OLIGOCLASE (AB 4 ANi). 95 



position of extinction. The interference figure in convergent light 

 shows a bisectrix normal to the plane of the bladed crystal bundle, 

 the bisectrix being the direction of vibration of the ray having the 

 greatest index of refraction, ;-. The angle between the optic axes is 

 larger, and the plane of the optic axes lies in the longer diameter of the 

 crystal blade. This is the optical orientation of Ab 4 Ani* when the 

 feldspar plate is tabular parallel to the second pinacoid (oio), the 

 common case. 



The angles between the crystal edges of the thin plates, the position 

 of the plane of optic axes, and the generally low extinction angles ex- 

 hibited by longitudinal cross-sections of the bundles show that the 

 feldspar crystals are tabular parallel to (oio) and elongated parallel 

 to the crystal axis a, and are probably bounded by the planes (201) 

 and (304) or by (201) and (203). 



* Michel-Levy, A., op. cit., Plate in. 



