396 



0. Andersen — Aventurine Feldspar. 

 Table 12 



Microcline ; n = 1" 523 





Measured 



P 



<P 



Form 



i r 



Calculated 



from 



measurements 



Calculated 



from 

 axial ratios 



Measured 



Calculated 



from 

 axial ratios 





Pole 



: Normal to 001 ( p P and ^ P ) 











47° 40' 



19° 20' 



20° 47' 



20° 57' 



56° 



19' 





112 



50 17 



18 14 



21 5 



56 



42 



56° 38' 





51 24 



16 34 



20 50 



56 



55 







Pole 



: Normal to 010 ( p M and M 









150 



46 21 



14 5 



18 47 



18 41 



64 



20 



63 57 



52 5 



7 59 



18 13 



63 



50 



The form (110) (embracing 110 and 110) was established by 

 the microscopic measurements : $ P = 56 - 2° ; <£ M = 64'3°. 



The edges of the lamellae after (001) were sometimes parallel 

 to the «-axis and also occasionally parallel to the J-axis. More 

 frequently, however, they formed oblique angles with the 

 a-axis. Angles of 40° and 70° between the a-axis and the 

 elongated edges of lamellae (or the outlines of the groups of 

 lamellae described) were measured. Lamellae after (010) were 

 often oriented with one of the edges perpendicular to the a-axis 

 more seldom forming +50° with the o-axis. The lamellae after 

 (112), (112), (150) and (150) were generally so small and 

 irregular that no definite measurements of the orientation of 

 their edges could be made. 



Miscellaneous Occurrences. 



Besides the varieties described in the preceding pages, a 

 number of other specimens were examined more superficially.* 

 All these specimens showed a very weak aventurization and 

 the measurements were only approximate. The results for all 

 were that the aventurization on (001) was due to lamellae 



* In the literature we find mentioned a considerable number of occur- 

 rences besides those described or referred to in tbis paper. No actual 

 descriptions of these aventurine feldspars have been given, however, and it 

 is therefore of minor interest to list the references, most of which can be 

 easily found in standard handbooks of mineralogy or treatises on gems. See 

 for instance : Max Bauer, Edelsteinskunde ; G. F. Kunz, Gems and Precious 

 Stones of North America. 



