54 CLAYS AND CLAY INDUSTRY. 



Silica. 1 



This is present in clay in two different forms, viz., uncombined 

 as silica or quartz, and in silicates, of which there are several. 

 Of these one of the most important is the mineral kaolinite, which 

 is found in all clays and is termed the clay base or clay substance. 

 The other silicates include feldspar, mica, glauconite, hornblende, 

 garnet, etc. These two modes of occurrence of silica, however, 

 are not always distinguished in the ultimate analysis of a clay, 

 but when this is done, they are commonly designated as "Free" 

 and "Combined silica," the former referring to all silica except 

 that contained in the kaolinite, which is indicated by the latter 

 term. This is an unfortunate custom, for the silica in silicates is 

 properly speaking combined silica, just as much as that con- 

 tained in the kaolinite. A better practice is to use the term sand 

 to include quartz and silicate minerals, other than kaolinite, which 

 are not decomposable by sulphuric acid. In the majority of 

 analyses, however, the silica from both groups of minerals is 

 expressed collectively as total silica. 



The percentage of both quartz and total silica found in clays 

 varies between wide limits, as can be seen from the following 

 examples. Wheeler gives a minimum 2 of 5 per cent, in the 

 flint clays, and the sand percentage as 20 per cent, to 43 per cent, 

 in the St. Louis fire clays, and 20 per cent, to 50 per cent, in the 

 loess clays. Twenty-seven samples of Alabama clays analyzed by 

 the writer contained from 5 per cent, to 50 per cent, of insoluble 

 residue mostly quartz. 3 In seventy North Carolina clays 4 the 

 insoluble sand ranged from 15.15 per cent. to> 70.43 per cent. 



The following table 5 gives the variation of total silica in several 

 classes of clays, the results being obtained from several hundred 

 analyses : 



1 See also description of the minerals quartz, feldspar, kaolinite, and mica 

 above. 



2 Mo. Geol. Surv., Vol. XI, p. 54. 



3 Ala. Geol. Survey, Bull. No. 6, 1900. 



4 N. Car. Geol. Survey, Bull. No. 13, p. 24, 1898. 

 c Bull. N. Y. State Museum, No. 35, p. 525. 



