192 F. W. Clarke — Constitution of the Zeolites. 



There still remain several zeolites to be considered, together 

 with some allied minerals, which seem to be compounds of a 

 different type from those represented in the preceding table. 

 Two of these zeolites, ptilolite and mordenite, I have already 

 , discussed in a former paper ;* and another, the more recently 

 discovered laubanite, appears to be related to them. In 

 ptilolite and mordenite we have salts of the bibasic acid, 

 H„Si„0 6 , while laubanite is a metasilicate of precisely similar 

 type. The two first named minerals shade into each other as 

 mixtures of certain fundamental salts, while laubanite is dis- 

 tinct. The formulae are as follows : 



Ptilolite and Mordenite. Laubanite. 



/ Si A\ Ca / Si A~ R ' / Si0 s\ C a 



Al-Si,0 6 /^ a Al-Si 2 B -R' Al-Si0 3 / ua 



\si Q 5 \si 2 6 ^Si0 3 



Al-Si„0 6 -H Al-Si 2 5 -H Al-SiO.,^p 

 \Si 2 5 -H \Si 2 5 -H \Si0^ a 



+ oH 2 +6H 2 +6H 2 



The relationship is so evident that it needs no farther explana- 

 tion. Laubanite can be so written as to throw it into the 

 normal series ; but this constitution is certainly preferable. 



Analcite, another zeolite, may be written li 4 !Na 2 Al 2 X 8 , with 

 \ of X = Si 3 8 . This would bring the mineral into the second 

 group of normal zeolites, and into close relationship with 

 natrolite. Analcite, however, is isometric, like the anhydrous 

 leucite ; and Lemberg has shown that by the prolonged action 

 of alkaline solutions, analcite and leucite are mutually con- 

 vertible the one into the other. Hence, the two species should 

 be regarded as closely akin, and of similar type, and their 

 simplest formulae are as follows : 



Leucite. Analcite. 



Alf Si °s Alf Si0 * +H„0 



\SiO s -K \Si0 3 -Na 2 



Both minerals, however, occur altered into feldspars ; and it 

 is an open question whether their formulas should not be 

 polymers of those given above, in order that the alterability 

 may become interpretable. The same question arises with 

 spodumene, a compound of similar formula, which splits up 

 easily into albite and eucryptite. These points I shall con- 

 sider elsewhere. 



Three other minerals, which are sometimes classed as zeolites, 

 but which contain no aluminum, are okenite, gyrolite, and 

 apophyllite. These species are undoubtedly related ; for 



* This Journal, August, 1892. 



