80 



The colour of the mineral is a pure black. Even the 

 thinnest splinters can hardly be said to be translucent. The 

 powder is bluish grey. 



Thin sections orientated parallel to the second pinacoid are 

 translucent with bluish green colour. One extinction-direction 

 makes, in the obtuse /9-angle, an angle of 10° with the c-axis. 

 This angle of extinction is, consequently, a little less than with 

 arfvedsonite from other localities. The pleochroism is very strong. 

 The light is most strongly absorbed in the aforesaid direction, 

 and the axial colour is a deep greenish blue. In the direction 

 perpendicular to this the axial colour is pale greyish green. 



Sp. gr. = 3,021. 



Arfvedsonite of type i generally occurs on Narsarsuk to- 

 gether with needle-like aegirine crystals of secondary formation 

 and is evidently of the same age as these. Sometimes the 

 arfvedsonite crystals are implanted on feldspar individuals with- 

 out being accompanied by aegirine needles. Also in this case 

 the mineral is of secondary origin. Most of the specimens on 

 which this kind of arfvedsonite occurs, have been collected at 

 the locality No. 2, but the mineral has also been found at the 

 localities Nos. 4 and 8. 



Type II (Riebeckite?). 

 The majority of the arfvedsonite crystals of the second 

 type are about P'^ in length and the same in breadth. They 

 exhibit a very simple combination of faces, the forms present 

 being only the three following: 



a = {lOO}, c={00l}, w = {llO}. 

 The prism of the third order is always predominant and often 

 occurs alone in the vertical zone (Fig. 10, Plate IV). Sometimes 

 also the first pinacoid occurs, always, however, represented by 

 very narrow faces. The second pinacoid, on the contrary, has 

 not been observed ; the prismatic faces meet at the sides at 

 acute angles. At the ends the crystals are bounded by a single 



