204 DESCRIPTIONS OF MINERALS. 



MAGNESIUM. 



Magnesium occurs, in nature, as an oxide or hydrated 



oxide, and in the condition of sulphate, borate, nitrate, 

 phosphate, carbonate and silicate. 



The sulphates and nitrate of magnesia are soluble in 

 water, and are distinguished by their bitter taste ; the 

 other native magnesian salts are insoluble. The presence 

 of magnesia, when no metallic oxides are present, is indi- 

 cated by a blowpipe experiment, explained on page 87. 



Periclasite.— Periclase. Magnesium Oxide. 



Isometric. In small grayish to dark-green imbedded 

 crystals, with cubic cleavage. H. nearly 6. G.— 3-674. 



Composition. Mg (or the same as for magnesia alba of 

 the shops), with a little iron. B.B. infusible. Soluble in 

 acids without effervescence. 



From Mount Somma, Vesuvius, Italy. 



Brucite. — Magnesium Hydrate. 



Rhombohedral. In foliated hexagonal prisms and plates; 

 structure thin foliated, and thin laminae easily separated 

 and translucent ; flexible but not elastic. Also fibrous. 

 Lustre pearly. Color white, often grayish or greenish. 

 H.=2-5. ft. =2*35. 



Composition. Mg0 2 H 2 = Magnesia 690, water 3 1*0 ==100. 

 B.B. infusible, but becomes opaque and alkaline. Soluble 

 in hydrochloric acid without effervescence. 



Diff. It resembles talc and gypsum, but is soluble in 

 acids ; it differs from heulandite and stilbite also by its in- 

 fusibility. 



Ohs. Occurs in serpentine at Hoboken, N. J. : in Rich- 

 mond County, X. Y. ; in Dutchess County, X. Y. , at 

 Brewster's ; at Texas, in Pennsylvania ; also at Swinaness, 

 in Unst, one of the Shetland Isles. 



The fibrous variety has been called nemalitej it resembles 

 amianthus ; it occurs at Hoboken. 



Hydromagnesite. A pearly crystalline, or earthy, white, hydrous 

 carbonate of magnesia, from Hoboken, N. J., Texas, Pa., and elsewhere. 



Spinel contains oxygen and magnesium along with aluminum. See 

 page 195. Magnesium is also present in some magnetite, a variety of 

 which is called magnoferrite. 



