CLAYS or NEW YORK 525 



Silica 



Three types of silica may be recognized in clay: 1) quartz; 2) 

 tliat wMcli is combined witb alumina and water in kaolinite; 3) that 

 which is combined with one or more bases in silicate minerals. In 

 chemical analysis the first and third are sometimes grouped together 

 under the head of " sand/' or at times erroneously spoken of as 

 " free " silica.. The silica included under the term sand is prac- 

 tically insoluble in sulfuric acid and caustic soda. This fact is 

 utilized in the rational analysis of clay to extract the kaolinite or 

 clay substance, which is soluble in sulfuric acid and caustic soda. 



Quartz is present in every clay so far as analysis shows, but in 

 variable amounts. Cook ^ found a minimum of .2^, and gives 

 5fo as the average in the Woodbridge fire clays. Wheeler ^ gives 

 the minimum as .5^ in the flint clays, and the sand percentage 

 as 20^^-43;^ in the St Louis fire clays and 20^-50^ in the Loess clays. 

 27 samples of Alabama clays analyzed by the writer contained from 

 5/^ to 50^ of insoluble residue mostly quartz.^ 



In 70 ISTorth Carolina* clays there were from 15.05^ to 70.43^ in- 

 soluble residue; while in three samples, of which a rational analysis 

 was made, the percentage of sand was from 24.55^ to 56.58^. The 

 quartz varied from 16.58^ to 49.06^, with the feldspathic residue 

 from 7.52^ to 16.05^. 



In European clays similar variations in the amount of sand and 

 quartz are observable. Thus a clay from Hainstadt, Germany 

 contains 67.03/^ of quartz (Ziegler Kalender. Berlin 1896), while 

 one from Ruppersdorf showed .26^. (.Seger's Ges. Schrift. p. 891) 



The following table saves the variation in the total silica in four 

 types of clay : 



Quality Min. Max. Aver. 



Brick clays . 34. 35 90 . 877 59 . 27 



Pottery clays 45 . 06 86 . 98 45 . 83 



Fireclays 34.40 96.79 54.304 



Kaolins , 32.44 81.18 55.44 



IN. J. geol. sur. 1878. Clays of New Jersey, p. 213. 



2 Mo. geol.. sur. 1896. 11: 54. 



3 Ala. geol. sur. 1900. Bulletin no. 6. 



4]Sr. C. geol. sur. 1898. Bulletin no. 13, p. 24. 



