THE MINERALOGRAPHY OF THE FELDSPARS 365 



these are forthcoming the writer cannot offer diagrams which are as 

 accurate as desired. 



Kozu and Suzuki' report that the indices of a moonstone from 

 Korea (Orei.g Ab32.7 Ans.4) vary among the several specimens at hand 

 and even from place to place within the same crystal. To find 

 zonals in orthoclasic feldspars is to be expected but here the indices 

 may vary as much as .002 between two crystals from the same local- 

 ity. This indicates some of the difficulties in the way of a satisfac- 

 tory index of refraction diagram for the potash-soda-lime feldspars. 



EXTINCTION ANGLES 



Similar remarks apply to values of extinction angles. Here 

 again a peg model undoubtedly suggests two series. The plotting 

 of the data is given in Figure 6. 



In Part I (p. 229) it was stated that "as the amount of the sodium 

 component [in orthoclase] increases .... the (001) extinction angle 

 changes from zero to about three degrees." These potash-soda 

 feldspars were called monocKnic. Professor A. N. WinchelP has 

 called the attention of the writer to this obvious error. The extinc- 

 tion angles of strictly monoclinic minerals measured on the trace of 

 the (010) face in cleavage fragments or sections parallel to (001) 

 must necessarily be zero. However, it may be said that the writer 

 employed the term "monoclinic" as a general distinctive name 

 for the orthoclase-barbierite series as contrasted with the triclinic 

 microcline-albite series. It was thought unwise to introduce a new 

 term. As a matter of fact, the statement that the angle varies from 

 zero to three degrees may mean that the specimens so plotted were 

 mixtures of the two series and hence the term triclinic should be 

 applied instead. If the (001) extinctions remain zero it may be that 

 it is because this specimen is a representative of pure orthoclase- 

 barbierite feldspar. Thus the microscopic determination of the 

 potash-soda series is compKcated by (i) the effect on the physical 

 properties by the presence of the lime component, (2) by the 

 presence of nephehte-carnegieite, (3) by the possible dimorphism 



^ S, Kozu and M. Suzuki, "Optical, Chemical and Thermal Properties of Moon- 

 stone from Korea," Sci. Repts. of Tohiiko Univ., Ser. Ill, Vol. I (No. i, 1921), p. 20. 

 ' Personal communication, May 28, 192 1. 



