MINERALOG Y OF CLA YS. 41 



to 2.6 and its hardness is 2 to 2^. It is naturally 

 white in color and plastic when wet but very slightly 

 so. The microscope shows the kaolinite to be collect- 

 ed in little bunches which can be broken apart by 

 grinding and thereby increasing the plasticity.* 



Kaolinite is nearly infusible but a slight addition of 

 fusible impurities lowers its refractoriness. A mass 

 of kaolinite is called kaolin, and pure kaolin is practi- 

 cally unknown. 



Many kaolins contain very minute scales of white 

 mica, which under t'he microscope are hardly distin- 

 guishable from kaolinite. It is not to be inferred that 

 kaolinite always occurs in hexagonal plates, for in 

 some clays scales of six sided outline are almost want- 

 ing. 



QUARTZ. 



This mineral is present in sedimentary clays most- 

 ly in the. form of rounded grains, and sometimes in 

 crystals, while in residual clays the particles are most 

 commonly angular. It is an extremely hard mineral, 

 which will scratch glass and possesses a shell l^ke or 

 conchoidal fracture, it is practically not attacked by 

 the common acids, but is affected by alkaline solu- 

 tions. This is one of the few mineral components of 

 clay which, at times, occurs in grains of sufficient size 

 to be recognized by the unaided eye. It may be color- 

 less but the surface of the grain is not infrequently 

 stained by a tlrn film of iron oxide. Feldspar might 

 be mistaken for it, but the latter will not scratch 

 glass. 



Flint or non-crystalline silica is sometimes present 

 in clays. It usually has a muddy color and a con- 

 choidal fracture. 



*G. H. Cook, Clays of New Jersey, Geological Survey, 1878. 



