KAOLIN IX WISCONSIif. 5 



These figures correspond to the formula AI2 O3, 2 Si Ov -{- 2 Ho 0, 

 one deduced by Forchhammer for kaolin as long ago as 1830, from 

 a comparison of a series of analyses of crude and washed articles. 



We may then designate as kaolin any native hydrated silicate of 

 alumina having the above percentage composition, or any native 

 material composed of a mixture of such a silicate with quartz 

 fragments, and fragments of undecomposed rock. Some of the 

 raw kaolins are almost pure kaolinite, whilst others contain as 

 much as fifty to sixty per cent, of foreign matter. 



Origin of Kaolinite. — The mineral kaolinite, when considered as 

 the base of large clay masses, appears always to have resulted from 

 the decomposition of minerals of the felspar group. In verj^ small 

 quantities, it is true, the same substance is known to be an altera- 

 tion product of other minerals than the felspars, e.g. beryl, stauro- 

 lite, leucite; still all of the large kaolinite masses have originated 

 by the alteration of some of the felspars. This alteration may have 

 been caused by several agents, by far the most important of which 

 has, however, been carbonated water, or water carr^ang carbonic 

 acid in solution. The felspars are silicates of alumina with an al- 

 kaline ingredient, which may be either potash, soda, or lime. Ob- 

 taining carbonic acid from the atmosphere, and to some slight ex- 

 tent from direct organic decay, the surface waters, thus reinforced, 

 infiltrating through the seams of the felspar-bearing rocks, (gran- 

 ite, gneiss, porphyry,) act gradually upon the alkaline silicates, 

 forming first carbonate of lime, if lime be present in the rock, 

 which dissolving as bicarbonate in the carbonated Avater, is carried 

 away. More slov/ly are taken up and leeched out the alkalies as car- 

 bonates, or as silicates, if the amount of carbonic acid is only small, 

 which will be the more usual case. Part of the silica thus set free 

 always remains as colloid or hydrated silica, and may be detected 

 in samples by its solubility in alkali. The amount of colloid sil- 

 icaf remaining will depend directly on the supply of carbonic acid, 

 being greater as the carbonic acid is more plenty. Still remaining 

 after the leeching process are now certain proportions of alumina 

 and silica, to which is added a certain pi'oportion of water. These 

 three combining and crj^stallizing, form the hydrated silicate of al- 

 umina, kaolinite. The theoretical change from orthoclase felspar 



""Dana's Mineralogy — ^imder orthoclase, p. 361. 

 fBischof Chem. Geol. Vol. II, p. 183. 



