40 GENERAL DISCUSSION OF CLAYS. 



cepted, the purer clays are usually light colored, while 

 the impure ones are yellow, red, or brown. 



Organic matter however, frequently masks the iron 

 coloration, and makes it often difficult 

 to determine the refractory nature of 

 the material. Some clays , which bum 

 perfectly white may be colored black by organic 

 matter as in the case of the sand clay from Pegram. 

 Ferrous compounds not infrequently impart a gray or 

 bluish tint to clay, and at times the lower part of a 

 clay bed may be gray while the upper portion is yellow 

 or red, due to the oxidation of the iron contained in it. 



THE MINERALOGY OF CLA.YS. 



Most clays are so fine grained that it is impossible 

 to determine the mineral constituents ynth the naked 

 eye, and their recognition even microscopically, is 

 sometimes a matter of diffculty. At the same time 

 however, there are certain minerals, which are either 

 present in all clays or are to be found in a great many 

 of them, and these will be mentioned in the order of 

 their abundance. 



KAOLINITB. 



The mineral kaolinite is looked upon as the base of 

 all clays, and while it is not wanting so far as we know 

 in any of them, nevertheless, it is not as abundant as 

 we have been apt to consider it, nor are the charact- 

 eristic properties of clay wholly due to it. 



Kaolinite, whose formula is Al^Og, 2Si02, ^H^O, or 

 silica 46.3 per cent., alumina 39.8 per cent., water 13.9 

 per cent is a white scaly mineral crystallizing in the 

 monoclinic system, the crystals presenting the form 

 of small hexagonal plates. Its specific gravity is 2.2 



