HYDROUS SILICATES — SEOLITE SECTION. 303 



Composition. CaAl 16 Si 6 + b' aq— Silica 57*4, alumina 16*5, 

 lime 8-9, water 17-2=100; but with a little Na 2 or K 2 in 

 place of part of the Ca. B.B. exfoliates, swells up, and 

 curves into fan-like forms, and fuses to a white enamel. 

 Decomposed by hydrochloric acid without gelatinizing. 



Diff. It cannot be scratched with the thumb-nail, like 

 gypsum. It is distinguished from heulandite by its crys- 

 tals. 



Obs. Occurs mostly in trap-rocks ; also on gneiss and 

 granite. Found on the Faroe Ids. ; Isle of Skye ; Isle of Ar- 

 ran, and elsewhere, Scotland ; Andreasberg, Hartz ; the Ven- 

 dayah Mts., Hindostan. Found sparingly at the Chester 

 and Charlestown syenite quarries, Mass.; at New Haven, 

 Thatchersville and Iladlyme, Conn., and other points in the 

 Connecticut Valley trap ; at Phillipstown, N. Y.; at Bergen 

 Hill, N". J.; in trap, in the copper region of Lake Superior ; 

 in beautiful crystallizations at various points in Nova 

 Scotia. 



The name stilbite is derived from the Greek stilbe lustre. 

 It has also been called desmine, and in Germany heulandite, 

 where heulandite has been called stilbite. 



Heulandite. 



Monoclinic. In right rhomboidal prisms like the figure, 

 with perfect pearly cleavage parallel to P and other 

 planes vitreous in lustre. P on M or T=90° ; M on T 

 = 129° 40'. Color white ; sometimes reddish, gray, 

 brown. Transparent to subtr°nsl cent. Folia brit- 

 tle. H. =3-5-4. G. =2-17- «-2. 



Composition. CaA10 16 Si 6 + 5 aq = Silica 59*1, alu- 

 mina 16*9, lime 9 -22, water 14*8 = 100. Contains 

 1 to 2 per cent, of JS T a 2 or K 2 in place of part of the 

 Ca. Blowpipe characters like those of stilbite. In- 

 tumesces and fuses, and becomes phosphorescent. Dis- 

 solves in acid without gelatinizing. 



Biff. The very pearly lustre of the cleavage face is a 

 marked characteristic. Distinguished from gypsum by its 

 hardness ; from apophyllite and stilbite by its crystals ; and 

 from the latter species also in not occurring in radiated 

 crystallizations. 



Obs. Found in amygdaloidal cavities and fissures in 

 trap ; occasionally in gneiss, and in some metalliferous 



