ALUMINA. 



'M5 



Varieties. Sapphire. Tliis name is applied to the transparent blue varieties. 



Oriental Ruby, is transparent, and of a blood-red or rose-red colour. 



Emenj. Massive ; nearly opaque ; bluish grey and greyish black. 



Common Corundum. Massive and crystallized ; of various colours ; translucent. 



Adamantine Spar. This is brown and faintly translucent. 



Composition. Sapphire — Alumina 9S.50, oxide of iron I .00, lime 0..50 {Klaproth). 



Emery — Alumina 86.00, silica 3.00, lime 3.00, oxide of iron 4.00 (Tennant). In its 

 pure form, therefore, this mineral is an oxide of aluminum, the formula of which, according 

 to Kane, is Al O;. 



Geological Situation. The finest sapphires and rubies occur in alluvial soil in Ceylon, 

 &c. Common corundum and adamantine spar are said to be found at foreign localities, in 

 granite. In tliis State, and in New-Jersey, corundum has heretofore been found only in white 

 limestone. 



Uses. The transparent varieties are highly esteemed as gems. The common corundum, 

 and emery, are employed for polishing steel, cutting gems, etc. 



Fig. 254. 



localities. 



Orange County. Corundum has been found, by Dr. 

 pjg_ 253. / I \\ Horton, in the vicinity of the village of Amity. The 



colours are nearly white, pale and deep blue, and pale 

 and deep rose-red. It occurs in crystals and grains. Fig. 

 253 and 2.54 represent two of the forms which have been 

 /' observed, but the sides are rough, and cannot be well 

 ' measured. It is associated with hornblende, spinelle and 

 red oxide of titanium, in white limestone. This mineral 

 is also found at Newton in New-Jersey. 



SPINELLE. 



Spincllc Cleavdand and Bcuilant. — Spiiiell. T/iomsoii. 

 hedral Corundum, or Spinel. Jameson. - 



Fig. 2j5. 



— Spinel. P/tillips, Shcpard and Dann. — Dodeca- 

 ■ Dodeeaedrischcr Korund. J\]ohs. 



Description. Colour red, sometimes green, black, blue, yellow, 

 brown, and nearly white. Streak white. It occurs regularly crys- 

 tallized. The primary form is a regular octahedron. Fig. 255. 

 P on P' 109° 28' K) ' (Hauij). Cleavage parallel with the primary 

 from difficult. Fracture conchoidal. Lustre splendent and vitreous. 

 Form transparent to translucent on the edges. Hardness 8.0. 

 Specific gravity from 3.50 to 3.00. Infusible by the blowpipe 

 alone; with borax, it fuses slowly into a transparent glass. The 

 solution obtained by an acid after treatment with caustic potash, 

 affords a precipitate of alumina after the addition of hydrosulphate of ammonia. 



