128 SOILS AND MANURES 



definite intermediate compounds and the composition of 

 the same are often entirely unknown. 



Decomposition of Minerals. Among the most important 

 of the chemical changes which take place in the soil are 

 those by which the constituents of insoluble minerals are 

 rendered available to plants. Carbonic acid and water 

 take part in the reactions which are also greatly accelerated 

 by the processes of mechanical pulverisation previously 

 described. 



The formation of kaolin from felspars may be taken as 

 typical of the changes which complex silicates undergo. 

 In the case of anorthite, the lime felspar, it may be repre- 

 sented by the following equation 



Ca A1 2 Si 2 8 + 2 C0 2 + 3 H 2 = Si 2 6 A1 2 (OH) 4 + Ca H 2 (C0 3 ) 2 



Anorthite. Kaolin. 



The nature of the change may be more easily understood 

 by comparing the constitutional formulae which have been 

 proposed for the two substances. 



n j SiO.O A10.A10 f SiO.O A10.0H 2 



1 SiO.O CaO t SiO.O A10.0H 2 



Anorthite. Kaolin. 



It will be seen that, if these are correct, the change could 

 be accomplished by the transference of an atom of 

 aluminium to the place occupied by the calcium the 

 latter being eliminated by the action of C0 2 and subse- 

 quent hydration thus 



The composition of orthoclase differs from that of 

 anorthite and it does not undergo change so readily. It 

 is one of the most insoluble of rock-forming minerals, but 



