F. E. Wright — Optical Properties of Roseoelite. 307 



The refractive indices were determined by the use of refrac- 

 tive liquids of known index. Because of the strong absorption 

 and structure of roseoelite, however, the phenomena thereby 

 observed were not sufficiently distinct for accurate measure- 

 ments and the refractive indices for sodium light may be in 

 error ± 0*003:7 = 1-704, /3 = 1-685, a = 1-610. Compared 

 with the other members of the mica group these refractive 

 indices are remarkably high and are evidently due in part to 

 the high vanadium content. 



The optic axial angle was measured on a number of different 

 flakes and found to vary noticeably from flake to flake. 

 Measurements were made by use of (1) the double screw 

 micrometer eyepiece on sections normal to the acute bisectrix, 

 (2) the improved Fedorow stage, and (3) the substage aperto- 

 meter. In sodium light the observed values ranged from 

 2E = 42° to 69°, while in lithium light 2E varied from 34° to 

 60°, and for the green mercury light (546/a/a) values of 2E up 

 to 75° were obtained. The axial dispersion is strong with 

 2V v >2V r . Dispersion of the bisectrices was not detected 

 with certainty. An abnormal shift in interference colors near 

 the position of total extinction was noted in some of the grains 

 and may have been due either to a slight dispersion of the 

 bisectrices or to the overlapping of crinkled flakes. On many 

 of the sections the negative bisectrix was found to be inclined 

 up to 4° with the axis of the centered microscope. This may 

 have been the result, in each case, of a slight tilting of the 

 cleavage flake, on which crumpling and warping of the plane 

 surface is the rule. The crinkling is most pronounced along 

 lines about normal to the cleavage traces of (010) and is often 

 so regular that it resembles, at first glance, polysynthetic 

 twinning lamellae. The gliding movement has probably taken 

 place along the usual gliding plane p (205) which plays an 

 important role in the production of compression figures on 

 mica. 



With the aid of percussion figures and the unusually distinct 

 cleavage after (010), the plane of the optic axes was found to 

 be perpendicular to the plane of symmetry. Roseoelite 

 belongs, therefore, to the first group of micas of E. Eeusch. 

 Its optical orientation is b = 7, a ^ /3, either zero or a small 

 angle not over 4°. 



The birefringence is strong, and the interference colors are, 

 as a rule, high. On basal cleavage flakes the selective absorp- 

 tion of the mineral is so strong that abnormal interference 

 hues in green and olive-brown appear and dominate in every 

 section. A peculiar apple-green interference color is, in fact, 

 characteristic of roseoelite. 



