121 Brongniart on the Formation of Porcelain Clay, 



Brongniart has found, as the result of the various analyses, 

 that what is termed kaolin, or true porcelain clay, is always a 

 product of the decomposition of felspar in the primitive rocks, 

 which, however, does not preclude clays in general being also 

 formed by the weathering and decomposition of other minerals. 

 An opinion different from this has been expressed by Fuchs, 

 who assumes that kaolin is formed from a peculiar mineral 

 resembling felspar, which he has analyzed and found to con- 

 sist of N S 3 + C S 2 + 9 A S. It is, however, difficult to 

 understand in what way the large quantity of silicate of lime 

 contained in it can be dissolved and carried away. 



The kind of primitive rock in which the kaolin is most fre- 

 quently formed, and in which it is purest, is pegmatite or gra- 

 phic granite, which is composed of a lamellar compound of 

 quartz and felspar. It is, however, also formed in common 

 granite, in gneiss, as well as in diorite and porphyry. The 

 kaolin of Passau and St Yrieux occurs in gneiss, that of Auer- 

 bach and Zetlitz in granite. At St Yrieux a portion of it is 

 met with in diorite, and at Mori in porphyry. 



In regard to the circumstances which cause the conversion 

 of felspar into kaolin, the change seems chiefly ascribable to 

 perhaps a hydro-electrical influence exercised on a neighbour- 

 ing rock, which is always met with in close proximity to the 

 spots where the kaolin is found. This rock is of a red colour, 

 in consequence of its containing a large quantity of oxide of 

 iron. This circumstance was first remarked by Gehlen, when 

 he examined the repository of kaolin at Passau. Kuhn has 

 adduced another striking proof upon this subject. Near Sosa 

 in Saxony, kaolin is found. A vein of quartz which cuts 

 through the granite, has on both sides a thick sahlbandoi iron- 

 ore, and on both sides of the vein the felspar of the granite is 

 converted into kaolin, which is of excellent quality. Brongniart 

 lias found in every locality where he has examined reposi- 

 tories of kaolin, that they are surrounded by a very ferruginous 

 rock. It may also be added that the granite of Carlsbad, in 

 which large felspar-crystals occur, which, though still retain- 

 ing the crystalline form of felspar, have been converted into 

 kaolin, is strongly coloured red by oxide of iron. 



As to the formation of the kaolin, and the unequal propor- 

 tions in which the remaining silicic acid and alumina are 

 therein combined, he states tho following probabilities : — 1. 



