392 



O. Andersen — Aventurine Feldspar. 



specimens examined by me consisted of microcline perthite 

 made up of copper red microcline, and colorless albite. 

 The microcline showed an exceedingly fine cross hatching 

 invisible with low magnification and in places hardly de- 

 tectable even with a high magnifying power of the micro- 

 scope. The albite forms coarse inclusions extended approxi- 

 mately parallel to the c-axis but of very irregular cross sections. 



The hematite lamellce. — The reflecting lamellae were 

 restricted to the microcline, the albite never containing any. 

 They were very small, seldom more than - 05 mm in diameter^ 

 of rather regular outlines, hexagonal or rhomb-shaped, or some- 

 times forming linear strips. 



Orientation of the lamellae. — Thegoniometric measurements 

 are stated in Table 9. The following faces were planes of 

 orientation: (001), (010), (112), (112), (150), (150).* After 

 (112, (112), (150) and (150) there were numerous lamellae caus- 

 ing a distinct aventurization on the cleavage faces ; after (001) 

 and (010) there were very few. 



Table 9 



Microcline: n = l - 523 



Form 



Measured 



Calculated 



from 



measurements 



Calculated 



from 

 axial ratios 



Measured 



Calculated 



from 

 axial ratios 







Pole 



: Normal to 001 ( p P and <p F ) 





112 



49° 2' 



18° 50' 



20° 59' 



20° 57' 



56° 15' 



56° 38' 



54 22 



15 10 



21 4 



56 32 







Pole 



: Normal to 010 ( p M and M ) 





150 



40 39 



17 9 



18 15 



18 41 



64 27 



63 57 



56 11 



6 48 



18 46 



64 10 



Many of the lamellae after (112) and (112) were oriented 

 with the projection of their elongated edges on (001) approxi- 

 mately parallel to _the a-axis. The elongation of the lamellae 

 after (150) and (150) was often approximately parallel to the 

 c-axis. 



*For the sake of comparison with the plagioclase, the positive and nega- 

 tive faces of forms like (112), that is 112 and 112, are put down as distinct 

 forms, although their angles are referred to the monoclinic axes of ortho- 

 clase (see p. 364 and Table 1) and therefore are the same. 



