Decowposltloii of lro)i Pyrites. 15 i 



spearbccided crvstuls, needles, twins, etc., which the iiatunil 

 forms of this mineral present to the naked eye. Within these 

 interstices, air and atmospheric moisture are condensed over the 

 entire area and locked up in the most constant and intimate 

 contact-, or these invisible cavities may be partially occupied by 

 other sulphides, clay, quartz, etc., whose presence is shown by 

 chemical analysis. Both the area exposed to attack and its du- 

 ration must enormously exceed those possible on the ordinary 

 exterior surface of a visible ci'ystal ; while color, lustre, and 

 density are equally decreased by the porous texture of the mass. 

 If we can assume such a molecular constitution, the well-known 

 facility of decomposition Avill be easily understood. On the 

 otlier hand, the a'ssociated high specific gravity and resistance to 

 decomposition shown by tiie well ci-ystallized forms of the min- 

 eral, such as those from the chalk, are well explained by the 

 corresponding compactness of texture and absence of interstices, 

 attending the metasomatic alteration or replacement of the min- 

 eral by ])yrite. 



With pyrite, on the other hand, we may i)erhaps assume 

 a naturally compact texture, when pure, from the crystal- 

 lographic symmetry of forces implied in its isometric con- 

 stitution. The resulting absence of interstitial cavities and im- 

 purities, and therefore of an internal area ex])osed to condeusa- 

 tion of oxvgen and consequent decomposition, are naturally ac- 

 companied by the high color, lustre, density, and freedom from 

 alteration observed in well crystallized forms of the mineral, 

 liut the genei'al detei'ioration of the common varieties of the 

 mineral in all these properties, and their close approach to those 

 of maj-casite, api)ear to imply a general intermixture with that 

 mineral, excei)t in regions of high local metamorphism, such as 

 Elba, Colorado, etc. In the fibrous, and esjiecially in the gran- 

 ulac forms of ]iyrite, such as constitute the usual nodular and 

 ]-adiaiing forms of the mineral, the intermixture of marcasite 

 becomes at last visible, at least under the microscope, and these 

 forms are notorious for ready decomposition. But the absorp- 

 tion of oxygen, shown in the preceding analyses, indicates that it 

 may also be true that even pure pyrite, in so finely divided a 

 condition, may yield far more easily to oxidation. The early 

 observer, Ilenkels,^ recognized the fact that the rapidity of vit- 



^ Pyritologia, Leipzig, (1754). 805 and 790. 



