Crystallization of Parisite. 549 



dicular to the prismatic axis which was sufficiently thin to 

 yield, in convergent light, using a powerful illumination, a 

 faint biaxial interference figure, was out by the firm of Voigt 

 and Hochgesang. The hyperbolae move well out of the field 

 on rotation of the preparation, indicating 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 

 hyperbolae in figures from this section are faintly colored red 

 and blue ; also interference brushes obtained from random sec- 

 tions are strongly colored red or blue, indicating a strong dis- 

 persion, the exact character of which has not been made out. 

 From the above it appears that the axial-plane in this riebeck- 

 ite lies perpen dicular to b 010, an unusual relation for a horn- 

 blende, while the acute bisectrix lies inclined by not over four 

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

 index of refraction for the yellowish ray, by the immersion 

 method, gave a value of 1/695 (sodium). 



Chemical composition. — Material for a chemical analysis 

 was obtained from a single large crystal which appeared excep- 

 tionally free from impurities. This was broken up and most 

 carefully picked over by hand under a powerful glass. When 

 examined under the microscope it appeared agreeably free 

 from alteration and included grains, except some inevitable 

 black dust and a few black oxide particles as well as traces of 

 aegirite and microcline. The average of closely agreeing 

 duplicates is as follows : — 



Per cent. Molec. ratios. 



Si0 2 51-79 0-833 



TiO a ... 1-28 0-015 



f% - '"J. -097 



FeO 21-43 1 



MnO 1-15 I h 



CaO 1-28 f 661 



Mc;0 -10 J 



Na„0 6-16 ) . '. 



K a O 1-10 J ' H1 



F. -20 



H 2 —115 -10 ]■ 



H 2 + 115._ 1-30 



Total .101-08 



Less = 2F -09 



101-17 

 Spec, gr 3-391 



Am. Jour. Sci. — Fourth Series, Vol. XXXI, No. 186. — June. 1911. 

 37 



