122 SOMERS: MICROSCOPIC EXAMINATION OF CLAYS 



set was refired for 8 hours to 1,150° C, and a second set for 10 

 hours to 1,300° C. They were then ground to thin sections 

 and examined under the microscope. 



Quartz grains usually stand out with much greater clearness 

 in the burned than in the raw clay, due to the fact that the 

 hydrous aluminum silicates tend to mat or fuse together to a 

 fine-grained ground mass which holds the quartz. In some speci- 

 mens a fluxing action appears to have taken place between the 

 fine-grained material and the silica, resulting in a corrosion of 

 the quartz, but this is comparatively rare. 



Hydromica on heating to 1,150° C. either practically disap- 

 pears, forming an isotropic mass, or else it loses the greater 

 part of its interference color. The only exception to this was 

 where the hydromica grains were much larger than usual, in 

 which case it was noticed that the central portion of the grains 

 retained usually the original interference color. 



This change of the hydromica on heating would seem to sug- 

 gest that it furnishes some of the flux for the clay, and other 

 things being equal, there may be a connnection between the de- 

 gree of densification at the temperature mentioned, and the quan- 

 tity of hydromica present. 



Kaolinite when not fluxed, appears to retain its shape and at 

 least a good part of its original interference color. Tourmaline 

 and probably epidote disappear even at 1,150° C, but the rutile, 

 zircon, and probably titanite seem to be unaffected even at 

 1,300° C. The persistence of the rutile can be plainly seen even 

 through the particles are very small. 



Sillimanite was noticed in a Florida white clay fired at 1,300° 

 C, where the conditions happened to be just right for its develop- 

 ment. That it has formed from the large flakes of kaolinite 

 or low-grade hydromica is clearly indicated by one composite 

 flake of the two minerals. 



The actual reason for its development is not evident, other 

 clays carrying similar micaceous flakes, and burned at the same 

 time, not showing the sillimanite in the burned product. 



