CHEMICAL PROPERTIES OF CLAY. 47 



formula of A1 2 3 , 2Si0 2 , 2H 2 0, which corresponds to a com- 

 position of Silica (Si0 2 ), 46.3 per cent.; Alumina (A1 2 3 ), 

 39.8 per cent.; Water (H 2 0), 13.9 per cent. It is rarely found 

 in pure masses, but when isolated is found to be a white, pearly 

 mineral, the crystals forming small hexagonal plates, which are 

 often found to° be collected into little bunches, that can be sep- 

 arated by grinding. When the mineral kaolinite forms large 

 masses, the name of kaolin is applied to it. 1 It is plastic, and is 

 also highly refractory, fusing at cone 36. The amount of 

 kaolinite present in clays varies, some white kaolins containing 

 over 98 per cent., while other sandy impure clays may have less 

 than 20 per cent. 



Associated with kaolinite, there have been found one or more 

 species of allied minerals, which are all hydrated silicates of 

 alumina. They are known as halloysite, rectorite, newtonite, 

 allophane, etc. Some have been found in the form of crystals, 

 and others have not. 



Rutile. — The oxide of titanium (Ti0 2 ) rutile is of wide- 

 spread occurrence in clays, and is usually found on chemical 

 analysis, when proper tests are made. Rutile grains can be seen 

 under the microscope in many fire clays, and the analyses show 

 the presence of titanium oxide to* the extent of nearly two per 

 cent. 2 The presence of this mineral, however, is unfortunately 

 too commonly ignored in the analysis of clay, and yet, as will 

 be shown later, its effect on the fusibility of the clay is such that 

 it should not be neglected in the higher grades, at least. 



Calcite. — This mineral is composed of carbonate of lime, and, 

 when abundant, is found chiefly in clays of recent geological 

 age, but some shales also contain considerable quantities of it. 

 It can be easily detected for it dissolves rapidly in weak acids, 

 and effervesces violently upon the application of a drop of 

 muriatic acid or even vinegar. When in grains large enough 

 to be seen with the naked eye, it is found to be a translucent 



1 The material termed kaolin, which is found in the Woodbridge district of 

 New Jersey, is not such, but a quartz sand with a considerable percentage of 

 white mica and clay. 



" See the Fire Clays and Fire-Brick Industry, Chap. XVI. 



