Decomposition of Iron Pyrites. 381 



which the analyst could not find it in his heart to crush up for 

 proper examination, and therefore weighed it as a whole. 



In order to throw more light on this question, I present at 

 the close of this paper a large number of determinations of spe- 

 cific gravity, on the mineral crushed to a coarse powder, in dis- 

 tilled water at 15° C. (60° F). 



The results thus far obtained have demonstrated a coincident 

 change of color and of density in varieties of pyrite possessing 

 differing powers of resistance to oxidation. As to the significa- 

 tion of such change in density, it will be generally accepted that 

 in every mineral possessing strong power of crystallization, the 

 figure for specific gravity will remain invariable in all pure sam- 

 ples, probably to the third place of decimals. A determination 

 made on coarsely powdered mineral, with the usual precautions, 

 and at standard temperature, can only vary on account of en- 

 closed cavities, the partial decomposition or alteration of the 

 mineral, its original enclosure of foreign matter, isomorphous 

 replacements of some one or more of its normal constituents by 

 lighter or heavier substitutes, or on account of an internal re- 

 arrangement of its molecules into a dimorphous form of different 

 density. 



The influence of the enclosure of cavities is shown by the 

 wide variation of the figures obtained by weighing entire crys- 

 tals, e. g., many of those of pyrite examined by Kenngott and 

 V. v. Zepharovich, and the one by Eammelsberg, on pyrite of 

 Compostella. 



The influence of decomposition on the specific gravity has 

 been abundantly illustrated already, as in the figures for the 

 partially decomposed pyrite of Compostella, etc. 



The influence of the enclosure of gangue, etc., is manifested 

 in the lessened figures in analysis No. 1±, and in the following 

 analyses of both pyrite and marcasite, by C. Mene, 35 each set 

 arranged in the order of increasing amount of impurities. 



35 Compt. Rend. (1867), LXIV, 870. 



