42 CLAYS AND CLAY INDUSTRY. 



Clays contain a great many different chemical compounds of 

 more or less definite chemical composition, and often having 

 a definite form. Each of these represents a mineral species, 

 possessing definite physical characters, which could be easily 

 seen if the grains of clay were large enough. The latter is the 

 case, however, with only a few of the scattered, coarse grains, 

 which a clay may contain, and, consequently, it is necessary to 

 use a microscope in order to identify the various mineral grains 

 present in any clay, as even a powerful hand glass cannot 

 ordinarilv distinguish them. 



MINERALS FOUND IN CLAY. 



The number of different minerals present in a clay is often 

 large and depends partly on the mineralogical composition of 

 the rock or rocks from which the clay has been derived, and 

 partly on the extent to which the mineral grains in the clay have 

 been destroyed by weathering. As the result of this weathering 

 action, new minerals are sometimes formed, and we can thus 

 recognize two> groups of minerals in the clay, viz., primary and 

 secondary constituents. Those which are probably in most cases 

 of primary character include quartz, feldspar, calcite, gypsum, 

 mica, pyrite, dolomite, iron ores, hornblende and rutile. The 

 chief secondary ones are kaolinite, together with closely allied 

 mineral species and limonite, but calcite, quartz, 1 gypsum and 

 pyrite may also be of secondary origin. 



Quartz. — This mineral, which is silica chemically, is found 

 in at least small quantities in nearly every clay, whether residual 

 or sedimentary, but the grains are rarely large enough to be 

 seen with the naked eye. They are translucent or transparent, 

 usually of angular form in residual clays, and rounded in sedi- 

 mentary ones, on account of the rolling they have received while 

 being washed along the river channel to the sea, or dashed about 

 by the waves on the beach previous to their deposition in deeper 

 still water. Ouartz mav be colorless, but it is often colored 



1 Some writers argue that much of the quartz found in soil is of secondary- 

 origin, but this is often difficult to prove. 



