34 THE SOU, SOLUTION 



The acid of the formula H.AlSi 3 O 8 is not known and is 

 probably entirely instable under ordinary conditions, and breaks 

 down with the separation of silica, to form the minerals pyro- 

 phyllite, kaolinite or kaolin, and diaspore according to the follow- 

 ing equations : 



H.AlSi 3 O 8 SiO 2 = H.AlSi 2 O 6 (Pyrophyllite) 

 H.AlSi 3 O 8 2SiO 2 = H.AlSiO 4 (Kaolinite) 

 H.AlSi 3 O 8 -- 3SiO 2 = H.A1O 2 (Diaspore). 



All three of these minerals and their corresponding salts have 

 been found in nature as alteration products of orthoclase. It 

 is probable that, under soil conditions, the principal metamorphic 

 product of feldspar is kaolin (or kaolinite when it is crystalline), 

 hydrated aluminum oxide being of much less importance 1 and 

 pyrophyllite of doubtful occurrence. A still more interesting 

 case, perhaps, because of the well recognized tendency of mag- 

 nesium salts to form basic compounds, is the alteration of 

 pyroxene, amphibole and olivine with the formation of a chlorite 

 or serpentine, common associations in nature, which may be 

 represented 



MgSi0 3 -f HOH 5 MgSiO 3 .wMg(OH) 2 -f SiO 2 . 



It is tacitly assumed in the foregoing statements that the 

 reaction between a silicate mineral and water is a reversible 

 reaction. This is not definitely known to be the case, for the 

 formation of the ordinary silicate rock-forming minerals in the 

 wet way at ordinary temperatures has as yet been realized in 

 only a few cases. The assumption has, however, some experi- 

 mental support. Minerals have been often made in the wet way 

 at somewhat elevated temperatures, especially interesting cases in 

 this connection being the formation of orthoclase by Friedel and 

 Sarasin 2 at slightly elevated temperatures, and the formation of 



1 See Ueber die Bildung von Bauxit und verwandte Mineralien, von 

 A. Liebrich, Zeit. prakt. Geol., 1897, 212-214. 



* Sur la reproduction par voie aqueuse du feldspath orthose, par 

 Friedel et Sarasin, Comptes rendus, 92, 1374 (1881). 



