84 



The angle which the two different cleavages make with each 

 other could be measured with great accuracy on the reflecting 

 goniometer. It was found to be 18° 26'. This agrees very 

 well with the angle (18° 25^/2'), which the cleavages of aegirine 

 make whith those of arfvedsonite, when the two minerals are 

 in parallel position. Consequently, we have here an arfvedso- 

 nite with aegirine inclusions. A microscopical section, orientated 

 parallel to the common plane of symmetry of the two minerals, 

 shows this very distinctly. The arfvedsonite is easily recog- 

 nized by its strong pleochroism and its angle of extinction of 

 about 15°. The aegirine, on the other hand, is recognizable 

 by its weak pleochroism and an extinction angle of only a few 

 degrees in the direction opposite to that of arfvedsonite. 

 The two minerals are, consequently, orientated with a common 

 vertical axis and a common plane of symmetry and the respec- 

 tive a-axes inclined in the same direction. Thus they are not 

 in twin position with reference to each other. It is very pro- 

 bable that the aegirine portions are the result of changes in the 

 molecular arrangement of the arfvedsonite substance. Tlie 

 aegirine portions, which are wholly enclosed by the arfvedsonite, 

 are of varying size, elongated in the direction of the vertical 

 axis, irregularly bounded, and terminated at the ends by nume- 

 rous splinters. The arfvedsonite portions are pure and without 

 inclusions, the aegirine, on the contrary, often contains nume- 

 rous small black grains, which may be of magnetite. The 

 aegirine portions are often orientated in zones perpendicular to 

 the vertical axis ^). At the extreme end the section consists of a 

 zone of pure aegirine, which, where it borders on the crocido- 

 lite, is quite colourless and without inclosures. This colour- 



^) W. C. Brögger and к. J. V. Steens trup describe (Zeitsch. f. Kryst. Vol. 

 16, p. 405, 1890) arfvedsonite crystals Avhich are more or less com- 

 pletely altered into aegirine. These crystals, which were not, hoAvever, 

 from Narsarsuk, but from the sodalite-syenite on the firth of Tunugdliarfik, 

 are probably in the main of the same character as those described above. 



