CLAYS OF NEW VORK 503 



MIJSTEKALOGY OF CLAYS 



Tlie niTmber of mineral species which may exist in clays is very 

 great, and depends partly on the mineralogic composition of the 

 parent rock, and the extent to which decomposition has proceeded 

 in the clay mass. 



The characters of the common minerals found in clays together 

 with their more important features, are here given, arranged ap- 

 proximately in the order of their abundance. Any one of these 

 may however at times become a predominating constituent. 



Kaolinite. Pormula: Aip., -^ SiOg, 2 BgO, or silica (SiOa) 

 46.3^ alumina (Al-A) 39.8^, water (HgO) 13.9)^ 



This is a white, pearly mineral, crystallizing in the monoclinic 

 system, the crystals presenting the form of small hexagonal plates. 

 Its specific gravity is 2.2-2.6; its hardness 2-2.5. It is naturally 

 white in color and a mass of it is plastic when wet, buit very slightly 

 so. The occurrence of kaolinite in crystals has been noted from 

 iSTational belle mine, Red Mountain, Col. (H. Reusch, Jahrh. f. 

 mineralogie, 1887, 2 :70) and from Anglesey (A. Dick, Min. 

 mag. 1876, 8:15). A microscopic examination shows the plates of 

 kaolinite collected in little bunches. Their separation by grinding 

 increases the plasticity.^ 



If kaolin be formed into briquets, of the same shape as those 

 used in testing cement, its tensile strength, as determined by pulling 

 these briquets apart in a testing machine, is usually 5-15 pounds 

 the square inch — a very low degree compared with the tensile 

 strength of more plastic clays. 



Kaolinite is nearly infusible, but a slight addition of fusible 

 impurities lowers its refractoriness. 



Many kaolins contain very minute scales of white mica, which 

 it would be difficult to distinguish under the microscope from 

 kaolinite. Since white mica in a very finely divided condition is 



1 Clays of New Jersey, N. J. geol. sur. 1878. G. H. Cook. 



