HYDROUS SILICATES—SEOLITE SECTION. 303 
Composition. CaAl O.,Si,+6 aq=Silica 57-4, alumina 16°5, 
lime 8:9, water 17°2=100; but with a httle Na,or K,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 
eranite. 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 Hadlyme, 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 sfi/bé 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°; Mon.) 
= 129° 40’. Color white ; sometimes reddish, eray, 
brown. ‘Transparent to subtranslucent. Folia brit- | \ 3 
tle. H.=3°5 S28. Be 
Composition. CaAlO,,Sig+5 aq=Silica 59:1, alv- 
‘mina 16°9, lime 9°22, water 14°8=100. Contains 
1 to 2 per cent. of Na, or K, 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. 
Diff. ‘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 

