Effect of Temperature, Acidity, etc. 425 



Table XII. 



Influence of acid on the crystalline form of ZnS at 325° . All solutions con- 

 tained 2% ZnSOiJHiO and were precipitated by H^S from 0'5 g. 

 NHiCNS. 





Final 







Product 





Initial 



Composition of Crystaline 





H 2 S0 4 



H 2 S0 4 

 in wt. % 



Time 





Part 



% Amorphous 



in wt. % 



% wurtzite % sphalerite 





3'0 



0*9 



1 day 







100 



15 



3-0 



1-1 



1 " 







100 



trace 



5-0 



2-0 



1 " 







100 







6-0 



2-3 



1 " 



60 



40 







1'5 



3-0 



1 " 



30 



70 







9-0 



3-3 



2 " 



50 



50 







9-0 



4-2 



1 " 



100 











100 



4*6 



1 " 





no precipitate 



of ZnS 



from higher acid concentrations contain higher percentages of 

 wurtzite. 



2. Conditions necessary for the genesis of either form of 

 zinc sulphide alone. — Conclusions drawn from experiments 

 which resulted in products consisting entirely of wurtzite or 

 entirely of sphalerite were much more satisfactory, since the 

 microscopic analyses of such products were naturally more 

 certain. At three of the temperatures investigated, 250°, 300° 

 and 325°, it was found that when the final acid concentration 

 remained above a certain definite limit, wurtzite* was the only 

 crystalline form obtained. At two of the temperatures (300° 

 and 325°) a final acid concentration was determined below 

 which sphalerite* was the only crystalline form obtained. 

 When the final acid concentration lay between these limits, 

 both wurtzite and sphalerite were found in the products. 



At 250° (Table X) practically pure wurtzite was obtained 

 whenever the final acid remained as high- as 1*2 per cent; if 

 the acidity fell to 0*6 per cent the percentage of wurtzite was 

 reduced to 50 per cent, but further reduction in the quantity 

 of wurtzite was impossible since weaker acids gave only an 

 amorphous product. It seems certain that if a crystalline 

 product could be obtained, a concentration of less than 0*5 per 

 cent sulphuric acid would be necessary for the formation of 

 pure sphalerite at this temperature. 



At 300° (Table XI) it was possible to determine the final 

 acid concentrations necessary for the formation of either crys- 

 talline form alone. Practically pure sphalerite was obtained 



* Amorphous zinc sulphide was sometimes admixed. 



