358 0. Andersen — Aventurine Feldspar. 



p. As will be shown further on, however, all the lamellae that 

 cause aventurization on (001) have practically the same angle 

 p P (about 21°) and those producing aventurization on (010) 

 have an angle p M only a little smaller (around 19°). The differ- 

 ences in the angles * and r will therefore be small. Fig. 2 

 may then be considered a fair representation of the general 

 relations between the angles i and r in aventurine feldspars 

 where the aventurization is observed on the cleavage faces 



Fig. 3. 



--i \'j 



(001) and (010). The incident rays are marked with small 

 letters and the corresponding reflected rays with the same let- 

 ters distinguished by prime signs. It is seen that rays a p of 

 angle i = 90° are reflected in the direction p a' at a very small 

 angle r. For an angle p of 21° all the incident and reflected 

 rays fall on the same side of the face normal jt? ?i. For smaller 

 angles p they may, in part, fall on opposite sides. The direc- 

 tion e p represents the ray which after the refraction at the 

 surface A B will coincide with the normal to the reflecting 

 lamellae. Incident rays along the line e p will, therefore, 

 emerge along the same line, that is, the angles i and r for these 

 rays are equal and e p represents, in a way, the axis of incidence 

 for the whole reflecting system feldspar-lamellae. Rays falling 

 on the surface A B in any direction between p a' and p g will 

 not pass out through the same surface after the reflection from 

 the lamellae, because they will be totally reflected at the surface. 



