Decoinjwsition of Iron Pyrites. 165 



apparent evidence of contraction or fracture of the material 

 during its foi-mation. and of the filling in of the clefts by later 

 deposit, sometimes of juirer resistant material, and sometimes 

 of imj)ure matei'ial which yields first to oxidation. Hexagonal 

 figui'es are often thus produced by the mutual intersection of 

 the cubic and octahedral cleavages of the mineral. This consti- 

 tution is only seen in the lighter unstable varieties, with specific 

 gravity below 4.98. 



d. {Plate 9, Fig. 4). A spongy cellular suiface, deeply fretted 

 into an irregular minute network. This clearly shows the in- 

 timate and uniform intermixture throughout, in large propor- 

 tion, of a constituent yielding readily to corrosion. Such a 

 structure characterizes the light pyrite common in bituminous 

 coal, in which the impurities apparently consist of both mar- 

 casite and carbonaceous matter, and lower the specific giavity 

 to 4.90. In this constitution we have unmistakable proof of 

 great instability. 



e. An irregularly pitted and furrowed surface, with distinct 

 differences in the color, lustre and surface of the constituent 

 grains. These phenomena indicate the enclosure of gangue- 

 matter, quai-tz, or other impurities, with a decrease of the spe' 

 cific gravity to the low^est extreme, as in No. 102 at the bottom 

 of the Table. Though the pyrite present, in cases like this, 

 may itself possibly be pure and free from marcasite, the con- 

 ditions of the structure are peculiarly favorable for rapid oxida- 

 tion, and such specimens are probably always highly unstable. 



While therefore I have not succeeded in determining, for the 

 marcasite enclosed in pyrite, morphological characteristics by 

 which to establish its presence directly, its minute particles re- 

 veal themselves, as to pro^iortion and mode of distribution, by 

 their influence on the erosive etching of the enveloping 

 groundmass. This seems then to afford a useful means of in- 

 vestigation, if due care be given and sufficient area of etched 

 surface be subjected to the microscopical examination. 



III. Determifatioi^ of the Density of Pyeites. 



It has been already manifest that the records of the few ex- 

 periments, which have yet been made on the specific gravity of 

 these minerals, furnish us with little satisfactory information 

 on many important points : e. g., the purity of the material un- 



