OCCURRENCE AND ALTERATIONS OF SODALITE. 295 



The increase in volume of the natrolite, gibbsite, and calcite (rhombo- 

 hedral; sp. gr., 2.7135) as compared with cancrinite is 8.64 per cent. 

 The reaction is that of hydration and breaking- up of a complex com- 

 pound into several simpler compounds requiring greater volume, and is 

 therefore typical of the zone of katamorphism. 



SODALITE GROUP. 

 SODALITE, HAiJYMTE, AXD AOSELITE. 



The sodalite group includes — 



Sodalite: 



NaC1.3NaAlSi0 4 . 



Isometric. 



Sp. gr., 2.14-2.30. 



Hauynite: 



Na 2 Ca(NaS0 4 . Al) Al 2 Si 3 12 . 



Isometric. 



Sp. gr., 2.4-2.5. 



Noselite: 



Na 4 (]SraS0 4 .Al)Al 2 Si30 12 . 



Isometric. 



Sp. gr., 2.25-2.4. 



SODALITE. 



occurrence. — Sodalite is sodium aluminum silicate with some chloride. 

 Sodalite occurs as an original constituent in the igneous .rocks, both surface 

 and deep seated. It is not known in the secondaiy rocks or their metamor- 

 phosed equivalents. In fact, the occurrence of sodalite is almost identical 

 with that of nephelite, which mineral is one of its sources. 



Alterations. — The alteration products of sodalite are also identical with 

 those of nephelite, except that nephelite passes into sodalite, and the 

 reverse reaction is not recorded, although, as noted below, it is believed to 

 occur. The alterations of sodalite into minerals similar to those into 

 which nephelite alters is natural, as sodalite is made up of the nephelite 

 molecule with the addition of sodium chloride. 



Sodalite alters to the same zeolites as does nephelite, viz, to hydro- 

 nephelite (hexagonal; sp. gr., 2.263), natrolite (orthorhombic ; sp. gr., 2.2— 

 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 



