no 



24. Zinnwaldite (Polylithionite). 



J. Lorenzen examined and described a species of mica 

 from Kangerdluarsiik, which he found to contain a considerably 

 greater percentage of lithia than any other known kind of mica; 

 on account of this he called it polylithionite M- Optically 

 this mineral described by Lorenzen does not differ from zinn- 

 waldite, i. e. its optic axial plane is parallel to the crystallo- 

 graphic plane of symmetry. It occurs associated with steen- 

 strupine and albite in sodalite - syenite on the firths of 

 Kangerdluarsuk and ïunugdliarfik, often in very large tabular 

 individuals arranged in rosettes having no regular crystallo- 

 graphic boundaries. 



The lithia -bearing mica from Narsarsuk which is to be 

 described below, has been previously observed in the Lützen 

 collection, where, however, it was found in very small quantity. 

 On the material available it was, however, possible to ascertain 

 the agreement in the optical orientation with that of zinnwaldite : 

 the plane of the optic axes was found to be parallel to the second 

 pinacoid, and the angle between the axes was comparatively 

 large. It was further shown that the mineral contained a large 

 amount of lithia. Sufficient material for a quantitative analysis 

 could not be procured. Nor were any determinable crystals 

 met with -). 



In the collections which I have brought home from Nar- 

 sarsuk this mineral occurs on a large number of specimens, 

 but always in small quantities. It occurs developed as more 

 or less distinct crystal tables, which seldom are more than 5"" 

 n breadth and l""™ in thickness. They are generally nearly 

 •round or present an irregularly lacerated edge. This irregular 

 outline often seems to be of secondary origin, as such imper- 

 fect crystals for the most part occur on specimens that have 



') This Journal. Vol. 7, 1893, p. 43. 



'') Zeitschr. f. Kryst. Vol. 23, 1894, p. 367. 



