109 



23. Biotite. 



With the exception of poiyh'thionite, which is of secondary 

 formation, mica is a mineral of rare occurrence on Narsarsuk. 

 Only a few plates which are portions of larger crystals of black 

 mica have been met witli. As these have been found lying 

 loose, not attached to any other minerals, it cannot be deter- 

 mined how the mineral originally occurred in situ. No doubt, 

 however, this mica is one of the primary minerals of the 

 pegmatite. 



The largest crystal plate found here is о^г*^" wide, and is 

 bounded at the outline by the faces of the second pinacoid, 

 {010} , and of the prism of the third order, (lio). As these 

 make angles of about 120° with one another, the cleavage plates 

 have the form of hexagonal tables. In tolerably fresh portions 

 the colour is a pure black sometimes with a greenish reflection. 

 Only very thin scales are translucent with a greenish brown colour. 

 The mineral is nearly optically uniaxial. The black cross of 

 the axial image opens only very little, when the plate is revolved. 

 If the mineral is to be regarded as biaxial, the axial plane is 

 parallel to the second pinacoid. 



However, this mica is almost wholly altered into chlorite. 

 Only small portions have retained their lustre, cleavability and 

 translucency. For the most part the mineral is dull, brittle and 

 opaque. The altered portions are partly dark ash-gTcy, partly 

 of a reddish brown copper colour. 



It is very probable that this mica is closely related to the 

 lepidomelane from Langesund in Norway, described by Brög- 

 ger^). But no chemical examination of the scanty material 

 has been made. 



Zeitschr. f. Kryst. Vol. 16, 1890, p. 189. 



