SILICATES, TITANATES, ETC. 483 



Twinning plane the base, penetrating twins common; Cleavage, 

 be;i-\, c lessso; Brittle; Fracture, uneven ; II. =4.5; G. = 2.44- 

 2.50; Color, white, jiray, yellow, or brown ; Streak, white; Luster, 

 vitreous; Translucent to transparent; a = 1.503; y = 1.508; 

 y - a = .005 ; Optically ( + ) ; Bx. = b ; Bx A ft = 60 in 

 front. 



B.B. Whitens, crumbles, and fuses at 3.5 to a white, transpar- 

 ent ^lass. Yields water in the closed tube. Very little affected 

 with HC1 ; fused with soda and freed of silica and aluminium, the 

 filtrate yields a white precipitate with dilute sulphuric acid (ba- 

 rium). 



General description. Crystals are usually twinned in cross- 

 shaped, interpenetrating, or contact twins, in which the composition 

 face is the base, yielding a crystal orthorhombic in appearance, in 

 which the unit prism m appears as if a pyramid face, and the faces 

 b and c as if prisms. This habit of twinning is also characteristic 

 of other zeolites, as phillipsite and stilbite. 



The barium and strontium zeolites are rare and local in their 

 occurrence. In the United States harmotome occurs associated 

 with stilbite in places, in the gneiss of Manhattan Island, New 

 York. 



STILBITE 



Stilbite. (Nag . Ca) Al 2 Si 6 Oi 5 . 6H 2 O ; Monoclinic ; Type, Digonal 

 Equatorial ; ft : b : 6 = .7622 : 1 : 1.1940 ; p = 50 50' = 001 A 100 ; 

 100 A 110 = 30 35' ; 001 A 101 = 89 30' ; 001 A Oil = 42 47' ; Com- 

 mon forms, c (001), b (010), m (110), e (Oil) ; Twinning plane, the 

 base penetration twins ; Cleavage, basal perfect ; Brittle; Fracture, 

 uneven; H. = 3.5-4; G. = 2.1-2.2; Color, white, gray, yellow, 

 brown, or red ; Streak, white ; Luster, vitreous, b pearly ; 

 Transparent to translucent; a = 1.494; (3 = 1.498; -y = 1.500; 

 y -a = .006; Optically ( - ); Axial plane, _L 100; Bx. A c = 

 31 10' in front ; 2 V = 33. 



B.B. Exfoliates, curls, and fuses at 2-2.5 to a white enamel. 

 Yields water in the closed tube. Decomposes with HC1 without 

 gelatinizing. 



General description. Crystals flattened parallel to 010 and 

 joined by this face in complex bundles of parallel or divergent ag- 



