134 ilOAV CROPS PEED. 



pite, perhaps, excepted,) and the ampliiboles and pyrox- 

 enes, which are often easily disintegrated, also yield 

 kaolin ; but in the case of these latter minerals, the result- 

 ing kaolinite is largely mixed with oxides and silicates of 

 iron and manganese, so that its properties are modified, 

 and identification is difficult. Other hydrated silicates of 

 alumina, closely allied to kaolinite, appear to be formed in 

 the decomposition of compound silicates. 



Ordinary (lays, as pipe-clay, blue-clay, brick-clay, etc., 

 are mixtures of kaolinite, or of a similar hydrated silicate 

 of alumina, with a variety of other substances, as free 

 silica, oxides, and silicates of iron and manganese, carbon- 

 ate of lime, and fragments or fine powder of undecom- 

 posed mii'.erals. Fresenius deduces from his analyses of 

 several Xassau clays the existence in them of a compound 

 having the symbol Al„ O^ 3 SiO,+H,0, and the follow- 

 ing composition joer cent. 



Silica, 57.14 



Alumina, 31.72 ' 

 Water, 11.14 



100.00 



Other chemists have assumed the existence of hydrated 

 silicates of alumina of still different composition in clays, 

 but kaolinite is the onh'^ one which occurs in a pure state, 

 as indicated by its crystallization, and the existence of 

 the others is not perfectly established. (S. W. Johnson 

 and J. M. Blake on Kaolinite^ etc.. Am. Jour. Sci., May^ 

 1867, pp. 351-362.) 



d. The Zeolites readily suffer change by weathering ; 

 little is known, however, as to the details of their disinte- 

 gration. Instead of yielding kaolinite, they appear to be 

 transformed into other zeolites, or retain something of their 

 original chemical ccmstitution, although mechanically dis- 

 integrated or dissolved. We shall see hereafter that there 



