MINERALOGY OF CLAYS. 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.* 



Eiiolinite 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 the 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 iflfrequently 

 stained by a th'n 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, Geolcgioal Survey, 1878. 



