366 



University of California Publications. 



[Geology 



The color and absorption on one hand, the strong dispersion 

 on the other hand, make the determination of the optic orienta- 

 tion and of the color of birefringence very difficult. The use of 

 the gypsum plate is to be recommended, but at the same time 

 the determination should be controlled by the mica plate, and 

 especially by the character of the figure in convergent light. 



The angle of extinction is difficult to determine in white light 

 because of the dispersion of the optic normal, besides the hori- 

 zontal dispersion of the bisectrix {bp:b v = 6°) . Palache gives 

 a : C = 13%° ; Rosenbusch gives b : t = -20° ( max . -30° ) . I have 

 determined in different specimens C :c = +71° max. 74°), or 

 b:c = -19° (in Palache's slides, -16°). It is certain, how- 



Fig. 1. — Stereographic projection on (010) of the optical orientation of 

 common glaucophane Gl, uniaxial glaucophane u, and eross- 

 ite Cr. 



ever, as in glaucophane, that the maximum angle of extinction 

 measured in the slides does not correspond to the angle on (010), 

 but to a plane near the prism face, as Daly has demonstrated. 

 Measurements on cleavage lamellae gave 15°-20°, according to 

 the color and the size of the angle of the optic axes.* y — a, = 

 0.008, y — /? = variable, very small. The axial plane is almost 

 parallel to the base ; both pinacoid sections show a cross (by using 

 the oil immersion) which by rotation of the stage passes over into 

 a hyperbola with widely separated arms. The angle of the optic 



* We often find in the literature glaucophanes with large angle of 

 extinction (16°-20° in glaucophane of New Caledonia and M. Vanoise, 

 21°-28° in glaucophane of Syra and Saasthal, and even more), but we do 

 not know either the chemical composition or the exact optic properties of 

 these. 



