Effect of Temperature, Acidity, etc. 431 



5. The acid concentration required to give rise to pure mar- 

 casite or pure wurtzite falls with the temperature and is close 

 to neutrality for marcasite at ordinary temperature, and proba- 

 bly so for wurtzite. 



6. Several conditions other than acidity and temperature 

 were varied in the formation of wurtzite, where the process 

 was necessarily more complicated ; these were zinc concentra- 

 tion, addition of sodium sulphate to the solutions, and hydro- 

 gen sulphide pressure. ISone of these had any influence 

 except as they affected the acidity. 



7. At the temperatures of 25° and 200° from sulphuric acid 

 solutions and at 300° from hydrochloric acid solutions, we 

 obtained a product containing 95 per cent of marcasite as com- 

 pared with the purest natural marcasite we have had in our 

 hands. Since this determination depends on the quantity of 

 iron dissolved from the mineral under definite conditions, and 

 different natural specimens vary somewhat, it may be that this 

 product is pure synthetic marcasite. 



8. Some new data on the genesis of the natural minerals are 

 cited. 



The authors wish to extend their thanks to Mr. C. E. Sie- 

 benthal of U. S. Geological Survey for important field data 

 and mineral specimens, and to Mr. J. B. Umpleby, also of the 

 Geological Survey, for a specimen of wurtzite. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C, June 30, 1914. 



