08 Washington and Wright — Feldspar from Linosa. 



The general tendency of this involved twinning is to produce 

 a hexagonal network of interpenetrating lamellse, but ordi- 

 narily one set predominates and the other sets appear only 

 dimly outlined in the background. 



As a result of this complicated twinning, single individual 

 grains suitable for optical work were rarely observed, and were 

 then too small for accurate measurements. Optic axial angle 

 determinations were influenced particularly by this condition, 

 and although much time was spent in searching for suitable 



Fig. 3. 



Fig. 3. Polysynthetic twinning on artificial soda anorthite. Nicols 

 crossed. Magnification 75 diameters. 



sections, the angular values obtained varied considerably. The 

 most probable value for 2V is about 36°±5°. The optic 

 The refractive indices were determined 



= l-520±-003. 

 several 



character is negative. 



by the immersion method : a = 1*516 ±'003, 7 

 The birefringence is weak and was measured 

 tions, the highest value for a-y being 0*0042. 



The specific gravity of soda anorthite was 

 the pycnometer method of Day and Allen, 

 2*571 at 25° was obtained. 



on 



sec- 



determined by 

 and the value 



