382 0. Andersen — Aventurine Feldspar. 



Table 2 



Ab 92 An 8 ; n = 1'535 



Form 



Measured. 



Calculated 



from 



measurements 



Calculated 



from 

 axial ratios 



Measured 



Calculated 



from 

 axial ratios 







Pole : Normal to 001 ( p P and P ) 









112 



62° 18' 



10° 21' 



20° 58' 



20° 39' 



58° 30' 



59° 



0' 



62 50 



10 10 



21 



57 22 



112 



62 43 



12 14 



21 40 



21 49 



56 30 



56 



3 



62 52 



12 22 



21 43 



57 



021 



'66 52 



27 24 



46 23 



46 49 



















Pole: 



Normal to 010 



(p M and M ) 







150 



44 54 



17 15 



19 16 



19 24 



63 24 



63 26 



150 



45 7 



16 35 



19 6 



19 16 



64 22 



1 a = 93° 24'. 



forming +53° or +63° with the a-axis. They formed com- 

 paratively broad strips and penetrated a number of the twin- 

 ning lamellae of the feldspar. Lamellae after (021) were often 

 oriented with their elongated edges approximately parallel to 

 the a-axis. Most of the lamellae after (150) and (150) formed 

 narrow linear strips parallel to the c-axis ; the angle between 

 these strips and the a-axis was measured at about 63*5°. The 

 lamellae after (010) formed the larger flakes observed in sec- 

 tions after (010). 



Albitefrom near Media, Delaware County, Pennsylvania* 



The feldspar. — The cleavage pieces were fresh, transparent, 

 grayish or colorless. 



Extinction angle on (001) = + 1° 

 " " " (010) = + 16° 



/3 = 1-535 

 Composition : Ab 9l An 9 , albite. 



The feldsparconsisted of single individuals, sometimes with- 

 out any twinning but generally with very narrow twinning 

 lamellae after the albite law inserted at regular intervals. 

 * Specimens from U. S. National Museum, No. 79828. 



