HAROLD L. ALLING 
232 
‘“soda orthoclase”’ is 
ranges of composition. In general the term 
clear in its meaning. 
Rogers' points out a danger in employing the term soda 
orthoclase in that it is somewhat ambiguous, for ‘‘soda-orthoclase 
may mean an orthoclase in which a portion of the potassium is 
replaced? by sodium or it may mean that sodium compound cor- 
responding to orthoclase.” For the latter it is better “to use a 
distinctive name for the monoclinic feldspar in which sodium pre- 
dominates molecularly? over potassium.” For such a mineral 
the term barbierite has been proposed. The writer fully appreci- 
ates the necessity for such distinctions, and maintains that soda 
orthoclase should mean an orthoclasic feldspar with some sodium 
component dissolved in it (Winchell’s [K,Na]AISi,Os) while 
sodium orthoclase should refer to barbierite. 
The limits of the range here proposed for soda orthoclase are 
K-feldspar 90, Na-feldspar 10, to K-feldspar 70, Na-feldspar 30. 
There exists considerable uncertainty regarding the composi- 
tion of anorthoclase. Rosenbusch* gives as the range of this 
mineral the following ratios: Na-feldspar 67, K-feldspar 33- 
Na-feldspar 82, K-feldspar 18. These ratios suggest that anor- 
thoclase is analogous to soda orthoclase, that is, it is a feldspar 
consisting chiefly of the sodium component with an appreciable 
amount of potash feldspar dissolved in it. Yet a study of the 
available chemical analyses of “‘anorthoclase’’ would suggest that 
these limits should be extended farther toward the potash side of 
the diagram, embracing in many cases the range where, under 
equilibrium conditions, perthite occurs. It is quite reasonable 
therefore to consider that many anorthoclases are undercooled 
metastable solid solutions of the two alkali components. A clear 
tA. F. Rogers, ‘‘The Nomenclature of Minerals,” Proc. Amer. Phil. Soc., LII 
(1913), 610. 
2 It is well to recall the objection to an expression of this kind. 
“Replaced” is unsatisfactory in a physical-chemical sense. 
Professor Johannsen informs me that he has used ‘‘proxied by” instead of 
“replaced by.” ‘There is no misunderstanding of that term. 
3 The application of X-rays to crystal structure has shown that molecules do not 
exist as such in solids. 
4 Rosenbusch-Iddings, Microscopic Physiography of the Rock-Forming Minerals, 
Pp. 340. 
