 

 their constitution, and it was with a view of adding to out knowledge of 

 tiie chemical behavior of two of these sulphides that I undertook the 

 series of investigations about to be described. 



The compound FeS.^ is found in nature in two well-known forms— the 

 one Pyrite (the isometric mineral) and the other Marcasite (the ortho- 

 rhombic mineral). Since the separation of the two names from the gen- 

 eral term, pyrites, it has been recognized that the orthorhombic form is 

 lighter in color and also of lower specific gravity than the isometric form. 

 From early times, also, the greater tendency of "white pyrites," or mar- 

 casite, to decompose in the air was well known. 



Pyrite, the form which resists atmospheric weathering most thoroughly, 

 is of a bright brass-yellow color and metallic lustre, breaking with an 

 uneven conchoidal fracture, bat bright on the surface of fracture. It 

 crystallizes in the isometric system in forms showing generally pentagonal 

 hemihedrism. Its specific gravity ranges from 4.8 to 5.2, averaging some 

 what over 5. The brass-yellow crystals are generally quite unaltered in 

 the air. 



