.:^- 



160 



ALUMINA. 



there is one crystal formerly belonging to the Jardin des 

 Plantes of Paris, for which he gave £3000 sterling. 



The largest oriental ruby known was brought from China 

 to Prince Gargarin, governor of Siberia ; it afterwards came 

 into the, possession of Prince MenzikofF, and constitutes now 

 a jewel in the imperial crown of Russia. 



2. Combined with other oxyds. 



SPINEL. 



Monometric. In octahedrons, more or less modified, and 

 dodecahedrons. Figure 1, is the octahedron with truncated 

 12 3 4 



edges ; figure 3, the same with beveled edges ; figure 2, the 

 dodecahedron. Occurs only in crystals ; cleavage octahedral, 

 but difficult. Figure 4 represents a twin crystal. 



Color red, passing into blue, green, yellow, brown and 

 black. The red shades often transparent and bright ; the 

 dark shades usually opaque. Luster vitreous. H=8. 

 Gr=3-5— 3-6. 



Composition : of a red spinel, from Ceylon, alumina 69*0, 

 magnesia 26*2, protoxyd of iron 0*7, silica 2*0, chromic acid 

 1*1. Essentially alumina and magnesia. Infusible alone, 

 and with difficulty with borax. 



Varieties. The following are the varieties of this species 

 that have received distinct names : The scarlet or bright 

 red crystals, spinel ruby ; the rose-red, balas-ruby ; the 

 orange-red, rubicelle ; the violet, almandine-ruby ; the green, 

 chlorospinel ; while the black varieties are called pleonaste. 

 Pleonaste crystals contain sometimes 8 to 20 per cent, of 

 oxyd of iron. 



Dif. The form of the crystals and their hardness dis- 

 tinguish the species. Garnet is fusible. Magnetic iron ore 



What is the usual crystalline form of spinel? What is its hardness? 

 What are its colors ? Of what dees it essentially consist ? Mention the 

 colors and names of some of the varieties ? 



