196 Lindgren, Hamilton, Palache — Melanovanadite 



Crystal Form. — The greater thickness of the needles is 

 about 0.5 mm. ranging down to 0.1 and 0.01 mm. The 

 color is black, luster almost submetallic, streak very dark 

 reddish brown, the hardness is 2.5, the specific gravity 

 3.477 at 20° C. The habit of the crystals is prismatic, 

 parallel to c, with monoclinic symmetry. The principal 

 faces consist of a flat striated prism, the longer diagonal 

 being parallel to the fr-axis, minor pinacoidal faces and 

 usually well-developed terminal faces of pyramids and 

 smaller domes. There is a perfect cleavage parallel to 

 the clinopinacoid. Although small the crystals can be 

 measured. 



Optical Properties. — Under the microscope the crystals 

 remain black except in very thin prisms which are trans- 

 lucent with brown color. Flat cleavage pieces parallel 

 to the clinopinacoid only become translucent when the 

 thickness is about 0.003 mm. and then show maximum 

 extinction of about 15°. Resting on the prism the crys- 

 tals become brown translucent with a thickness of about 

 0.03 mm. and then show lower extinctions of 12 c to 13°, 

 while these resting more nearly on the orthopinacoid 

 extinguish at lower angles ranging down to 0°. The 

 perfect cleavage being perpendicular to the poorly devel- 

 oped orthopinacoid extinctions of 0° are rarely seen. 

 Exact optical measurements are difficult on account of 

 the deep color. Obscure hyperbolae show on the prism 

 faces and it is probable that the plane of the optic 

 axes is perpendicular to the perfect cleavage (010). The 

 absorption is very strong. The a ray is visible with dark 

 yellowish brown color through the prism faces and the 

 orthopinacoid and has according to a determination 

 kindly made by Professor C. H. Warren a coefficient 

 of refraction of a little less than 1.74; while the /? and y 

 rays are somewhat higher but cannot be measured exactly 

 on account of the strong absorption. The /3 and y rays 

 therefore lie in the 010 plane of perfect cleavage and 

 their absorption is so strong that such cleavage pieces 

 only become translucent in extremely thin plates, with 

 dark reddish brown color, /? and y differing slightly in 

 depth of tint. The double refraction is strong. 



Composition. — An analysis by L. F. Hamilton gave the 

 following composition: 



