VOLUME XXXI NUMBER 5 



THE 



JOURNAL OF GEOLOGY 



'July- August IQ2J 



THE MINERALOGRAPHY OF THE FELDSPARS 

 PART 11— {Concluded) 



HAROLD L. ALLING 



University of Rochester, Rochester, New York 



THE SODA-LIME SERIES 



The problem of the isomorphism of the plagioclase feldspars has 

 not been fully understood. That they are a series of solid solutions 

 is a matter upon which there is little disagreement, but how can 

 albite and anorthite be isomorphous? On this question Wherry^ 

 has reached the conclusion that it is a case of atomic isomorphism, 

 with one aluminum atom occupying a sort of nuclear position in 

 both end-members; the second aluminum atom of anorthite being 

 definitely a replacement of one silicon atom of albite. This is indi- 

 cated by the formulas: NaAl(Si308) and CaAlCAlSi^Og).^ 



^E. T. Wherry, "The Plagioclase Feldspars as a (Case of Atomic Isomorphism," 

 Amer. Min., Vol. VII, No. 7 (July, 1922), 1 13-21. 



^ The Asches in The Silicates in Chemistry and Commerce, p. 295, say: "Tschermak 

 assumed that the [plagioclase] feldspars were isomorphous mixtures of two silicates — 

 albite and anorthite — to which he gave the following formulas: Albite NaAlSiSijOg; 

 Anorthite CaAlAlSi208." If this is correct, the idea expressed by Wherry is not a new 

 one at all and the credit should be given to Tschermak. See E. T. Wherry, "Volume 

 Isomorphism in the Silicates." Amer. Min., Vol. VIII (1923), pp. 2-3. F. Zambonini 

 (H. S. Washington), "The Isomorphism of Albite and Anorthite," Amer. Min., Vol. 

 VIII (1923), pp. 81-85. T. Sterry Hunt, Amer. Jour. Sci., Vol. XVIII (1854), p. 270; 

 Phil. Mag., Vol. IX (1855), p. 354- 



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