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



tions, and which seems to be fully justifiable from all points of 

 view. The acid radicals Si0 4 and Si 3 9 are also regarded as 

 mutually replaceable, the one by the other ; a belief which 

 may now be looked upon as fairly well established by the evi- 

 dence of the feldspar and scapolite groups, as studied by 

 Tschermak, and of the mica group as interpreted by myself. 

 These two fundamental conceptions are the foundation stones 

 upon which I have tried to build. 



In a paper upon the chemical structure of the natural sili- 

 cates,"* the substitution theory above outlined was applied to 

 the feldspars, although the formulae were not written out in 

 full. Albite, the soda compound, AlXaSi 3 8 , was regarded as 

 a first substitution derivative of the unknown salt Al 4 (Si 8 8 ) 3 , 

 and anorthite as the corresponding orthosilicate containing 

 lime. In each case the simplest possible formula is tripled, 

 giving the subjoined expressions representing the two feld- 

 spathic salts : 



/Si 3 0,^Xa 3 /Si0 4 ^Al 



Al-Si 3 (X = Al Al-Si0 4 = Al 



\Si 3 8 = Al \SiO.= Ca 



I 

 Ca 



I 

 /Si0 4 = Ca 

 Al-Si0 4 =Al 

 \Si0 4 -Al 



From these formulae the formulae of many zeolites are 

 easily derived ; providing we assume that the soda salt may be 

 replaced by its equivalent ortho-compound, and the calcium 

 salt by the corresponding trisilicate. The sodium ortho salt 

 would be isomeric with nepheline ; but the calcium compound 

 containing Si 3 O s is purely hypothetical. Upon this basis, if 

 we temporarily regard all water in the zeolites as water of 

 crystallization, and represent the groups Si0 4 and Si 3 8 by the 

 general symbol X, the greater number of the minerals under 

 consideration fall easily into two groups, having the following 

 generalized formulas : 



i i A1 6 X 6 R' 6 , n. aq., + 

 L ' J A1 3 X 3 R' 3) n. aq. 



A1 4 X 6 R' 12 , n. aq., + 

 A1„X 3 R' 6 , n. aq. 



For the first group, with varying amounts of water, we have 

 thomsonite, gismondite, edingtonite, phillipsite, harmotome, 

 chabazite, levynite, gtnelinite, hydronephelite, and offretite. 



*TT. S. Geological Survey, Bulletin No. 60. 18ST-88. 



