148 Decomposition of Iron Pyrites. 



posed to the air on the surface of a stone ; if huried beneath the 

 surface, the ferrous sulphate is likely to be immediately convert- 

 ed into a comparatively insoluble ferric sulphate, and from that 

 the ferric oxide may be immediately deposited ; if the matrix is 

 calcareous or maguesian, the ferric oxide is deposited at once in 

 place by precipitation, and a hepatic ])seudomorph finally results. 

 This material m;iy consequently detain small quantities of lime 

 and magnesia within its pores. Prof. Erastus G. Smith has re- 

 ported' these traces in his careful analysis of such alimonite- 

 eube, from the Trenton limestone near Beloit, Wis. 



In the impure forms of pyrite, inclining to rapid oxidation, the 

 soluble ferrous sulphate is transported farther away, through 

 the matrix, before further oxidation and precipitation take 

 place. The extent of the discoloration thereby produced is lim- 

 ited only by protection from air and moisture ; the formation 

 of hepatic pseudomorphs is accomplished only in the pres- 

 ence of an excess of basic precipitant, such as " rock sa[)" satu- 

 rated by calcium-carbonate.' 



E. General Conclusions in Eegard to Variation in 

 Decomposition. 



A consideration of tlie facts connected with the paragenesis of 

 the three iron pyrites, especially of the general intermixtures, 

 intercrystallizations, and mutual replacements of the two more 

 common species, marcasite and pyrite, leads to the conclusion of 

 the constancy of their association in the most intimate forms. 

 Wherever the least deviation is noticed from the ordinary phy- 

 sical properties of either mineral, the presence of the other may 

 be at once suspected. The opacity of both preventing the ap- 

 plication of most optical tests, resort must be had to the close 

 examination of other physical properties. We are thus led to 

 the following general conclusions : 



First, In regard to marcasite, the tendency to decomposition 

 and its absence are plainly associated with other physical piop- 

 ertics. When the mineral occurs in the rarer condition of com- 

 pact well formed crystals, with brilliant lustre, grayish-white 



' Am. J. Sci., (1886), XXXI, 376. 



'2 During publication, I have found this theory ah'eady suggested in a 

 brief note to Pyrite, in Dana's System of Mineralogy, (1883), 64. 



