WARREN AND PALACHE. — QUINCY PEGMATITES. 153 



Ray near c'=a Ray // to b = c Ray near a = b 



For sections 0.03 mm. For 0.03 mm. thick- For thickness un- 



or under, deep blue to ness, very dark smoky der 0.03 mm., yellow, 



bluish smoky green. For green to almost black. For 0.03 and over, 



over 0.03 mm. nearly or brownish yellow with 



quite black. a greenish shade. 



Absorption a < c much greater than 6. For many sections inter- 

 mediate in position between the front and side pinacoids, a peculiar 

 dull, grayish blue (some might call this a drab or even see a violet tone 

 of color) is seen. This is particularly true of thin cleavage fragments. 

 In many sections parallel to the clinopinacoid it has been observed 

 that the distribution of color is not uniform, the blue being seen in 

 streaks parallel to the cleavage, or lying along lines crossing the cleav- 

 age, suggesting in appearance minute cracks along which there has 

 been some slight chemical change. In such cases the remainder of the 

 section had a dull green color. In the riebeckite from the pegmatites 

 at least, such variation in color does not appear to be connected with 

 any significant change in the chemical composition. Tests with the 

 sensitive tint on very thin cleavage fragments show always a negative 

 elongation. The extinction in 010 sections does not exceed 4 or 5 de- 

 grees, measured on the prismatic cleavage. Its accurate determination 

 is rendered difficult by the strong natural color and strong dispersion 

 of the mineral. A single section perpendicular to the prismatic axis 

 was obtained sufficiently thin to yield, in convergent light, using a 

 powerful illumination, a faint biaxial interference figure. The hyper- 

 bolae move well out of the field on rotation of the preparation, indicat- 

 ing a large axial angle. The axial plane bisects the acute angle of the 

 cleavages. This is substantiated by the interference figure obtained 

 from the 010 section which is clearly that of an obtuse bisectrix with 

 the axial-plane lying parallel to the cleavage direction. The hyper- 

 bolae in figures from this section are faintly colored red and blue ; also 

 interference brushes obtained from random sections are strongly colored 

 red or blue indicating a strong dispersion, the exact character of which 

 has not been made out. From the above it appears that the axial 

 plane in this rieheckitelies perpendicular to b, 010, an tmusual relation 

 for a hornblende, while the acute bisectrix lies inclined by not over 4 

 or 5 degrees to c\ and is negative. A determination of the index of 

 refraction for the yellow ray b, by the immersion method, gave a value 

 of 1.695 (sodium). 



Chemical Composition. — Material for a chemical analysis was ob- 

 tained from a single large crystal which appeared exceptionally free 



