THE NONMETALLIC MINERALS. 



421 



ALUM SLATE OR SHALE is a somewhat indefinite name given to 

 fine-grained arenaceous rocks consisting essentially of siliceous ami 

 fieldspathic sands and clays with disseminated iron pyrites. The fol- 

 lowing analyses from Bischofs Chemical Geology will serve to show 

 their varying composition: 



(I) An alum slate from Opsloe. near Christiania, Norway, (11) from 

 Bornholm, and (III) from Garnsdorf, near Saalfeld, Prussia. Concern- 

 ing No. Ill it is stated that "on the roof of the adit, driven into the 

 slate, there are almost everywhere yellow or white opaque stalactites, 

 and more rarely a green transparent deposit is produced. Both con- 

 sist of hydrated basic sulphate of alumina and peroxide of iron. In 

 the former, iron predominates; in the latter, alumina. Both substances 

 are quite insoluble in water. 



From shales and slates of this type the alum is obtained by crushing 

 and allowing to undergo prolonged weathering or submitted to a roast- 

 ing process. The essential part of the reaction consists in oxidizing 

 the bisulphide to the condition of a sulphate and finally into iron 

 sesquioxide, with separation of free sulphuric acid which attacks the 

 alumina, forming an equivalent quantity of sulphate of aluminum or 

 alum. So far as is known this process is not carried on at all in the 

 United States. 



The alum shale of the English Upper Liassian formation consists of 

 hard blue shale with cement stones. On exposure to the air it grad- 

 ually becomes incrusted with sulphur, and occasionally with alum. 

 In composition the alum shale is as follows: 



Iron sulphide 8. 50 



Silica 51. 16 



Iron protoxide 6. 11 



Alumina 18. 30 



Lime 2. 15 



Magnesia 0. 90 



Sulphuric acid 2. 5 



Potash Trace. 



Soda Trace. 



Carbon 8. 29 



Water... . 2.00 



Total. 



99.91 



