CLAYS OF KEW YOKK 535 



little of it, and partly because it was thoiiglit to be like kaolinite in 

 its behavior. Where the mica percentage is very low, say Ifo or 2^ 

 and is in a very finely divided condition, it can be neglected, but 

 where it reaches 5^ or more it does not seem proper to class it as 

 clay substance, for the reason that mica tends to decrease the plas- 

 ticity, which effect is greater the coarser the mica. It does resem- 

 ble kaolinite in refractoriness. In many of our washed kaolins 

 now on the market there is very little mica, but some contain 

 8^-10^, which does not always yield to sulfuric acid treatment. 



If now a kaolin containing clay substancei, quartz and feldspar 

 be treated first with sulfuric acid, the kaolinite is decomposed into 

 sulfate of alumina and hydrous silica. The former is soluble in 

 water, the latter is removed by subsequent treatment with caustic 

 soda, and we have the insoluble residue consisting of quartz and 

 feldspar. In this residue the alumina is determined, and from this 

 the amount of feldspar is calculated, viz : 



102 : 556 ; : a : x 



molec. wt molec. wt weight of 

 alumiDa orthoclase alumina 



This is subtracted from the insoluble residue, and the differ- 

 ence is the quartz. 



There is still another way of conducting a rational analysis, 

 which is chiefly applicable when the clay contains other minerals 

 beside the kaolinite, feldspar and quartz, such as carbonate of 

 lime and magnesia, and appreciable amounts of ferric oxid and 

 such mica as is attacked by sulfuric acid. This second method is 

 Seger's method as elaborated by Langenbeck, and may be illus- 

 trated by the following example, a fire clay from Ohio. 



