OXIDES 



377 



Chromite is used in the making of ferrochrorne, in the steel in- 

 dustry. It is the source of all chromium salts, which are used 

 extensively MS pigments and oxidi/ers. The powdered mineral is 

 made into bricks with tar as a binder, which are used infurnaces 

 where a refractory lining is required to resist the corrosive action of 

 the fused charge. 



( 'hromite, like magnetite, resists decomposition by weathering 

 and is therefore a component of many residual sands, some of 

 which are of commercial value for their chrome content. 



Other members of the spinel group are rare in occurrence and are 

 unimportant minerals. Gahnite, ZnAl 2 O 4 , the zinc spinel, is found 

 at Franklin, New Jersey; at the Canton mine, Georgia; in 

 Mitchell County, North Carolina. When the molecule contains 

 manganese and iron as well as zinc it is known as dysluite, 

 (Zn . Mn . Fe) (Al . Fe) 2 4 , a variety of gahnite also found at Stir- 

 ling Hill, New Jersey. 



Streak, white; Luster, vitre- 



a = 1.747; P = 1.748; -y = 



2E = 84 43'; Axial plane = 



CHRYSOBERYL 



Chrysoberyl. Be(AlO) 2 , BeO = 19.8 ; Al 2 0s = 80.2 ; Or- 

 thorombic ; Type, Didigonal Equatorial ; a : b : c = .470 : 1 : 

 0.580; 100 A 110 = 25 10'; 001 A 101 =50 59'; 001 A 011 = 

 30 1' ; Forms, a (100), o (111), m (110), n (121) ; Twinning plane, 

 031; Cleavage, Oil distinct and 010 less so; Brittle, fracture 

 uneven ; H. = 8.5 ; G. = 3.5-3.85 ; Color, various shades of green, 

 brown, yellow, and sometimes red ; 

 ous; Transparent to translucent; 

 1.7565; -y-a = .009; 2V == 45 20'; 

 010; Bx. = b; Optically (+) 



B.B. Infusible; powdered and ignited with cobalt solution 

 becomes blue. Dissolves slowly in the S. Ph. bead. Insoluble in 



acids. 



General description. Crystals are usually tabular parallel to 

 the macropinacoid, with striations parallel to the c axis. Twins 

 are common, both contact and penetrating, often repeated, yielding 

 a six-sided pseudohexagonal shape. In these twins the face a (100) 

 is common to all the individuals and will locate the direction of the 

 c axis in each by its striations. 



Chrysoberyl is a rare mineral found in granites and pegmatites, 

 where it is associated with such minerals as tourmaline, beryl, 



