HYDROUS SILICATES — GENERAL SECTION. 293 



I. GENERAL SECTION. 

 Pectolite. 



Monoelinic, isomorphous with wollastonite. Usually in 

 aggregations of acicular crystals, or fibrous-massive, radiate, 

 stellate. 'Color white, or grayish. Translucent to opaque. 

 Tough. H.=5. a.=2'68-2-8. 



Composition. R 3 Si, in which R^^-Hi JNa 2 f- Ca, = Silica 

 54-2, lime 33-8, soda 9-3, water 2-7=100. In the closed 

 tube yields water. B.B. easily fusible. Decomposed by hydro- 

 chloric acid, and the solution gelatinizes somewhat when 

 evaporated. 



I)iff. Its fibrous forms and its blowpipe reactions are 

 distinctive. 



Obs. Occurs mostly in cavities or seams in trap or basic 

 eruptive rocks, and occasionally in other rocks. Found 

 at Ratho Quarry, near Edinburgh, Scotland ; at Kilsyth ; 

 Isle of Skye ; in the Tyrol ; in fine specimens at Bergen 

 Hill, N. J. ; a compact variety at Isle Royale, Lake Supe- 

 rior. 



Okenite and Gyrolite are related hydrous calcium silicates. Okenite 

 is from the Faroe Islands, Iceland, and Greenland, and gyrolite from 

 the Isle of Skye, and from Nova Scotia 25 miles southwest of Cape 

 Blomidon. 



Laumontite. 



Monoelinic, with the angles nearly of pyroxene ; I A 1= 

 86° 10'. Cleavage parallel to the clinodiagonal section and 

 to I perfect. Also massive, with a radiating or divergent 

 structure. 



Color white, passing into yellow or gray, sometimes red. 

 Lustre vitreous, inclining to pearly on the cleavage face. 

 Transparent to translucent. H. =3*5-4. G.= 2*25-2*36. 

 Becomes opaque on exposure through loss of water, and 

 readily crumbles. 



Composition. CaAl 12 Si 4 + 4 aq = Silica 50*0, alumina 

 21-8, lime 11-9, water 16'3 = 100. B.B. swells up and fuses 

 easily to a white enamel. Decomposed by hydrochloric acid, 

 and the solution gelatinizes on evaporation. 



L>iff. The alteration this species undergoes on exposure 

 to the air at once distinguishes it. This result may be 

 prevented with cabinet specimens, by dipping them into a 

 solution of gum arabic. 



