374 0. Andersen — Aventurine Feldspar. 



section may not be in a favorable position for showing inter- 

 ference colors and when the feldspar is in the most favorable 

 position the vibration directions of the " nicols '* may form 

 only a small angle with each other. Remembering this the 

 behavior of lamellae of large angles p on rotating the micro- 

 scope stage presents no difficulty. 



Double Refraction. 



Lamella? parallel to (or forming small angles p with) the sec- 

 tions extinguished simultaneously with the feldspar between 

 crossed nicols. Lamellae of large angles p, on the other hand, 

 remained light when the feldspar extinguished and thus proved 

 to be anisotropic. This is the relation to be expected if we 

 consider the lamellae uniaxial crystals in plates after the base. 

 Attempts to discover axial figures in the isotropic lamellae 

 failed, but this is not surprising when we remember the ex- 

 treme thinness of the lamellae, most of which were actually no 

 thicker than about 1/100 of the thinnest rock sections (see 

 p. 370), and also consider the disturbing influence of the bire- 

 fringent feldspar in which the lamellae were enclosed. 



The course of the rays through lamellae showing double 

 refraction' is seen in fig. 12 (a p h op'" a'"). The rays are 

 never transmitted through the lamellae in a direction near the 

 plane of the lamellae. If the lamellae are considered to be 

 hematite or other uniaxial crystals in basal plates, we see that 

 the transmitted rays will never have the vibration direction e. 

 Consequently we shall in no case obtain the maximum double 

 refraction and the path difference of the transmitted rays will 

 depend on the thickness of the lamellae and on their angle with 

 the section (angle p). 



Chemical Tests. 



Scheerer* found that the originally red powder of sunstone 

 from Tvedestrand turned white on heating with hydrochloric 

 acid, the filtrate containing iron oxide. In the course of the 

 present study similar tests were made on samples of various 

 aventurine feldspars with the same result. Microscopic ex- 

 aminations of the powder showed that the red (pink) color was 

 due to the presence of the thin reflecting lamellae described 

 and the discoloring by treatment with HC1 was due to the 

 solution of these lamellae. These tests, therefore, show that 

 the lamellae of the aventurine feldspars contain iron oxide. 



*Pogg. Ann!, lxiv, 160, 1845. 



