Decomposition of Iron Pyrites. 383 



in nature, the mineral becomes coated by the rusty stains and 

 crusts of iron oxide already described, or even entirely trans- 

 formed into it, while the enclosing rock in its vicinity may be 

 stained or even corroded and disintegrated by the action of the 

 free acid. Where the enclosing or neighboring material con- 

 tains alumina, as in the common pyritiferous shales, whitish 

 crusts of aluminum sulphates and of true alums effloresce over 

 the weathered surfaces. This action is shown on very many 

 of the rock-cuttings all over New York Island, by white crusts 

 of astringent taste, on surfaces of the common pyritiferous 

 gneiss, which have been exposed to the weather but a few years. 

 The pyritiferous slates of the Mesozoic in Virginia, 36 the black 

 shales of the Coal formation in Pennsylvania, the Cretaceous clays 

 at South Amboy, N. J., and the shales of Strafford and Thet- 

 ford, Vt., contain pyrite of the same perishable nature ; and at 

 the latter localities, advantage has been taken of this property 

 to manufacture copperas and alum in large quantities, by the 

 exposure of the mineral in moistened heaps to the action of the 

 atmosphere. 



It may also be remarked, in regard to the destination of sul- 

 phuric acid set free fry pyritous decomposition, that, on its ex- 

 posure to the air, the introduction of organic matter, as dust or 

 in solution, must have resulted in its final deoxidation and 

 escape as hydrogen sulphide, sometimes perhaps with a partial 

 deposit in the form of free sulphur. 



Again, Senft refers to the other mode of decomposition in 

 pyrite :— 



" In the air becoming variegated in color, and finally covered 

 Avith a compact smooth brown crust; which then protects it 

 against farther weathering." 



In this process the mineral has lost its entire content of sul- 

 phur, which has been replaced by oxygen and by water, so that 

 the iron has reached the maximum of oxidation and generally 

 of hydration, probably thus : 



4 Fe S 2 + 15 O 2 + 11 H 2 = H 6 Fe 4 O 9 •+ 8 H 2 S 0\ 



Where crystals of pyrite have been altered in this way, a 

 mineral has been produced, retaining the form, exterior surface- 



36 O. J. Heinrich, Trans. Am. Inst. Min. Eng. (1877-78), VI, 274. 



