188 Washington — Kaersutite from Linosa and Greenland. 



(Wright) with observed limits of ± 2' ', measured on cleavage 

 fragments with a two-circle goniometer with reducing attach- 

 ment. The reflection signals were fairly sharp and satisfactory. 

 No cleavage or parting after 100 or 001 was noted with cer- 

 tainty, and whenever cleavage is not developed the fracture is 

 conchoidal. The hardness is 6 and the mineral is very brittle. 

 Before the blowpipe it fuses readily to a black, slightly mag- 

 netic bead. 



Fig. 1. Kaersutite from Linosa. Etch pit on 110, HF. x 480. 



The specific gravity was carefully determined with the pyc- 

 nometer on about 2 grams of selected fragments, entirely free 

 from adhering bits of scoria or feldspar and quite free from 

 inclusions so far as could be seen with a lens. With this mate- 

 rial the density at 13° was found to be 3*336 (Washington), a 

 figure which may be accepted as representing the true value. 



Etch figures on m (110) were produced by immersing cleav- 

 age fragments in hot commercial hydrofluoric acid (on a steam 

 bath at 100°) for a period of 30 seconds. Further action was 

 stopped by plunging the platinum basket containing the frag- 

 ments into cold water. Under these conditions of experiment 

 well-formed etch pits resulted, from 0*01 to 0'04 mm long and 

 about half as wide. Different stages of development are illus- 

 trated in figures 1-3, which are reproductions of photomicro- 

 graphs of the figures in reflected light. In each case the vertical 

 edge of the photograph is parallel to the prism-axis. A com- 

 parison of these figures with those obtained by Daly,* and later 

 by Wright,f shows that they resemble in certain features the 



* Ft. A. Daly, Proc. Amer. Acad. Arts and Sci., vol. xxxiv, pp. 333-429, 

 1899. 



+ F. E. Wright, Tschermak's Min. Petr. Mitth. vol. xix, pp. 308-320, 

 1899. 



