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PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Hydrated Silicas are formed by the deposition of silica from 

 aqueous solutions. From many silicates water dissolves silica, 

 which may be deposited as various forms of hydrated silica. 

 Hydrated silicas are probably present in some soils. 



Hydrated Silicates are produced by combination of water with 

 primary or secondary silicates, with or without loss of matter by 

 solution. The complex hydrated silicates are more easily decom- 

 posed, and otherwise enter into reactions much more readily than 

 the primary silicates, from which they were formed. They are 

 also more easily decomposed by acids. Hydrated silicates appear 

 to be of considerable importance in the soil. A great number of 

 hydrated silicates are known. 



Zeolites are hydrated silicates of alumina, with varying amounts 

 of potash, soda, lime, etc., produced by hydration and decomposi- 

 tion of many varieties of minerals. They are easily decomposed 

 by acids. A large number of zeolites are known. They take 

 part in some important reactions which occur in the soil, especially 

 the fixation of potash. 



Chabazite (HK) 2 CaAl 2 Si 5 O 15 +6H 2 O, stilbite (CaNa) 2 Al 2 Si 6 - 

 Q 16 +6H 2 O, analcite Na 2 Al 2 Si 4 O 12 +2H 2 O and prehnite H e Ca 2 - 

 Al 4 Si 3 O 12 , are hydrated silicates belonging to the zeolite class. 

 They are soluble in hydrochloric acid, the silica being separated. 

 When brought in contact with salts of potash, they remove some 

 of the potash from solution and replace it with equivalent 

 quantities of lime or soda. For example, the minerals named 

 below 1 were treated with a strong solution of sulphate of potash, 

 washed and subjected to analysis : 



1 Texas Station, Bulletin 106, p. u. 



