230 HAROLD L. ALLING 
containing its maximum amount of the potash component, but 
recourse to indices of refraction furnishes a fairly accurate means. — 
The optical mineralogy of this system should be studied in con- 
junction with the chemical analyses, which for convenience should 
be recast in terms of the three components. 
It is chiefly from the examination of some sixty specimens of 
natural potash-soda feldspars that the writer offers the diagrams 
(shown in Fig. 7) of the extinction angles of the potash-soda 
series. They are tentative and suggestive, nothing more. The 
dimorphism of the sodium component is not represented; for no 
information is at hand. Consequently barbierite has been left out 
of consideration. 
Some of the uncertainty about the accuracy of the curves is 
because of the degree to which the lime component affects the 
extinctions. All of the specimens examined contained anorthite 
in solution up to 10 or 15 per cent of the whole and the problem 
was to correct the extinctions for the simple binary. The extinc- 
tion angles of the plagioclase series, together with those of the 
potash-soda feldspars, whose chemical composition was known," 
enabled the writer to plot equal extinction contours (isogonic lines) 
for that portion of the ternary system (K-, Na-, and Ca-feldspars) 
represented by natural specimens. These in turn made it possible 
to draw the extinction curves for the pure binary system. 
Classification of the Potash-Soda Series.—Most mineralogists 
recognize that orthoclase usually contains an appreciable amount 
of soda, and that albite carries some potash. Winchell,? for ex- 
ample, gives orthoclase, KAISi,Os; soda orthoclase, (K,Na)AISi,Og; 
albite, NaAlSi,0s; and anorthoclase, (Na,K)AISi,Os. While these 
expressions in general convey fairly definite meanings, it is well 
to remember that there is no definite ratio between the K and Na 
in homogeneous crystals which are members of the series, and 
that no single chemical formula can be given for them. Such 
feldspars with the potash and soda components in appreciable 
amounts should be considered as solid solutions limited to certain 
t Dana, System of Mineralogy, sixth ed. (1892), p. 324. 
2N. H. and A. N. Winchell, Optical Mineralogy (1909), p. 210. 
