PK/.XCIPLES OF ROCK WEATHERING 86/ 



removal of the lime set free, as a carbonate, provided the water 

 still contains a sufficient amount of carbonic acid. This replac- 

 ing 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 requiring for solution only six times its 

 weight of water at ordinary temperatures. Silica, on the other 

 hand, even in its most soluble form, requires 1 0,000 times its 

 weight of water for solution. If, therefore, the decomposition of 

 feldspar 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 xttwo" ^^ ^^e water present, the excess could not 

 be dissolved, but would remain mixed with the kaolin. 



The case is very different when the decomposition of feld- 

 spar is effected by fresh water containing only minute quantities 

 of carbonic acid. 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 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. 



H. P. Murakozy ' has shown that in the decomposition of rhy- 

 olite from Nagy-Mihely, the sanidin passes into kaolin and opal, 

 the latter separating out as hyalite in veins or impure concretion- 

 ary forms. Through this abstraction of silica there is an apparent 

 proportional increase in the amounts of alumina and alkalies. 



It follows from the above considerations that in the decompo- 

 sition of feldspar into kaolin, more of the silica separated remains 



" Abstract of F. Becke, Neues Jahrbuch, 1894, i Band, 2 Heft, p. 291. 



