O. Andersen — Aventurine Feldspar. 371 



sections after (001) ) : (1) In vibration directions perpendicular 

 to the section lines of the lamellae — colors varying from yellow- 

 to strong brown red or blood red (depending on the thickness 

 of the lamellae). (2) In vibration directions parallel to the 

 section lines of the lamellae — colors dark brown of nearly the 

 same tinge in all lamellae (independent of their thickness). 

 The change is thus apparently stronger in the thinner than in the 

 thicker lamellae. There is no distinct change in the quality of 

 the colors but rather a change in the tints of the same brownish 

 color from dark in one direction to light in the other. 



This change in tints has been observed by previous authors 

 and explained as pleochroism.* As the reflecting lamellae have 

 been considered hematite in tabular crystals after the base a 

 strong pleochroism a> > e (co dark brown, e light brown) has 

 been generally adopted as one of the characteristic qualities of 

 hematite. A somewhat closer consideration of the observa- 

 tions described will, however, show that this explanation is not 

 correct. 



If the lamellae are hematite crystals in plates after the base, 

 those parallel to the surface of the feldspar section, or forming 

 small angles (p) with the same, must show the absorption color 

 of the vibration direction co. According to the observations 

 on such lamellae their absorption colors vary between yellow 

 and brown red. According to the observations on lamellae of 

 large angles p, on the other hand, the color of the vibration 

 direction co (direction parallel to the section lines of the 

 lamellae) should be dark brown with very little variation, 

 whereas the colors of e (or strictly a direction between e and co) 

 should vary between yellow and brown red. In other words 

 the absorption colors of co in the lamellae with small angles p 

 correspond to the colors of e in the lamellae with large angles/). 

 This indicates that the lamellae have only a weak pleochroism, 

 if any, and the dark absorption colors for vibration directions 

 parallel to the section lines of the lamellae of large angles p 

 must be explained in the manner outlined below. 



Fig. 12 shows the light rays passing through a cleavage piece 

 A B G D containing 'a lamella EFGH of angle p = 75°. 

 The light of the incident ray a p h is supposed to be unpolar- 

 ized. By the influence of the lamella the refracted and 

 reflected rays become polarizedf with the reflected ray h p' a' 



*F. Kinne, Neues Jahrb. Min., 1890, i, p. 183. 



f The phenomenon of polarization by reflection and refraction is too well- 

 known to need any explanation. It may only be recalled that the polariza- 

 tion, in the case of metallic substances like the lamella? here considered, is 

 never complete either in the reflected or in the refracted ray, but reaches 

 a maximum for a certain (always large) angle, the main angle of incidence, 

 and becomes insignificant for small angles of incidence. C. Forsterling 

 (Neues Jahrb. Min., B. B., xxv, 360, 1908) determined the main angle of 

 incidence for hematite at 71 "-73° for rays of medium wave lengths. 



