NO. 1867. CRYSTALLIZED VARI8CITE FROM UTAH.—8CHALLER. 417 



trix, identical in direction with the crystallographical axis c, is the 

 axis of minimum velocity of light. The optical relationship can, there- 

 fore, be expressed in the conventional form as follows : 



c = Z = c 

 The axial angle is large; 2E was measured as about 113°. Dis- 

 persion was not marked and its kind could not be determined. 



Viewed in transmitted light under the microscope the crystals are 

 colorless if very thin, or pale green if somewhat thicker. The color- 

 ing pigment is not uniformly distributed, but is generally arranged 

 in irregular cloudlike areas. In a few cases the coloring seemed to 

 bear in its distribution some relation to the shape of the crystal, and 

 in these cases the color was most intense near the edges of the crystals. 

 The pleochroism is distinct but slight, and its relation to the crystal 

 directions is shown in the following scheme : 



a (X) = colorless. 

 h (Y) = pale green. 

 c (Z) =pale green. 

 The weakness of the pleochroism is in striking contrast to that of 

 the heated lavender-colored material in which, as described further 

 on, the pleochroism is strong. 



REFRACTIVE INDICES. 



The indices of refraction were measured by the method of oil 

 immersion for ;- and a. The values found are ;-= 1.577 and 

 «'= 1.547. Therefore 7-— or = 0.030, which is very close to the 

 average value (0.032) foimd by actual determination of the bire- 

 fringence. This birefringence was measured accurately on the flat 

 brachypinacoid, and the average value obtained is 0.032. The value 

 was measured on 11 crystals mth the following results: 



Birefringence (y—a) measured on b (010). 



Crystal No. 



Thickness 

 in milli- 

 meters. 



94428°— Proc.N.M.vol.41— 11 27 



Birefrin- 

 gence. 



0.034 

 .029 

 .035 

 .027 

 .031 

 .037 

 .029 

 .034 

 .030 

 .030 

 .034 



.032 



