190 Washington — Kaersutite from Zinosa and Greenland. 



pinacoid, and the extinction angle was measured only on the 

 cleavage face m (110). By using the etch iigures as a basis for 

 orientation, the extinction angle c A c was found to be +1'4° 

 (in the acute angle ft) in white light,* as indicated by the arrow 

 in figure 1. This direction presupposes the standard crystallo- 

 graphic orientation of the amphiboles by Tschermak. This 

 extinction angle is noteworthy because of its positive character, 

 and it is readily discernible, although so small that (in the 

 absence of corroborative data) it might be considered to be 

 due to observational error alone in the deep-colored Hakes. 

 The dispersion of the bisectrices is very slight, and practically 

 the same value was obtained by using sodium light as that for 

 white light. In convergent polarized light the interference 

 phenomena are only moderately clear and distinct. The optic 

 axes lie in the plane of symmetry (010), and the optical charac- 

 ter is negative. 



The refractive indices were determined on a very perfect 

 cleavage flake with an Abbe total refractometer, a solution of 

 sulphur in methylene iodide, with refractive index 1*7882, 

 being used. The observations were made in sodium light, and 

 the following values obtained : 



Average angle 

 observed. 





Equivalent refrac- 

 tive index. 



y = 67° 18' 





1-760 



/3 = 65° 8' 





1-730 



a = 62° 33' 





1-692 



a = 0-068, y 



-P = 



0-029 (3 — a = 0-0J 



The optic axial angle calculated directly from these refrac- 

 tive indices is 2Y = 79° 54'. 



The boundary shadow for a was much more distinct than 

 those for j3 or 7, and could be determined with greater accu- 

 racy. In the values given for j3 and 7 an error of ± - 002 is 

 easily possible. The fact that light waves vibrating along c and 

 b were strongly absorbed undoubtedly exerted an influence on 

 the relative intensity of the phenomena observed, and caused 

 the faintness of the /3 and 7 curves of total reflection. 



A somewhat smaller value for 2 V was obtained by meas- 

 uring the angle of the optic axes directly in another section cut 

 approximately normal to the acute bisectrix, by the method 

 described by Wright, f The deep color of the mineral impaired 

 the accuracy with which the determinations of the positions of 

 the optic axes could be made, so that the results are but approx- 

 imate at best. In two different portions of the same section 

 thus measured the values 2V = 71° and 72° were obtained. 



* Average of 10 measurements, with observed limits ±0*6°. 

 f F. E. Wright, this Journal, vol. xxiv, p. 317, 1907. 



