THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XX. — The Mineral Sulphides of Iron, by E. T. Allen, 

 J. L. Crenshaw, and John Johnston ; with Crystallo- 

 graphic Study, by Esper S. Larsen. 



Introduction. 



Application of Chemistry to the problems of ore depo- 

 sition. — The problems of ore deposition have claimed the 

 attention of geologists for a long time, both for economic 

 and for scientific reasons. The difficult questions as to the 

 origin of ores and the conditions of their genesis have been 

 zealously studied, and with notable success ; yet despite the 

 great advances which have been made, no one will question 

 that the scarcity of chemical data has been a serious drawback 

 in the development of the subject. A laboratory investigation 

 of some phases of it involves, indeed, difficulties which are still 

 to be surmounted, but the problems of the temperature ranges 

 within which the minerals have crystallized, the composition 

 of the solutions from which they have come, and the agencies 

 which have precipitated them are in general not only within 

 the bounds of chemical possibility but within the limits of 

 present day methods. 



The sulphides of iron. — The sulphide ores from a chemical 

 view-point are of very great interest, and geologically, they are 

 of high importance. The sulphides of iron, in particular, fre- 

 quently carry paying quantities of gold and nickel, and when 

 themselves barren, are so frequently associated with other 

 valuable ores as to hold a place of unusual significance in a 

 general consideration of the subject of ore deposition. The 

 chemical knowledge of these substances is still very meager. 

 Syntheses of pyrite and pyrrhotite were made long ago by 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 195.— March, 1912. 

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