320 DESCRIPTIVE MINERALOGY. 



GIBBSITE. 



[In honor of the late Col. George Gibbs.] 



Gibbsite. Cleaveland, Phillips, Shepard, Dana, Beudant, Mohs (App. 1 614). — Gibbsite, or Hydrate of Alumina. 



TTumison,. 



Description. Colour white, with a slight shade of green or grey. Streak white. It 

 occurs in reniforin, botryoidal and stalactitic masses. Composition fine granular. Lustre dull. 

 Slightly translucent. Hardness from 3.0 to 3.5. Specific gravity from 2.40 to 2.09. In- 

 fusible by the blowpipe, but gives out water. 



Composition. Alumina, 64.80, water 34 . 70 ( Torre^). Formula AlaOj+Aq. 

 Alumina 54.91, silica 8.73, peroxide of iron 3.93, water 33.60 (Thomson). The latter 

 supposes it to consist of a congeries of hydrates. 



Geological Situation. This mineral has heretofore been found only in beds of hematitic 

 iron ore. 



localities. 



Dutchess County. Gibbsite has been found in considerable abundance, investing hema- 

 tite at the Clove mine in the town of Union-vale. It is of a white or yellowish white colour, 

 and occurs in tuberose masses or stalactites which are slightly fibrous. Specific gravity, the 

 mean of two trials with specimens as pure as I could obtain, 2.305. The mineral is easily 

 reduced to powder, but it is tough, and some parts of it are much harder than others. It is 

 soluble in muriatic acid without effervescence. My specimen contained silica and oxide of iron, 

 but these may be accidental ingredients, as it is exceedingly diflicult to free the specimens 

 from hematite. 



This mineral was first discovered by Dr. Emmons, in a neglected mine of brown hematite 

 at Richmond in Massachusetts. Dr. Horton informs me that he has recently found it in 

 Orange county. These are the only known localities. 



ALUMINITE. 



Subsulphate of Alumina. Ckaveland? PMUips. — Websterite. Bev4ant? 



Description. This mineral occurs in reniform masses and botryoidal concretions, of a 

 white or yellowish white colour. Fracture earthy. Adheres to the tongue. Specific gravity 

 1.66. It gives out water by calcination. Fuses with diSiculty by the blowpipe. Soluble in 

 acids without effervescence. The solution gives with ammonia a gelatinous precipitate, which 

 is redissolved by pure potash or soda. 



Composition. Specimen from Halle in Prussia — Alumina 30 . 26, sulphuric acid 23 . 36, 

 water 46.33 {Stromeyer). 



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