562 Washington — Constitution of Some Salic Silicates. 



and (CaAl) respectively replacing isomorphously the five basic 

 hydrogen atoms of the aluinosilicic acid H,AlSi a O B , in accord- 

 ance with the third method mentioned' above. They are, 

 therefore, in conformity with the doctrine of isomorphism and 

 in harmony with the physical isomorphism of the feldspars. 

 These formulas do not seem to have been generally adopted or 

 even much mentioned, though Miers,* speaking of albite and 

 anorthite being respectively salts of polysilicic and ortho- 

 silicic acids, says : " But here again the close relationship 

 which undoubtedly exists between albite and anorthite finds no 

 expression. If one is to regard them as similar salts, the 

 radicle SiNa of albite must be replaced by the radicle AICa 

 of anorthite, so that Al plays the part of an acid." It will be 

 observed that these formulas are essentially those of Streng. 



Nothing in theoretical mineralogy is more certain than that 

 the sodi-calcic feldspars form a typically isomorphous series 

 and that the empirical compositions of the end members, 

 albite and anorthite, are well ascertained and are represented 

 by NaAH3i 3 6 and CaAl.,Si„O s . What is uncertain is the 

 arrangement and number of atoms in the molecule, that is the 

 constitutional formula, and of what acid or acids they are salts ; 

 though their physically perfect isomorphous relations should 

 leave no room for doubt that their formulas must be of identi- 

 cal type and that they are salts of the same mineral acid. 



Apart from the fact that minerals are known which are, or 

 can be interpreted to be, salts of trisilicic and orthosilicic acids, 

 H 4 Si 3 6 and H 4 Si0 4 , we seem to have no real ground for think- 

 ing that albite and anorthite must be interpreted respectively 

 as such salts, in accordance with the usually accepted Tscher- 

 mak's formulas. On the contrary, the now well-established 

 general facts and laws of isomorphism are decidedly against 

 this interpretation, as they are against others of the same 

 mineralogist's end member formulas which are almost univer- 

 sally accepted. Without wishing to detract from, and with 

 full recognition of, the brilliant and invaluable contributions 

 of Tschermak to the science of mineralogy, it may be said that 

 mineralogists have been brought up to accept this view of the 

 feldspar molecules, partly because of the great authority of its 

 originator, partly because it is simple and certainly can express 

 the composition of nearly all feldspars, partly because there is 

 as yet no experimental verification or disproof, and largely 

 because the seriousness of the deviation from the principles of 

 isomorphism is not duly appreciated. 



If we accept Streng's and Groth's suggestion of the pres- 

 ence of the totally quinquivalent integrals (R'Si) v and (R,"A1) V 3 

 with the decivalent integral (CaSi 2 ) x in some cases, it will be 



* H. Miers, Mineralogy, 1902, p. 209. 



