394 Decomposition of Iron Pyrites. 



amount to only 68 per cent, of the latter. The pseudomorphs 

 do almost always indicate a reduction of volume. . . . Since 

 magnetic pyrites is most frequently amorphous, the iron pyrites 

 resulting from it would present its own crystalline form, so that 

 there would not be any indication of the change that might have 

 taken place." 



So too in regard to eruptive rocks, in all the fresh basalts ex- 

 amined by E. Boricky, 67 the pyritous particles consisted of pyrr- 

 hotite, while, in those basalts which had been weathered and 

 altered, pyrite crystals took its place. 



It must be added, however, that with other conditions, even 

 at ordinary temperatures, the highest sulphide combination, 

 Fe S 2 , pyrite, has sometimes been formed at once in the presence 

 of an excess of hydrogen-sulphide ; e. g., in the well-known ob- 

 servation of Forchhammer, 68 at a running spring on the seashore, 

 where the ferruginous water was borne into a mass of decompos- 

 ing sea-weed ; also, in one instance, in the mud at the bottom of 

 a pond. 69 



So too, in thermal waters, pyrite appears to have been, and to 

 be now, the original deposit; e. g., in the highly crystalline and 

 exceedingly pyritiferous lodes of Colorado, etc., and the deposit 

 from warm springs at Aix la Chapelle, Burgbrohl, etc., and, in 

 the form of brass-yellow crystalline globules, in the sand and the 

 Eoman brickwork, at Bourbonne, France. 70 



Secondly, in the course of metamorphism of the rock, and 

 usually in proportion to its degree, the scattered particles of 

 pyrrhotite, usually Fe 7 S 8 or 6 Fe S + Fe S 2 , have suffered a 

 further alteration — it may be by the common solvent, carbonic 

 acid — by which the entire amount of sulphur was combined as 

 Fe S 3 , and the excess — over one quarter — of the amount of iron 

 was transported elsewhere. The new combination, Fe S a , was 

 most commonly effected under certain conditions — probably 

 great pressure and high temperature — which resulted in the 



67 Petrog. Stud. Bas.-gest. Bohmens (1873), 36, 258-259. 



68 Rep. Brit. Ass. Adv. Sci. 



69 Gmelin-Kraut, Handb. d. Chem., 6th ed., Ill, 333. 

 10 A. Daubree, Etudes syn. de Geol. exper. (1879), 89. 



