HYDROUS SILICATES — ZEOLITE SECTION. 



299 



TPf*f a 



The species was named after Dr. Thomas Thomson, of 

 Glasgow. 



Natrolite. 



Trimetric. In slender prisms, terminated by a short pyra 

 mid; iAi=:91 ; IaL over x =143° 20'. Also 

 in globular, stellated, and divergent groups, 

 consisting of delicate acicular fibres, the 

 fibres often terminating in acicular prismatic 

 crystals. 



Color white, or inclining to yellow, gray, 

 or red. Lustre vitreous. Transparent to 

 translucent. H.— 5-5*5. Gr. = 2'17-2 2o. 

 Brittle. 



Composition. Na 2 Al do Si 3 + 2 aq = Silica 

 47*29, alumina 26-06, soda 16*30, water 

 9*45 = 100. B.B. fuses easily and quietly to a clear glass ; 

 a fine splinter melts in a candle flame. Decomposed by hy- 

 drochloric acid, and the solution gelatinizes on evaporation. 



Diff. Distinguished from scolecite by its quiet fusion. 



Obs. Found in amygdaloidal trap, basalt and volcanic 

 rocks ; sometimes in seams in granitic rocks. The name 

 natrolite is from natron, soda. 



Occurs in Bohemia; Au vergne; Fassathal, Tyrol; at Glen 

 Farg in Fifeshire ; in Dumbartonshire ; Nova Scotia ; Ber- 

 gen Hill, X. J. 



Scolecite. Resembles natrolite, and differs in containing lime in place 

 of soda; also in having its slender rhombic glassy prisms longitudi- 

 nally twinned, as is shown by the meeting of two ranges of striaB at an 

 angle along or near the central line of opposite prismatic planes. The 

 lustre is vitreous or a little pearly. B.B. it curls up like a worm 

 (whence the name from the Greek skolex, a worm) and then melts. 

 From Staff a, Iceland, Finland, Hindostan. 



Mesolite. Another related species. 



Dimetric or Trimetric. 



Analcite. 



Occurs usually in trapezohedron 

 (fig. 1, also fig. 2). 



The appearance sometimes 

 seen in polarized light is 

 shown in figure 7, page 69 

 On account of this peculiar 

 behavior and indications of 

 a compound structure ob- 

 tained in a microscopic study 



