224 BOCK DISINTEGEATION AND DECOMPOSITION 



bonates, wMeh are dissolved. If the water thus impregnated, 

 on penetrating further below the surface, comes in contact with 

 calcareous silicates, another change will take place consisting of 

 a decomposition and replacement of these calcareous silicates by 

 the alkaline silicates, and a removal of the lime set free, as a 

 carbonate, provided the water still contains a sufficient amount 

 of carbonic acid. This replacing process and the retention of 

 the alkaline silicates is accounted for on the supposition that, 

 in their nascent state, they form new combinations with the 

 other silicates present, while the lime remains as a carbonate to 

 be removed or not, as the case may be. He further states that 

 the alkaline carbonates originating in the manner described 

 are among the most soluble substances known; the carbonate 

 of soda requires for solution only six times its weight of water 

 at ordinary temperatures. Silica, on the other hand, even in 

 its most soluble form, requires ten thousand times its weight of 

 water for solution. If, therefore, the decomposition of feld- 

 spar by such carbonated water were ever so energetic, there 

 would be sufficient water for the solution of the carbonate of 

 soda formed. But if the silica separated meanwhile amounted 

 to more than -nriiTir ^^ ^^® water present, the excess could not 

 be dissolved, but would remain mixed with the kaolin. 



The case is very different when the decomposition of feldspar 

 is affected by fresh water containing only the minute quantity 

 of carbonic acid derived from the atmosphere. By the action 

 of such water, only very small quantities of alkaline carbonates 

 are formed; consequently it is possible that the silica separated 

 at the same time, also small in quantity, may find enough water 

 for solution. In such cases the whole of this silica would be 

 removed with the alkaline carbonates, and pure kaolin would 

 be left. Such an action as this does not, however, appear to 

 take place; for the purest of kaolin nearly always contains an 

 admixture of quartz sand, or of free silica in some of its forms. 



K. V. Murakozy has shown^ that in the decomposition of 

 rhyolite from Nagy-Mihaly, the sanidin passes into kaolin and 

 opal, the latter separating out as hyalite in veins or impure 

 concretionary forms. 



It follows from this consideration that in the decomposition 

 of feldspar into kaolin more of the silica separated remains 

 mixed with the kaolin formed, the greater the quantity of 



* Abstract by F, Becke, Neues Jahrbueli, 1894, 1 Band, 2 Heft, p. 291. 



