ALTERATIONS OF PLAGIOCLASE FELDSPARS. 261 



tion as the feldspars are less siliceous. The alteration of a given feldspar 

 may be into two or more of the above minerals. Doubtless often orthoclase 

 and plagioclase together pass into other minerals. One such reaction has 

 been considered by Becke, and is given below. 



The less siliceous plagioclases, labradorite, bj^townite, and anorthite, 

 alter rarely to kaolin alone, but this mineral may separate simultaneously 

 with zoisite (orthorhombic ; sp. gr. 3.31) or epidote. 



The alteration of albite to kaolin and quartz, to gibbsite and quartz, 

 and of albite and gibbsite to paragonite, respectively, may be written as 

 follows : 



(1) 2NaAlSi 3 8 +2H 2 0+C0 2 =H,Al 2 Si,0 9 +48i0 2 +Na 2 C0 3 +k. 



(2) 2NaAlSi 3 8 +3H 2 0+CO,=2[Al(OH) 3 ]+6Si0 2 +Na,C0 3 +k. 



(3) NaAlSi 3 8 4-2Al(OH) 3 =NaH 2 Al 3 Si 3 12 +2H 2 6 + k. 



The decrease in volume in equation (1) of the kaolin and quartz is 4.89 

 per cent; in equation (2) the increase for the gibbsite and quartz is 1.58 per 

 cent; in equation (3) the decrease for the paragonite, as compared with the 

 albite and gibbsite, is 18.85 per cent. 



Analcite (isometric; sp. gr. 2.22-2.29) may be derived from albite 

 according to the following reaction : 



(4) 2NaAlSi 3 8 +2H,,0 = Na 2 Al. 2 Si < 12 .2H 2 0+2Si0 2 +k. 



The increase in volume is 20.82 per cent, supposing the silica separates as 

 a solid. 



Natrolite (orthorhombic; sp. gr. 2.20-2.25) may also be derived from 

 albite according to the reaction: 



(5) 2NaAlSi 3 O s +2H 2 = H 4 Na 2 Al 2 Si 3 12 +3Si0 2 +k. 



The increase in volume is 19.95 per cent, supposing the silica separates as 

 a solid. 



From anorthite a number of zeolites are derived. Clarke is one of the 

 latest authors who has discussed the relations of the zeolites to the feldspars, 

 and the chemical alterations given are obtained mainly from his paper." 

 The equations for the more common varieties may be written as follows: 



Thomsonite (orthorhombic; sp. gr. 2.3-2.4) is derived from anorthite 

 according to the following reaction: 



(6) 3CaAl 2 Si 2 8 +7H 2 0=Ca 3 Al 6 Si, j O M -7H 2 0+k. 



The increase in volume is 34.65 per cent. 



"Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, pp. 32-45. 



