176 ALLEN: ORE DEPOSITION 



original crystal had been subjected to a temperature the lower 

 limit of which lies somewhere between 350° and 450°, because at 

 300° marcasite crystals were found not to be changed to pyrite, 

 either by acid or alkaline solutions. 



Pyrrhotite can be readily formed by the decomposition of pyrite 

 in hydrogen sulphide at temperatures above 575°. The higher 

 the temperature is carried, the more sulphur is lost, and a product 

 formed at high temperatures, say at 800° to 1100° C, takes up 

 more sulphur when heated in hydrogen sulphide below that tem- 

 perature. Although it is somewhat aside from the main purpose, 

 of this article, viz., to discuss certain problems of ore deposition, 

 it may nevertheless be of sufficient interest to remark in regard 

 to the mooted question of the true chemical formula for pyrrho- 

 tite, that this mineral is really variable in composition, at the 

 same time preserving its homogeneity; in other words, ferrous 

 sulphide can dissolve a quantity of sulphur depending on the 

 temperature like any other solubility. At 600° the maximum 

 quantity dissolved is 6.0 per cent. We may if we please repre- 

 sent such a solid solution by the formula (FeS)S x . 



Under the experimental conditions thus far tested {i.e., heating 

 pyrrhotite in dry H 2 S) a solution containing only about 6.0 

 per cent can be obtained after which further sulphur changes 

 pyrrhotite into pyrite. The pyrrhotite of nature doubtless 

 formed from solution, yet it is remarkable that the highest per- 

 centages of sulphur in the natural mineral range in the neighbor- 

 hood of 6 per cent dissolved sulphur {i.e., about 40 per cent of 

 total sulphur). 



Geological observation leads to the conclusion that vein form- 

 ing solutions, doubtless polysulphides, change pyrrhotite to pyrite, 

 and phenomena are observed about contacts which lead to the belief 

 that the reverse action, the decomposition of pyrite into pyrrho- 

 tite and sulphur, occurs. In the neighborhood of contact intru- 

 sive masses, where the sulphides of iron occur, pyrrhotite is 

 found close to the contact where the temperature was highest 

 and pyrite in the colder zones. It is commonly held by geologists 

 that in nature pyrrhotite was formed from the same solutions 

 as silicates like olivine and augite. The experimental difficulties 

 are at present too great to attack this question. 



