HYDROUS SILICATES— ZEOLITE SECTION. 297 



II. ZEOLITE SECTION. 



The species of the Zeolite Section have beer; described as 

 having some relation to the feldspars in constitution. In 

 the feldspars, as explained on page 273, the following ratios, 

 for the protoxides, alumina, and silica which analyses af- 

 ford, occur: 1:3:4,1:3:6,1:3:8,1:3:9,1:3:10, 1:3:12. 

 So, among the zeolites, if the water be left out of considera- 

 tion, these are the ratios: 1:3:4 (in Thomsonite), 1:3:6 

 (Xatrolite, Scolecite, etc.), 1:3:8 (Analcite, Chabazite, 

 etc.), 1:3:10 (Harmotome), 1:3:12 (Stilbite, Heulandite, 

 etc.). This fact, added to the absence or nearly total ab- 

 sence of magnesium and iron, and presence instead of Na 2 , 

 K. 2 , Ca, Ba, make out a distinct relation to the feldspars, 

 whatever may be the part which the water sustains in the 

 compounds. Besides barium, strontium is sometimes pres- 

 ent, an element not yet known to characterize a species of 

 feldspar. 



These minerals were called zeolites because they generally 

 fuse easily with intumescence before the blowpipe, the term 

 being derived from the Greek zeo, to boil. Among those 

 described beyond, Heulandite and Stilbite, have a strong 

 pearly cleavage, and the latter is often in pearly radiations ; 

 Xatrolite, Scolecite, are fibrous and radiated, or in very 

 slender prisms ; Thomsonite occurs either radiated, or com- 

 pact, or in short crystals ; while Harmotome, Analcite, and 

 Chabazite, and the related Gmelinite, occur only in short 

 or stout glassy crystals, those of chabazite looking some- 

 times like cubes. 



The zeolites are sometimes called trap minerals, because 

 they are often found in the cavities or fissures of amygda- 

 loidal trap as well as related basic eruptive rocks. Yet 

 they occur also occasionally in fissures or cavities in gneiss, 

 granite, and other metamorphic rocks. They are not the 

 original minerals of any of these rocks ; but the results of 

 alteration of portions of them near the little cavities or fis- 



