266 A TREATISE ON METAMORPHISM. 



LEUCITE GROUP. 



Leucite is the only rock-making' mineral belonging to this group. 



LEUCITE. 



Leucite: 



KAlSi 2 6 . 

 Isometric. 

 Sp. gr. 2.45-2.5. 



occurrence. — Leucite is a common constituent of volcanic rocks, especially 

 the more recent ones. The fact that it is not abundant in the older volcanic 

 rocks is probably due to its ready alteration. It may have been present 

 originally. 



Leucite is not known as a constituent of the schists and gneisses 

 derived from the sediments. Leucite has been produced by pyro-chemical 

 methods from analcite and potassium chloride. This is a reversal of the 

 reaction in the case of change of leucite to analcite. 



Alterations. — Leucite frequently alters to analcite (isometric; sp. gr. 2.22- 

 2.29); to a mixture of orthoclase (monoclinic; sp. gr. 2.53-2.6) and kaolinite 

 (monoclinic; sp. gr. 2.6-2.63); to a mixture of orthoclase and muscovite 

 (monoclinic; sp. gr. 2.76-3.0); and to a mixture of orthoclase and nephe- 

 lite (hexagonal; sp. gr. 2.55-2.65). 



The change of leucite to analcite requires a substitution of sodium for 

 potassium; hence sodium carbonate or some other sodium compound must 

 be supposed to be present. Supposing sodium carbonate to be the com- 

 pound, the reaction is: 



(1) 2KAlSi 2 6 +Na,C0 3 +2H 2 0=:Na 2 Al 2 SiA 2 .2H 2 0+K 2 C0 3 +k. 



Ignoring the carbonates, the increase in volume is 10.74 per cent. 



The passage of leucite into orthoclase and kaolin is as follows, sup- 

 posing the freed potassium to unite with carbon dioxide: 



(2) 4KAlSi,0 G +C0 2 H-2H 2 0=2KA.lSi 3 O s +H. l Al 2 Si 2 9 +K 2 C0 3 +k. 



Supposing the potassium carbonate to be taken into solution, the decrease 

 in volume of the orthoclase as compared with the leucite is 38.57 per cent, 

 and the decrease of the orthoclase and kaolinite together as compared with 

 the leucite is 10.58 per cent. 



In a similar way the passage of leucite into orthoclase and muscovite 

 is as follows: 



(3) 6KAlSi 2 6 +C0 2 +H 2 0=3KAlSi 3 8 +KH 2 Al 3 Si 3 12 +K 2 C0 3 +k. 



