Or.ir.IN AND FOi:.Nf.\Tl<>\ ol' S(>ILS, lii'fi 



with liydrated silica, and often with grains of nndecomi>os- 

 ed mineral. If we compare the composition of pure pot- 

 ash feldspar with that of kaolinite, assuming, what is 

 probably true, that all the alumina of the former remains 

 in the latter, we find what portions of the feldspar have 

 been removed and washed away by the water, which, to- 

 gether with carbonic acid, is the agent of this change. 



Feldspar. Kaolinite. Liberated. Added. 



Alumina 18.3 18 3 



Silica 64.8 23.0 41.8 



Putasb 16.9 16.9 



Water 6.4 6.4 



100 47.7 58.7 6.4 



It thus appears that, in the complete conversion of 100 

 parts of potash felds])ar into kaolinite, there result 47.7 

 parts of the hitter, while 58.7" |„ of the feldspar, viz: 

 41.8° |„ of silica and 16.9° |^ of potasli, are dissolved out. 



The potash, and, in case of other feldspars, soda, lime, 

 and magnesia, are dissolved as carbonates. If much water 

 has access during the decomposition, all the liberated silica 

 is carried away.* It usually happens, however, that a por- 

 tion of the silica is retained in the kaolin (perhaps in a 

 manner similar to that in which bone charcoal retains the 

 coloring matters of crude sugar). The same is true of a 

 portion of the alkali, lime, and oxide of iron, which may 

 have existed in the original feldspar. 



The formation of kaolin may be often observed in na- 

 ture. In mines, excavated in feldspathic rocks, the fis- 

 sures and cavities through which surface water finds its 

 way downwards are often coated or filled with this sub- 

 stance. 



c. Other Silicious Minerals, as Lencite, (Topaz, Scapo- 

 lite,) etc., yield kaolin by decomposition. It is pi-obable 

 that the micas, which decompose with difficulty, (phlogo- 



♦ We have seen (H C. G., p. 121) that silica, when newly set free from combi- 

 natiou, is, at tirst, freely soluble in water. 



