214 Allen^ Crenshaw, Johnston, and Larsen — 



Application of data on pyrrhotite to geology. — Very little 

 of the chemistry of pyrrhotite worked out in these pages will 

 probably find application in geology. However, the following 

 points have a practical bearing. 



1. The readiness with which pyrrhotite changes to pyrite, 

 and the reverse, has been specially noted. The transformation 

 of pyrrhotite into pyrite through the agency of vein-forming 

 solutions which probably contained polysulphides, is an 

 observed fact in geology; but the dissociation of pyrite has 

 apparently not been noted, although certain phenomena in the 

 vicinity of contacts would suggest this. The most distinct 

 phenomenon which points to such a reaction is the almost con- 

 stant presence of pyrrhotite in contact metamorphic shales. 

 In such shales pyrite develops normally at some distance from 

 the contact, while close to it, pyrrhotite is universally present 

 and there is little, if any, pyrite. (Lindgren.) The occurrence 

 of pyrrhotite which could be proved to have formed in this 

 way would point to a temperature above 500°, — much 

 higher if the dissociation occurred under considerable pressure. 

 Pseudomorphs of pyrite after pyrrhotite do not seem to have 

 been observed, but they would probably be readily formed by 

 the action of polysulphides on pyrrhotite crystals. Pseudo- 

 morphs of marcasite after pyrrhotite have been described by 

 Schoudox and Schroeder* and by Pogue.t The synthetic 

 experiments (see p. 179) suggest that these were formed by 

 the addition of sulphur from slightly acid solutions containing 

 hydrogen sulphide and free sulphur in suspension. 



2. The possibility of the formation of pyrrhotite from 

 slightly acid solutions at temperatures as low as 80° has been 

 made clear in the foregoing pages. The crystals made in this 

 way were generally associated with crystals of pyrite and per- 

 haps marcasite, a fact which shows how readily the former unite 

 with sulphur. This is probably the reason why the pyrrhotite 

 of nature never seems to form under* the above conditions, for 

 such could only occur in surface solutions where there is more 

 or less access of air. This, with hydrogen sulphide, would 

 give free sulphur. 



3. Pyrrhotite is regarded in certain instances as a primary 

 constituent of eruptives. Its intimate intergrowth in such 

 cases with silicates such as augite and olivine strongly suggests 

 the conclusion that both have separated from a common magma. 

 Now it is well known that molten sulphides of this sort are all 

 but immiscible with molten silicates ; in other words, a system 

 like this would separate into two layers. It would be a matter 

 of great interest to determine whether the addition of water, 

 aqueous sodium sulphide or even of a more complex solution 



* Jahrest. Niedersachsisclien geol. Vereins Hannover, p. 132, 1909. 

 f Proc. U. S. Nat. Mus., xxxix, 576, 1911. 



