OXIDI S 



341 



senile, NiO, occurring at Johanngeorgenstadt, Saxony, are both 

 isometric minerals erystalli/iug in small cubes. 



M;i>sicit, PbO, is a massive yellow mineral found in several 



localities in Chihuahua. Mexico. 



OXIDES OF THi: ll,<>., TYPE 



CORUNDUM 



Corundum. Sesquioxide of aluminium, Al 2 0a ; Al = 52.9, 

 O = 47.1 ; Hexagonal; Type, Dihexagonal Alternating; c = 1.363 ; 

 0001 A 10ll = 57 34'; r.r' = 9356'; Common forms, a (1120), 

 r (1011), c (0001), v (4483), n (2243) ; Twinning plane r, polysyn- 

 thetic ; Cleavage, parting parallel to r and c at times perfect ; 

 H. = 9; G. = 3.95-4.1; Color, gray, red, blue, green, white, or 

 nearly black; Streak, white; Luster, adamantine; Transparent 

 to opaque; v = 1.768; = 1.76; <o - = .008; Optically (-). 



B.B. Infusible, fusing point 1880; the very fine powder heated 

 on coal in 0. F., moistened with cobalt solution, and again heated 

 becomes blue. Insoluble in acids, but dissolves slowly both in 

 borax and the S. Ph. beads. 



Fio. 414. Corundum Crystals from Montana. 



General description. Crystals when large are coarse, rough 

 prisms, swelling in the middle and drawn in at the ends or barrel- 

 shaped, terminated with the base ; such large crystals are opaque 



