CHEMICAL CONSTITUENTS OF THE SOU, 153 



The reaction may be represented as follows : 



Chabazite + K 2 SO 4 = K Chabazite + CaSO 4 . 



The reaction is reversible. When treated with sulphate of 

 lime, or other lime salts, the absorbed potash is partly replaced by 

 lime. 



K Chabazite + CaSO 4 K,SO 4 + chabazite. 



The extent of the change depends on the conditions of the 

 experiment and will be discussed under the topic of fixation by 

 the soil. It is possible that other hydrated silicates besides zeolites 

 take part in the fixation of potash. 



Finite is a hydrated silicate of alumina and potash, resulting 

 from the decomposition of felspar and some other minerals. 



Kaolinite is a hydrated silicate of alumina having the formula 

 Al a Si 2 O 7 2H 2 O. Kaolin is largely composed of kaolinite. Clay 

 contains kaolinite. Kaolin may be considered as a final product 

 of weathering of felspar and other silicates containing alumina. 

 The reaction which occurs in the formation of kaolin by the action 

 of water containing carbon dioxide upon felspar, may be written 

 as follows : 

 K 2 Al 2 Si 6 O 16 + H 2 O + CO 2 == Al_Si 2 O- + K 2 CO 3 + H 2 Si 4 O 9 . 



If lime or soda is present, its carbonate is formed in this decom- 

 position. The lime or soda may be converted into sulphate or 

 chloride if sulphur or chlorine is present in the rock. 



Chlorites are hydrated silicates of alumina, magnesia, or 

 iron. They are products of the decomposition of hornblende, 

 augite, and magnesium micas. Further decomposition results in 

 serpentine. 



Talc and Serpentine are hydrated silicates of magnesia. They 

 are soft, with a greasy feeling. Serpentine results from the decay 

 of olivine, and, though less often, from augite or hornblende. 



Glauconite is a hydrated silicate of alumina and iron, contain- 

 ing a small quantity of lime, magnesia, potash, soda, and phos- 

 phoric acid. It often occurs as grains of a green color, and is 

 termed green sand. Green sand marl is a mixture of glauconite 

 and calcium carbonate. 



Carbonates of Lime and Magnesia result from the action of 

 1 1 



