Washington — Constitution of Some Salic Silicates. 563 



found, as is here shown, that the constitution of all the members 

 of the feldspar, leucite, nephelite, and seapolite groups, and 

 nearly all the zeolites, can be interpreted in such a way as 

 readily to explain their composition, mutual relations, poly- 

 morphism, isomorphism, resistance to acids, and the relation 

 to the mutually common end alteration product kaolinite. 



On this interpretation the feldspars, lenads, and most zeo- 

 lites are regarded as salts, or isomorphous mixtures of salts, of 

 an alumino-silicic acid, H 5 AlSi 2 8 , or most probably a poly- 

 mer of this, the five hydrogen atoms being isomorphously 

 replaced by the integrals (R v Si) v , (R"A1) V , and (R"Si 2 ) x , R' 

 being Iv, Na, and Li, and R" being Na 2 , K„, Ca, and rarely Ba 

 and Sr. If one regards all the aluminum as basal this reduces 

 to the general formula of an orthosilicic acid, (H 4 Si0 4 ) n , but it 

 must be noted that it is now becoming generally recognized, in 

 accordance with the effects of mass action of complex acid 

 radicals, as shown by Pentield and by Groth, that aluminum and 

 often ferric iron function, at least in part, in the acid portion 

 of the mineral salt. 



The scapolites, on the other hand, while containing the same 

 integrals, which explains their relations to the feldspars, etc., 

 are interpreted as salts of a different alumino-silicic acid, 

 H 10 AlSi a O 12 , the hydrogens being entirely replaced by the radi- 

 cals (NaSi) 2 and (CaAl) 2 . Some modification of this most 

 simple statement may be necessary to explain the presence of 

 CI and S0 3 , as will be shown later. 



It must be supposed that the salts of the fellenadic acid 

 (H 6 AlSi 2 8 ) n are polymorphous, in general trimorphous, and 

 this acid may be called for convenience fellenadic acid. The 

 salts of the scapolitic acid seem, on the other hand, to belong 

 only to the tetragonal system, though dimorphism may possi- 

 bly be discovered in the future. It is noteworthy that, corre- 

 sponding to the difference in acidic type, tetragonal symmetry 

 does not seem to be represented among the feldspars or lenads, 

 and only rarely among the zeolites. • 



This interpretation of the feldspars, lenads, etc., somewhat 

 resembles that of F. W. Clarke,* who considers Si 3 8 and Si0 4 

 to be "mutually equivalent " and that many minerals which 

 are apparently orthosilicates or metasilicates are pseudo-com- 

 pounds or mixtures of the two acid radicals. The general cor- 

 respondence may be expressed by saying that where Clarke 

 assumes the presence of Si 3 O e (trisilicate) the suggested hypothe- 

 sis assumes that of (R'Si), and where he assumes SiO„ (ortho- 

 silicate) this assumes (R"A1). Clarke also, in common with 

 Groth, Rammelsberg, Tschermak, and many others, considers 

 the minerals under discussion as silicates, with the aluminum 

 *Bull. U. S. Geol. Surv., No. 125, 1895. 



