MAGNESIA. 249 



Composition. Magnesia 68.34, water 30.90, protoxide of manganese 0.64, protoxide of 

 iron 0.12 (Stromeyer). It is undoubtedly a compound of one atom of water and one of 

 magnesia. Formula Mg+Aq. 



Geological Situation. This mineral has heretofore been found in veins traversing serpen- 

 tine. It is usually accompanied by other magnesian minerals. 



LOCALITIES. 



Richmond County. Hydrate of magnesia occurs associated with talc and carbonate of 

 magnesia in the serpentine rock which forms the principal range of hills on the island. It 

 has been found in small specimens at the Quarantine. The first discovery of it in the United 

 States was at Hoboken, New-Jersey, where it is still obtained. By long exposure to the 

 atmosphere, it is converted into the carbonate of magnesia. 



Westchester County. This mineral is found associated with serpentine on the peninsula 

 east of New-Rochelle. It also occurs in a similar formation two miles north of the village of 

 Rye. It is in the form of small imperfect crystals or plates having a pearly lustre, and is 

 often invested with powdery carbonate of magnesia, undoubtedly formed by the action of the 

 atmosphere upon the hydrate. 



CARBONATE OF MAGNESIA. 



Carbonate of Magnesia. Cleaveland, Phillips and Tluymson. — Dolomite, or Macrotopous Limestone. Jameson. — 



Magnesite. Shepard. — Giobertite. Beudant. 



^'S' '^- Description. Colour white or yellowish white. It occurs 



in delicate acicular crystals, or in a white powder ; also in dull 

 opaque masses, with a flat conchoidal or earthy fracture. The 

 primary form is said to be a rhombohedron. P on P 107° 25^ 

 and 82° 35' {Beudant). Hardness sometimes about 3.5. Spe- 

 cific gravity from 2.56 to 2.88. Before the blowpipe, on 

 charcoal, it shrinks a little, and then acts on vegetable blues 

 like magnesia ; with borax or biphosphate of soda, it fuses into 

 a glass. It dissolves slowly, and with httle effervescence, in 

 cold nitric acid ; but more readily when heat is applied, espe- 

 cially if it is reduced to a fine powder. The solution thus 

 obtained is scarcely precipitated by oxalate of ammonia, but copiously by a solution of potash. 



I include under this name the minerals which are principally composed of carbonate of 

 magnesia, and which occur in needle-form crystals, or in a white powder coating the hydrate 

 of magnesia. When in the latter form, it is probably the same as the hydro-carbonate of 

 magnesia described by Dr. Thomson as from Hoboken in New-Jersey, and which is similar 

 in appearance to the magnesia alba of the shops. The Giobertite of Beudant is a nearly 

 pure carbonate of magnesia, which occurs in the magnesian rocks of Piedmont, the Tyrol, &c. 



MiN. — Part II. 32 



