4 GENERAL DISCUSSION OF CLAYS. 



with orthoclase, the potash feldspar, serving asi the 

 type of the groups as well as being the commonest 

 species. 



Under the influence of chemical action, which may 

 be the result of weathering or in some cases probably 

 of acid vapors ascendning from the interior of the 

 earth, the feldspar becomes decomposed, and the result 

 of this is that the potash of the feldspar is removed 

 partly in the form of solube carbonate, or perhaps 

 ■ silicate, or even fluoride, while the alumina and silica 

 remain and unite with water to form the hydrated 

 silicate of alumina, kaolinite, whose composition is 

 expressed by the formula Al^ O3, 2SIO2, 2H2O., or 

 in the proportion of sUica, 47.30 per cent.; alumina, 

 39.80 per cent. ; water 13.90 per cent. 



The change can be illustrated still better by the fol- 

 lowing in which the first column indicates the com- 

 position of the feldspar, the second the amount of 

 water taken up in the process of decomposition, ihb 

 third, the amount of matter removed in solution, andi 

 the fourth the relative amounts of the three ingredi- 

 ents of kaolinite. ^ 



Feldspar. Added. Dissolved out. Kaolinite. 



Alumina 18.3 0.0 18.3 



Silica 64.8 .... 41.8 23.0 



Potash... 16.9 .... 1«.9 



Water 6.4 .... 6.4 



Many clays approach quite closely to kaolinite in 

 their composition, and in some the percentage of 

 alumina even exceeds the theoretic amount, by one or 

 two per cent., and is evidently not due to errors of an- 

 alysis. 



It has been suggested by some that this may be due 

 to the presence of a certain amount of phol&rite, the 

 amorphous variety of kaolin,* and while this is pos- 

 sible the same composition might be shown by a cer- 

 tain amount of bauxite or alumina hydrate mixed in 

 with the clay. 



* Wheeler, Clays of Missouri, Missouri Geological Surrey, XI. 



