MAGNESIA. 



249 



Composition. Magnesia 68.34, water 30.90, proloxule 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 Iravcrsins serpen- 

 tine. It is usually accompanied by other magncsian minerals. 



LOCALITIES. 



Ricii.MOND 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 e.xposure to the 

 atmosphere, it is converted into the carbonate of magnesia. 



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

 cast 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 tlie action of the 

 atmosphere upon the hydrate. 



CARBONATE OF MAGNESIA. 



Carbonrttc ot" JMagncsia. Ciciivdaml, P/iiUips and 'IVuhnson. — Dolomite, or Macrotopoiis Limestone. Jtiinc:^itn. — 



INlagnesite. Shi:pard. — Giobcrtite. Bcutlant. 



F'§- '■-''■ Description. Colour white or yellowish white. It occurs 



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

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

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

 and 82° 35' [Dcudant). Hardness soinetimes 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 little 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 sulution of jjolasii. 



I include under this name the minerals which are principally comj)osed 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 Iloboken in New-Jersey, and which is similar 

 in appearance to the inagncsia alba of the shops. The Giolicrtitc of Beiulant is a nearly 

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



MiN. — PartII. 32 



