314 



DESCRIPTIVE MINERALOGY. 



ORDER 11. ALUMINA. 



I. Minerals in which the alumina is nearly pure, or combined with an acid. 



1. Corundum. 



2. Spinelle. 



3. Aiitomalite. 



4. Gibbsite. 



5. Akiminite ? 



II. Minerals chiefly composed of silica and alumi)ia, combined with some alkali, alkaline 



earth or metallic oxide. 



Corindoo. Haiiy und Boiiianf. — Corundum. Ckarchnd and Shcpard. — Sapphire. Thomson. — Rhombohe- 

 dral Corundum. Jameson. — Rhomboedrischer Korund. Mohs. 



Fig. 252 



Description. Colour blue, red, grey, white, green, yellow,, 

 brown and black. Streak white. It occurs regularly crystallized ; 

 also massive. The primary form is a rhombohedron. Fig. 252. P 

 on P 86° 6' ; P on V 93° 54'. Fracture conchoidal and uneven. 

 Lustre vitreous, sometimes inclining to adamantine. Transparent 

 to feebly translucent on the edges. Hardness 9.0. Specific gra- 

 vity from 3.97 to 4.16. Infusible by the blowpipe ; with borax, it 

 fuses slowly but perfectly into a colourless glass. It is not acted 

 on by acids. Becomes electric when rubbed. 



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