167 



A few other values of angles are, however, also given here. 



Measured Calculated 



m:m'= (nO):(lTO) = 90° 4' 90= 



ni'.a = (110): (100) = 44° 53' 45° 



rt:a' = (100):(100) = 89°52' 90° 



n:a = (210):(100) = I7°58' 18° 17' 



c:a =(001): (100) = 90° 3' 90° 



2j:c = (111): (001) = 36° 24' 36° 5Г 



2): a = (111): (100) = 65° 2' 65° 7' 



Twin crystals do not seem to occur. 



The specific gravity of narsarsukite was determined by the 

 direct weighing of a pure crystal hanging by a hair first in air, 

 then in distilled water. It was found to be 2,751 at a tempera- 

 ture of 20° C. The hardness of the mineral is a little above 7. 

 One can produce faint scratches on quartz with the mineral; 

 on the other hand it is more easily scratched by topaz. The 

 mineral has a most eminent cleavage parallel to the fundamental 

 prism 7П. In the direction of this cleavage the crystal tables very 

 readily cleave into cube shaped fragments. Owing to this cleavage 

 it is impossible to produce coherent thin sections perpendicular 

 to the principal axis. Even in sections parallel to the same 

 axis the cleavages are easily observable. Fracture, which, unless 

 special measures are taken, is always obtained in a direction 

 parallel to the c-axis, is columnar with vitreous lustre. The 

 prismatic faces often have a pearly lustre. 



In colour narsarsukite is honey-yellow with a tinge of reddish 

 brown in the clearest portions. On incipient weathering the 

 colour passes into ochre-yellow. Other crystals, which are in 

 part altered, are brownish grey. In sections parallel to the 

 principal axis the mineral is colourless. In an ordinary thin sec- 

 tion no pleochroism is observable. In thicker sections, on the 

 other hand, it is fairly distinct. The absorption is greatest 

 parallel to the c-axis, and the axial colour is reddish yellow. 



