60 CLAYS AND CLAY INDUSTRY. 



i. Carbonates. Calcite, dolomite. 



2. Silicates containing lime, such as feldspar, garnet. 



3. Sulphates. Gypsum. 



Whenever the ultimate analysis of a clay shows several per 

 cent, of lime (CaO), it is usually there as an ingredient of lime 

 carbonate (CaC0 3 ), and in such cases its presence can be easily 

 detected by putting a drop of muriatic acid or vinegar on the 

 clay. 1 When present in this form it is apt to be finely divided, 

 although it may occur as concretions or limestone pebbles; in 

 either case, it is usually restricted to drift clays, especially in 

 New Jersey. 



When lime is present as an ingredient of silicate minerals, 

 such as those mentioned above, its presence cannot be detected 

 with muriatic acid. It is doubtful, however, if many calcareous 

 clays contain much lime in this combination, and the fact that 

 practically all limy clays, shown to be such on chemical analysis, 

 give a strong test with muriatic acid, strengthens this theory. 

 Gypsum, which is found in a few clays, is often of secondary 

 character, having been formed by the action of sulphuric acid 

 on lime-bearing minerals in the clay. Since these three groups 

 of minerals behave somewhat differently, their effects will be 

 discussed separately. 



Effect of lime carbonate on clay. — Lime is probably most 

 effective in the form of the carbonate. When clays containing 

 it are burned, they not only lose their chemically combined 

 water, but also- their carbon dioxide, but while the water of 

 hydration passes off between 450 C (842 F.) and 6oo° C 

 (1112 F.), the carbon dioxide (C0 2 ) does not seem to go 

 off until between 6oo° C. (1112 F.) and 725 ° C. (1562 F.). 

 In fact it more probably passes off between 850 C. (1562 F.) 

 and 900 C. (1652 F.). The result of driving off this gas 

 in addition to the chemically combined water is to leave cal- 

 careous clays more porous than other clays up to the beginning 

 of fusion. 



If the burning is carried only far enough to drive off the 

 carbonic acid gas, the result will be that the quicklime thus formed 



1 See Minerals in Clay, Calcite. 



