ORDER VIII. PRISMATIC SCHEELIUM-ORE. 389 



Colour dark greyish- or brownish-black. Streak 

 dark reddish-brown. Opake. 

 Not very brittle. Hardness = 5-0 ... 5-5. Sp. 

 Gr. = 7-155, of a crystallised variety from 

 Zinnwald. 



Compound Varieties. Twin-crystals: 1. Face 

 of composition parallel, axis of revolution perpendi- 

 cular to Pr -f oo ; Face of composition parallel, 

 axis of revolution perpendicular to a face of ?r. 

 There is often a curious composition in the interior 

 of crystals parallel to all their faces. Massive : com- 

 position irregularly lamellar, easily separated, faces 

 of composition irregularly streaked ; also columnar, 

 the individuals being generally of a considerable 

 size, straight and divergent, and often rather strong- 

 ly coherent. Pseudomorphoses in the shape of py- 

 ramidal Scheelium-baryte. 



OBSERVATIONS. 



1. According to BERZELIUS, the varieties of the present 

 species consist of 



Tungstic Acid 78'77- 



Protoxide of Manganese 6-22. 



Protoxide of Iron 1 8-32. 



Silica 1-25. 



When pure it is expressed by Mn W 2 + 3 Fe W 2 , and 

 the proportion of its ingredients = 77*27 ' 5*84 : 16-89. It 

 decrepitates before the blowpipe, but may be melted in a 

 sufficiently elevated temperature into a globule, having 

 its surface covered with crystals possessing a metallic lustre. 

 It is pretty easily soluble in borax. 



2. This ore occurs very frequently along with pyramidal 

 Tin-ore, in veins and beds. It is met with, however, also 



