THE MINERALOGRAPHY OF THE FELDSPARS 2409 
they demonstrate, as no other method can, the nature of these 
minerals and the constants by means of which they are identified. 
Extinction Angles: Inasmuch as the extinction angles of 
the plagioclase and the potash-soda series constitute the most 
serviceable means of identification, it is important that some 
attempt be made to draw the isogonic lines of the orthoclase-albite- 
anorthite and the microcline-albite-anorthite systems for the two 
faces (o10) and (oor). This involves the use of four triangular 
diagrams, which are reproduced as diagrams 1 to 4 in Figure 12. 
When the extinction angle of natural specimen has been meas- 
ured an inspection of the proper diagram shows that considerable 
compositional range is indicated by its curve. The extinction 
angle, therefore, does not appear to constitute a conclusive identi- 
fication of composition. The common procedure is to ignore the 
least-abundant component and thus attempt to reduce the system 
_ toa simple binary. The writer is convinced, however, that this 
sacrifices considerable accuracy. It is much more accurate to 
assume the presence of a small amount of the third component 
rather than to ignore it altogether. As a result of examining 
nearly 1,300 analyses of feldspars the following empirical rule is 
offered: Albites contain an average of 6 per cent of the potash 
component; oligoclases, 8 per cent; andesines, 7 per cent; labra- 
dorites, 6 per cent; bytownites, 4 per cent; and anorthites, 3 per 
cent. Even though these figures are not constant (see “potash 
oligoclase’’ for example) yet the writer is convinced that if 
these are assumed the petrologist will be nearer to the truth 
than if the third component is ignored. A similar set of figures 
might be set up for the potash-soda side of the triangle. In soda 
orthoclase and soda microcline the percentage of the lime component 
is 20r3 percent. In potash albite and anorthoclases of hyperper- 
thitic composition the percentage of the lime component is greater, 
reaching in some instances as high a value as 15 or 18 per cent. 
This is suggested in diagram 5 in Figure 12 by the irregular line 
within the triangle. It represents the approximate average com- 
position of 954 natural feldspars actually recast and plotted. 
The point of intersection of this compositional line and of the 
measured extinction angle line represents the true composition of 
