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 O (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 folated hexagonal prisms and plates; 
structure thin foliated, and thin lamine easily separated 
and translucent; flexible but not elastic. Also fibrous. 
Lustre pearly. Color white, often grayish or Sco 
ete e5 8 Gn 
Composition. Mo oO: H,= Magnesia 69-0, water 31°0=100. 
B.B. infusible, but becomes opaque and alkaline. Soluble 
in hydrochloric acid without effervescence. 
Diff. It resembles tale and gypsum, but is soluble in 
acids ; it differs from heulandite and SeulDae also by its in- 
fusibility. 
Obs. Occurs in serpentine at Hoboken, N. J.; in Rich- 
mond County, N. Y.; in Dutchess County, N. Yj oh 
Brewster’s ; at Texas, in Pennsylvania; also at Swinaness, 
in Unst, one of the Shetland Isles. 
The fibrous variety has been called nemalite; 1t resembles 
amianthus ; 1¢ occurs at Hoboken. 
Hydromagnesite. A pearly crystalline, or earthy, white, hydrous 
carbonate of maguesia, from Hoboken, N. J., Texas, Pa., and elsewhere. 
Spinel contains oxygen and magnesium along with aluminum. See 
page 1)5. Magnesium is also present in some y magnetite, a variety of 
which is call ed ma gnoferrite. 


