THE MINERALOGRAPHY OF THE FELDSPARS 231 
EXTINCTION ANGLES 9010 
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Fic. 7.—Extinction angles of the potash-soda series. The upper diagram shows 
values of the extinctions for the simple binaries, microcline, Mi; albite, Ab; and 
orthoclase, Or; albite, Ab, on the two faces, (oro) and (oor). The lower diagram 
shows the alkalic portions of the ternary diagram, K-, Na-, Ca-feldspars. The 
values of the extinction angles are shown by isogenic lines. Both diagrams are 
tentative. 
