POTASH MANURES 281 



4. The Kock salt region, consisting chiefly of sodium 

 chloride. 



5. The Calcium region, consisting chiefly of gypsum 

 and anhydrite, both sulphates of calcium. 



The following are the more important forms of potash 

 salts produced at the mines : 



Sylvine. This consists of practically pure potassium 

 chloride or muriate of potash (KC1). It appears to be a 

 secondary product derived from the carnallite, and is 

 sometimes prepared from it artificially. Commercial 

 varieties contain from 70 to 95 per cent, of pure potas- 

 sium chloride. 



Sylvinite. This may be regarded as an impure form 

 of sylvine, but is of more variable composition. It con- 

 tains from 20 to 25 per cent, of potash, chiefly as 

 chloride, about 40 per cent, of common salt, and smaller 

 quantities of other impurities. 



Carnallite. Carnallite is a well-known double chloride 

 of potassium and magnesium. In the pure state its 

 composition corresponds to the formula (MgCl 2 , KC1, 6H 2 0). 

 The potassium chloride can be separated from the mag- 

 nesium salt by a process of crystallization, and is sold 

 as muriate of potash. 



Schonite. This salt may be regarded as the sulphate 

 corresponding to carnallite. It is represented by the 

 formula (MgS0 4 , K 2 S0 4 , 6H 2 0). It is >ot a large pro- 

 duct in the mines, but a substance of similar composition 

 can be prepared from kainite by dissolving out the 

 sodium and magnesium chlorides. It contains about 

 50 per cent, of sulphate of potash and a considerable 

 quantity of magnesium sulphate, but little or no 

 chloride. It is sometimes calcined, i.e., heated, to 

 drive off the water, which in the natural condition, 

 usually forms about 10 per cent, of the whole. This 

 niakes it more concentrated anid saves cost in transport, 



