Decomjjositiou of Iron Pyrites. 213- 



Resume. The general theoretical conclusions from this study 

 of the crystals of the iron-pyrites may be now summed up as 

 follows, in regard to the normal types of each mineral : 



1. Typical or normal pyrrhotite possesses a specific gravity = 

 4.6, and a ready tendency to oxidation, only moderated by a 

 notable replacement by, or intermixture with, marcasite or 

 pyrite. 



2. Normal marcasite has a tin-white color, a splendent lustre, 

 a density closely approaching 4.80, hardness of 6.5 on most sur- 

 faces and of about 6. on the faces of the macrodome (m P oo), 

 uneven fracture, and a tendency to rapid oxidation, indicated 

 by a greenish yellow, or variegated tarnish, dull or iridescent, 

 soon followed by efflorescence. 



3. Normal pyrite has a pale brass-yellow color, splendent lus- 

 tre, a density of about 5.01, uniform hardness of 6.5, con- 

 choidal fracture, and a strong resistance to oxidation, so slow a& 

 to render hepatic alteration common as the final result. 



4. The forms of iron pyrites occurring in nature are intimate 

 intermixtures of these three minerals ; rarely of pyrrhotite,. 

 however, on account of its ready metasomatic alteration into one- 

 or the other of the triad. These common mixtures of marcasite 

 and pyrite may originate by enclosure during crystallization ^ by 

 alteration, and by displacement, and pass progressively into- 

 complete paramorphs, well crystallized after the form of one or 

 the other mineral. 



5. The latent constitution of these composite minerals is in- 

 dicated by a variation in density, exactly proportionate in most 

 cases to the amount of each constituent, and by a similar vari- 

 tion in other physical properties, e. g., hardness, fracture, re- 

 sistance to decomposition, and even in color, in the case of the 

 paramorphs of marcasitic pyrite. 



IV. Practical Applications. We have yet to consider 

 briefly the pertinence of these facts to the choice and treatment 

 of several natural materials, used in the arts, which contain 

 varying mixtures of these pyrites. The distribution of pyrrho- 

 tite appears to be so limited, that we may confine our attention 



