502 MINERALOGY 



Kaolinite represents the ultimate product in the weathering and 

 hydration of a long series of aluminium silicates. The most impor- 

 tant source is in the alteration of the feldspars, known as kaoliniza- 

 tion, which is the result of percolating waters charged with carbon 

 dioxide. This is especially true of the alkali feldspars, as anorthite 

 does not form kaolin. The potassium is yielded as a carbonate, 

 leaving the aluminium silicate in the hydrated form as kaolin. 

 It is therefore a secondary mineral widely distributed in all rocks 

 which have suffered alteration and in all soils. It forms pseudo- 

 morphs after many minerals, or remains behind mixed with other 

 products of weathering, to form the soils ; or it is carried by run- 

 ning water and deposited as sedimentary beds, some of which are 

 very pure kaolin. Owing to the extreme fineness of the scales, they 

 remain longer in suspension, and are deposited apart from heavier 

 and coarser materials. 



Clay is a large component of .all sedimentary rocks except the 

 sandstones and carbonates. Under ordinary conditions of weather- 

 ing it is extremely stable and is not broken down, but under the 

 influence of heat and pressure it is dehydrated, yielding its alumina 

 and silica for the formation of minerals characteristic of meta- 

 morphic sediments. 



Allophane, Al 2 SiO 5 . 5 H 2 O, is an amorphous aluminium sili- 

 cate forming mammillary crusts and stalactites. Hardness 3, 

 harder than kaolin, and gelatinizes with HC1. 



Pyrophyllite, H2Al 2 Si 4 Oi 2 , is a silicate of aluminium resembling 

 talc in appearance, as to color, structure, luster, and feeling. It 

 may also be radiated and fibrous as well as massive, but when heated 

 it swells many times its volume and becomes blue when ignited 

 with cobalt solution. 



There are other silicates of aluminium related to kaolinite, 

 some of which are probably mixtures, and the composition of others 

 is uncertain. 



Commercially clay or kaolinite is the basis of all porcelain and 

 chinaware, and the impure varieties are used in the manufacture 

 of tile, drainpipes, and bricks. 



The purer varieties are obtained in Chester and Delaware 

 Counties, Pennsylvania, and in North Carolina, while a large pro- 

 portion of the ordinary clays is dug in New Jersey. 





