420 



Allen, Crenshaw, and Merwin- 



Table IX. 

 Showing that wurtzite is changed to sphalerite by heating ivith dilute acid. 



Time Temperature 



in of 



hours furnace 



Initial Final 

 H 2 S0 4 H 2 S0 4 



in wt. % in wt. % 



Product 



Composition of Crystalline 

 Part 



# Amorphous 



% wurtzite % sphalerite 



V 



300° 



2-0 



1-9 



100 







trace 



8 



300° 



3-0 



2-2 







100 



5-10 



1 



300° 



3-0 



2-5 



95 



5 



1-2 



H 



325° 



3-0 



1-9 



95 



5 



2-3 



n 



300° 



2-0 



1-1 



33 



6V 



trace 



12 



300° 



3-0 



2'0 



60 



40 



2-3 



11 



300° 



3-0 



2-4 



100 







5-10 



12 



325° 



30 



1-7 



10 



90 



5-10 



24 



300° 



2-0 



1-5 







100 



5-10 



24 



300° 



3-0 



2-0 







100 



5 



24 



300° 



3-0 



1-9 



50 



50 







25 



325° 



3-0 



1-1 







100 



trace 



temperatures, much of this first precipitate was crystalline and 

 contained a larger percentage of wurtzite than that portion 

 which was precipitated after the final temperature was reached. 

 The products which were obtained (Table IX) at the end of 

 7 or 8 hours must have been precipitated before the temper- 

 ature of the furnace (column 2) was reached. With one* excep- 

 tion they consist largely of wurtzite. 



At the end of 11 or 12 hours the percentage of wurtzite is, 

 in general, much less. This is, no doubt, partly due to the 

 fact that more zinc sulphide has been precipitated and this 

 must have been largely sphalerite since it was precipitated 

 after the maximum temperature had been reached. In every 

 case except one, however, the wurtzite had entirely dis- 

 appeared at the end of 24 hours. It is evident that the wurtz- 

 ite present at the end of 8 and 12 hours has been changed to 

 sphalerite by further heating with dilute acid.f This cannot 

 be explained as a simple case of an unstable form precipitat- 



* We have no explanation for this apparent exception. We have found 

 that the time at which precipitation begins varies considerably in different 

 experiments, and it may be that in this case no precipitation took place 

 until the temperature of the furnace was practically reached. 



fit might be objected that the more soluble wurtzite had been dissolved 

 away in these cases, leaving the sphalerite. This could not have occurred 

 since an excess of hydrogen sulphide was always found at the end of the 

 experiments and the weight of the products was always greater at the end 

 of 24 hours. The wurtzite had evidently been transformed and probably by 

 solution and recrystallization, since there is no evidence that the change 

 occurred in the solid state. 



