592 



MINERALS AND VEINS OF ^EGIRINE-SYENITE, 



the arfvedsonite is secondary, derived from the uralitisation of 

 the segirine, and should be avoided when pure material is required. 

 The crystals are jet black with splendent lustre best exhibited 

 on cleavage faces. I have followed the crystallographic orienta- 

 tion adopted by Rosenbusch.* 



The characteristic form is tabular on the b face, which is always 

 pre-eminently developed. The m form com- 

 pletes the prism zone, a faces not being 

 observed. Terminal faces are often present, 

 the usual forms (see fig. 3) being ■! 001 \- , 

 -{ Til }, ■{ 021 \- ; rarely other faces appear, 

 giving, however, only poor reflections. 



Two of these forms «J 2-5-1 } \ 2-10 1 \ appear 

 to be noted here for the first time. 



The largest crystal met with had a length 

 of 8 cms., breadth 3 cms. and thickness 2-5 mms. 

 More usually the better developed individuals 

 averaged 2 cms. long and from 1 mm. to 5 mms. 

 thick. Long narrow crystals were frequently 

 bent. The signal from the b faces was often 

 much confused, due chiefly to warping. The 

 angles varied slightly from one crystal to 

 another; for example, the (110a 110) with par- 

 „.„■„., , . ticularlv sharp signals varied through 3'. 



Fig. 3. —Orthographic J r & » 



aud ciinographic pro- This is no doubt morphotrophv due to replace- 



jection of an arfved- x x j ± 



souite crystal as com- ment amongst the vicarious constituents. 



rnoiily developed. ° 



* I have to thank Professor Eosenbusch for kindly enlightening me in the 

 matter of recent literature, leading to his interpretation of the crystallographic 

 orientation of the amphiboles and pyroxenes. Following Tschermak, he 

 regards the crystallographic orientation of the amphiboles, in the light of a 

 rotation about the vertical axis of 180° from that originally introduced by 

 Nordenskiold and contained in Dana, 1899 ed. 



For further proof that this is the correct view to take, see "A Comparative 

 Study of Etch-Figures. The Amphiboles and the Pyroxenes," R. A. Daly, 

 Proc. Amer. Acad, of Arts and Sciences. Vol.xxxiv., No. 15, 1899, tp. 422. 



For comparison, the crystal indices calculated by the old mehod, are 

 quoted as follows : — 



a : b : c = -55600 : 1 : '29222. 



/3=73°. 



