GUIDE TO THE MINERALOGIC. COLLECTIONS 



109 



Heulandite H 2 Ca Al 2 ( Si0 ;! ) ,. ! 3H 2 

 Heulandite is a hydrous calcium and aluminum silicate. 

 It occurs in monoclinic crystals, somewhat coffinlike in shape 

 with marked cleavage parallel to the clinopinaeoid and a pearly 

 Juster on the clinopinaeoid and cleavage surfaces. The crystals 

 are sometimes joined in parallel position, giving ridgelike forms 

 of the general coffinlike section. The luster is pearly to vitreous 

 and the color white, red or brown. 



Heulandite 1 occurs with the other zeolites in basaltic rocks and 

 gneiss. For localities see apophyllite. 



Stilbite (desmine) H 4 (Na 2 .Ca)Al 2 (SiO :! ) G +4H 2 

 Stilbite is a hydrous sodium, calcium and aluminium silicate. 

 It occurs in monoclinic crystals resembling those of heulandite 

 but usually more tabular parallel to the clinopinaeoid and with 

 a more strongly marked tendency to form aggregates which are 

 sheaf like (pi. 34^), globular or radiated in form. The broad faces 

 of the tabular crystals show a pearly luster, otherwise the luster 

 is vitreous; the color is white, red, brown or yellow. 



Stilbite occurs in the formations and localities common to the 

 zeolites, for which see apophyllite. 



Chabazite (Ca,Na 2 ) Al 2 (SiO,) 4 +6H 2 



Chabazite is a hydrous calcium, sodium and aluminium sili- 

 cate. 



It occurs in rhombohedral crystals with nearly square faces, 

 which give them somewhat the aspect of cubes. These faces, 

 however, are commonly striated parallel to the edges and are 

 often broken by the protuberance of an angle of the twinned 

 negative rhombohedron. The luster is vitreous and the color 

 white or flesh-red. 



Chabazite like the other zeolites is found generally in basaltic 

 rocks. It is abundant in several localities in Nova Scotia. 



Analcite NaAl(SiO :! ) 2 +H 2 



Analcite is a hydrous sodium and aluminium silicate. 



It occurs in isometric crystals, usually trapezohedrons (fig. 

 233). A variety from the Cyclopean islands near Sicily is cubic 

 in habit with small trapezohedral modifications. It is some- 

 times found in concentric groups about a single crystal as a 



