292 A TREATISE ON METAMORPHISM. 



NEPHELITE GROUP. 

 XEPHELITE AJiD CANCEIMTE. 



The nephelite group includes — 



Nephelite: 



NaAlSi0 4 . 



Hexagonal. 



Sp. gr. 2.55-2.65. 



Cancriniie : 



H B Na 6 Ca (NaC0 3 ) 2 Al,Si 9 S6 - 



Hexagonal. 



Sp. gr. 2.42-2.50. 



NEPHELITE. 



occurrence. — Nephelite is a sodium-aluminum silicate. Commonly the 

 sodium is in part replaced by potassium. Nephelite occurs in both ancient 

 and modern igneous rocks, both surface and deep seated. It is abund- 

 ant in the syenite-schists and syenite-gneisses of certain localities, but 

 is not known in the metamorphosed secondary rocks. This is doubtless 

 due to its ready alteration. Nephelite has been produced artificially at 

 220° C. by a reaction between kaolinite and an alkaline carbonate. As a 

 secondary product nephelite forms from leucite, but this alteration is not 

 an important source of the mineral. Nephelite is aho probably derived 

 from sodalite. 



Alterations. — The most frequently observed alteration of nephelite is to 

 the zeolites, and especially to hyclronephelite (hexagonal; sp. gr. 2.263), 

 natrolite (orthorhombic; sp. gr. 2.20-2.25), thomsonite (orthorhombic; sp. 

 gr. 2.3-2.4), and analcite (isometric; sp. gr. 2.22-2.29). Simultaneously 

 with the formation of some of the zeolites diaspore (orthorhombic; sp. gr. 

 3.3-3.5), or gibbsite (monoclinic ; sp. gr. 2.35), or kaolinite (monoclinic; sp. 

 gr. 2.615), or some combination of these, is frequently formed. 



The reaction for hyclronephelite is — 



(1) 6NaAlSi0 4 +7H 2 0+C0 2 =2(HNa 2 Al 3 Si 3 12 .3H 2 0)+Na 2 C0 3 -|-k. 



The increase in volume is 23.49 per cent. 



The alteration next in importance is to natrolite and gibbsite, or to 

 natrolite and diaspore. The reaction in the former case is: 



(2) 6NaAlSi0 4 +7H 2 0+C0 2 =2Na. 2 Al 2 H 4 Si 3 12 -)-2Al (OH) 3 +Na 2 C0 3 +k. 



