372 MINERALOGY 



(311), (322), (221), (332) rare; Twinning plane, 111; Cleavage, 

 octahedral imperfect ; Brittle, fracture conchoidal ; Streak, white ; 

 H. = 8; G. = 3.5-4.1; Luster, vitreous; Color, gray, brown to 

 black in opaque specimens, in transparent, red, blue, pink, green 

 yellow; n = 1.715. 



B.B. The light-colored varieties may change color ; infusible : 

 insoluble in acids ; in fine powder becomes blue with cobalt solu- 

 tion. 



General description. Crystals always well formed, of octahe- 

 dral habit, in combination with the rhombic dodecahedron and 

 cube ; other forms are rare. Pure spinel is white or light in color, 

 but as some of the magnesium is always replaced with other metals 

 of the isomorphous group, the various colors are due to these and to 

 impurities. In sections always pale in color, and owing to its high 

 index of refraction the surface is rough, showing a high relief. Crys- 

 tals are well formed with good outlines or in rounded grains, but 

 always fresh and free from decomposition. Some crystals have 

 the appearance of having been fused, with a glazed surface and 

 rounded edges ; this is a characteristic of all the species of the group. 



Spinel may occur as a primary accessory mineral in igneous rocks, 

 especially those rocks rich in magnesium or aluminium ; often as 

 picotite, a variety in which ferrous iron replaces the magnesium, and 

 chromium some of the aluminium. Spinel is often connected with 

 contact metamorphism and occurs in a granular limestone, from 

 Sparta in northern New Jersey to Amity in New York ; these are 

 well-formed gray and brown crystals associated with serpentine. 



In North Carolina it occurs with corundum, from which it may 

 be formed as a decomposition product. It may also result from 

 the alteration of olivine, garnets, or other minerals rich in magne- 

 sium or aluminium. 



Ruby spinel, so called from its resemblance in color to the true 

 ruby, and is nearly as hard, is used as a gem. The gem material is 

 associated with corundums in the gem-bearing gravels of Ceylon and 

 Burma ; of the various colors the " flame-red " and blue are the 

 most sought after. Ruby spinels occur also at Gold Bluff, Cali- 

 fornia. 



Artificial spinels may be formed by fusing the oxides or fluorides 

 of magnesium and aluminium with boric acid; various colors 

 may be produced by the addition of small amounts of iron and 

 chromium oxides. 



