121 



The crystals belong to the monoclinic system and have 

 the following axial ratios: 



a:b:c = 1,31639 : 1 : 0,8075 ; ß = 64° 42'. 



By reflected light neptunite is of pure black colour. In 

 thin microscopical sections and in thinnest splinters it is trans- 

 lucent with a blood- red colour. The plane of the optic axes 

 is perpendicular to the second pinacoid; the first mean-line 

 lias an inclination of 18° to the vertical axis within the obtuse 

 angle ß. The mineral is optically positive. It is strongly 

 pleochroic, and the absorption takes place according to the 

 scheme c>b>a. 



The specific gravity of the mineral is 3,234; the hardness 

 is 5 — 6. The mineral has an eminent cleavage parallel to the 

 prism of the third order. 



According to J. W. Clarke's interpretation M of the results 

 of the analysis the chemical composition is as follows: 



SiO.^—[Na,K] 



Ti<f.^^>{Fe,Mn) 



^Si0l — (Na,K) 



The neptunite crystals from the Liitzen collection that 

 formed the material of the above description Avere mainly of 

 one and the same character. The only variation in form 

 consisted in the presence or absence of certain subordinate 

 forms. Afterwards, however, G. Nordenskiöld discovered in 

 the same collection neptunite crystals of a habit quite different 

 from that of the crystals previously observed'^). These crystals 



^) Bull. U. S. Geol. Surv. No. 125, 1895, p. 97. 

 2) Geol. Foren. Förh. Vol. 16, 1894, p. 345. 



