222 Decomposition of Iron Pyrites, 



to prove that the isoQietric forms (cube, octahedron, andpyrito- 

 hedron), those generally indicating a marcasitic pyrite, are, on 

 the average, richer in pyrite than the orthorhombic and there- 

 fore more likely to resist decomposition. The modifications of 

 pyrite seem to have little bearing on this question, with the im- 

 portant exception that sharply defined pyritohedra, with 

 strongly striated faces, are, almost invariabl}^, purely pyritic and 

 stable. 



The color of the surface, immediately after fracture, is an 

 important indication — if decided brass-yellow, of predominance 

 of pyrite — if of the palest yellowish white or quite tin-white, of 

 richness in marcasite. 



A conchoidal fracture, if general, is characteristic of pyrite ; 

 if uneven or granular, an abundance of marcasite is indicated, 

 and sometimes that of some other intermixed impurity, very 

 frequently quartz — both tending to more rapid oxidation. 



Unusual hardness-, especially shown by the mineral striking 

 fire readily and constantly with steel, indicates the predominance 

 of pyrite, even in crystals of marcasite. 



The sulphurous odor of the fresh powder or freshly bruised 

 surface, and greenish streak appear to be further indications of 

 the presence of marcasite, probably of its abundance. Other 

 intermixed sulphides may contribute the same properties. 



The density of the mineral, if determined on pure crushed 

 crystals and with sufficient care, may be of the greatest value^ 

 in indicating the true constitution of the mixture of pyrite and 

 marcasite, the i)ercentages of each being readily obtained by ref- 

 erence to the tables already given. But a specific gravity 

 determination made upon large unbroken crystals, on too coarse 

 a powder, on massive or granular mineral, or on material not 

 scrupulously examined under the microscope in regard to purity, 

 may be a worthless reliance, only likely to mislead. So far as 

 may be judged from present information, the highest stability 

 can be expected only from samples of crystallized marcasite or 

 pyrite whose specific gravity exceeds 4.99, equivalent to at least 

 90 per cent, of pyrite ; though little danger from decomposition 

 may be expected down to a specific gravity 4.97, equivalent to at 

 least 80 per cent, of pyrite. 



Below that amount no stability can be safely relied upon. 



