Allen, Crenshaw, andMerwin — Temperature, Acidity, etc. 393 



Art. XXXIII. — Effect of Temperature and Acidity in the 

 Formation of Marcasite (FeSJ and Wurtzite (ZnS) ; A 

 Contribution to the Genesis of Unstable Forms ; by E. T. 

 Allen and J. L. Crenshaw. Microscopic Study ; by 



H. E. Merwin. 



Syllabus. 



I. Introduction. 

 II. Marcasite and Pyrite. 



A. Preparation. 



B. Purification of the synthetic products. 



C. Chemistry of the formation of iron disulphide. 



D. Analysis of the products. 



E. Other evidence on the nature of the products. 



1. Microscopic evidence. 



2. Color. 



3. Evidence concerning inclusions. 



4. Analagous behavior of zinc salts. 



F. Effect of temperature and acidity on the crystalline form of the 



products. 



1. The products at 25°. 



2. The products at 100°. 



3. The products at 200° and 300°. 



(a) Method of plotting. 



(6) Significance of the curves. 



(c) Irregularity at high acid concentration. 



(d) The contemporaneous precipitation of pyrite and 



marcasite. 



4. Influence of hydrochloric acid. 

 Gr. Synthetic marcasite. 



III. Wurtzite. 



A. Method of preparing wurtzite. 



B. The reduction of dilute sulphuric acid by hydrogen sulphide. 



C. Distillation of dilute sulphuric acid. 



D. Precipitation which takes place before the final temperature is 



reached. 



E. Effect of changing conditions other than acid concentration. 



1. Hydrogen sulphide pressure. 



2. Zinc concentration. 



3. Sodium sulphate. 



4. Time. 



F. Evidence that wurtzite is changed to sphalerite by heating with 



dilute acid. 



G. The influence of acidity in the formation of wurtzite. 



1. The question of a linear relation between the percentage of 



wurtzite and the final acid concentration. 



2. The conditions necessary for the genesis of either form alone. 



IV. Natural genesis of the sulphides of iron and zinc. 



A. Pyrite from cold alkaline solutions. 



B. Paragenesis of marcasite and wurtzite with calcite. 



C. Paragenesis of pyrite with calcite. 



Summary. 



