134 HOW CROPS FEED. 
pite, perhaps, excepted,) and the amphiboles 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 Clays, 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 minerals. Fresenius deduces from his analyses of 
several Nassau clays the existence in them of a compound 
having the symbol Al, O, 3 SiO,+H,O, and the follow- 
ing composition per 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 only 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. S7i., 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 constitution, although mechanically dis- 
integrated or dissolved. We shall see hereafter that there 
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