294 A TREATISE ON METAMORPHISM. 



Another alteration of nephelite of some importance is to socialite 

 (isometric; sp. gr. 2.14-2.30) — 



(8) 3NaAlSi0 4 -i-NaCl=NaC1.3NaAlSi0 4 +k. 



Supposing the NaCl to be added in solution, the increase in volume is 

 33.14 per cent. If the sodium chloride be present as solid halite (isometric; 

 sp. gr. 2.1-2.6), the increase in volume would be 15.64 per cent. 



While the change is not recorded, it is believed to be highly probable 

 that nephelite during mass deformation under deep-seated conditions may 

 change into feldspar, probably albite (triclinic; sp. gr. 2.62-2.65). This 

 reaction would require the addition of silica, as follows: 



(9) NaAlSi0 4 +2Si0 2 =NaAlSi 3 8 -fk. 



Supposing the silica to have been present as quartz (rhombohedral ; sp. gr. 

 2.653-2.654), the decrease in volume would be 0.41 per cent. 



The formation of the zeolites, and simultaneously the minerals gibbsite 

 or diaspore, equations (1) to (6), are all alterations of hydration, carbona- 

 tion, and expansion of volume, except that of thomsonite, equation (4), 

 which does not involve carbonation. It is therefore to be expected that 

 these are reactions which take place in the zone of katamorphism, and such 

 is the fact. As a result of the alteration of the nephelites to the zeolites in 

 this zone, a part of the sodium separates and probably goes into solution as 

 sodium carbonate, and thus we have one of the sources of this compound 

 which so frequently occurs in underground waters, especially in volcanic 

 regions. The formation of muscovite and kaolinite from nephelite is a 

 reaction involving hydration and carbonation and decrease of volume, and 

 therefore is characteristic of the zone of katamorphism. The formation of 

 socialite from nephelite is one which might take place in either physical- 

 chemical zone, only in the upper zone the sodium chloride would probably 

 be added in solution, while in the lower zone it would probably be derived 

 from solid halite. 



CANCEINITE. 



occurrence. — Cancriiiite is known only in the nepheline syenites. 



Alterations. — By Dana it is mentioned as altering to natrolite (orthorhombic ; 

 sp. gr., 2.225). The reaction, supposing the excess of alumina passes into 

 gibbsite (monoclinic; sp. gr., 2.35), may be as follows: 



H c Na 6 Ca ( NaC0 3 ) 2 Al 8 Si 9 3 6+ 6H 2 = 



3(Na,Al 2 Si 3 O 10 .2H 2 O)+2Al(OH) 3 4CaCO 3 +Na 2 CO 3 +k. 



