COMPOUNDS OF ALUMINUM:. 



195 



black. The red shades often transparent and bright ; the 

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

 G. =3-5-4-1. 



4. 



Composition. MgAl 4 = Mg + A10 3 = Alumina 72, mag- 

 nesia 28 = 100. The aluminum is sometimes replaced in 

 part by iron, and the magnesium often in part by iron, cal- 

 cium, manganese and zinc. Infusible ; insoluble in acids. 



Varieties. The following varieties of this species have 

 received distinct names : the scarlet or bright red crys- 

 tals, spinel ruby ; the rose-red, balas-ruby ; the orange-red, 

 rubicelle ; the violet, almandine-ruby ; the green, chloro- 

 spinel ; while the black varieties are called pleonaste. Pleo- 

 naste crystals contain sometimes 8 to 20 per cent, of oxide 

 of iron. Picotite is a variety containing 7 per cent, of 

 chromium oxide. 



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

 tinguish the species. Garnet is fusible. Magnetite is at- 

 tracted by the magnet. Zircon has a higher specific gravity 

 and is not so hard. The red crystals often resemble the 

 true ruby (red corundum), but the latter are never in octa- 

 hedrons. 



Obs. Occurs in granular limestone ; also in gneiss and 

 volcanic rocks. At numerous places in the adjoining coun- 

 ties of Sussex in New Jersey, and Orange county, of various 



