I)ecoiuj)Oi<itiofi of Iron Pyrites. 149 



color, and high specific gravity, it resists decomi)Osition as ef- 

 fectively as the stable form of pyrite and as most silicates. But 

 when it occurs in crystals with lower lustre and density, whose 

 color inclines to a greenish tint, or in finely granular, scaly, or 

 columnar masses, of lower lustre, density, and purity of color, 

 and in which a little clay and moisture can be detected by anal- 

 ysis, such varieties are certainly inclined to more or less rapid 

 decomposition by efflorescence. 



Secondly. The two minei-als, marcasite and pyrite, are very 

 commonly and abundantly associated (a) in more or less thorough- 

 ly intermixed masses, in which the grains or plates of each may 

 often be yet distinguished — {h) in nodules consisting of successive 

 enveloping crusts, com})osed alternately of each mineral, or of 

 mixtures of both — and (c) in crystals consisting of the substance 

 of one mineral, assuming the form characteristic of the other. In 

 most cases the constituents of such mixtures and associations 

 may be distinguished by their general physical properties : mar- 

 casite, for example, by its color, generally lower specific gravity, 

 somewhat inferior hardness, uneven fracture, and usual strong 

 tendency to decomposition. 



Thirdly. The crystals and nodules which are generally assign- 

 ed to pyrite, on account of the crystalline forms of that min- 

 eral (cubes, octahedrons, etc.), which the individual grains pre- 

 sent, seldom consist certainly of that mineral in a pure condition, 

 save when they also exhibit its characteristic brass-yellow color, 

 high density and lustre, conchoidal fracture, and strong resist- 

 ance to decomposition. When inclined to alteration, however, 

 they invariably exhibit either a paler yellow, whitish or greenish 

 color, alow density and lustre, uneven fracture, and tendency 

 to granular or fibrous texture. 



(Correlating then these three facts, the peculiar physical prop- 

 erties of marcasite, its common intermixture with pyrite, and 

 the presence of its other physical properties in most of the var- 

 ieties of pyrite which show ready alteration, the following con- 

 clusion seems inevitable. All specimens of pyrite in active de- 

 composition are not pure, hut are intimate intermixtures of mar- 

 casite and pyrite, prolally in the most minute, i. e., 7nolec2ilar 

 conditions of these minerals. 



^ye may here recall the interesting investigation of G. Briigel- 



