386 MINERALOGY 



and the concentrated HC1 solution yields no precipitate with 

 H 2 SO 4 . Yields no flame coloration in the forceps. 



General description. Magnesite is rarely crystalline. The 

 best crystals, which are simple rhombohedrons, occur in a chloritic 

 schist at Hall in the Tyrol ; they are of a light brownish color, 

 contain 10 per cent, of ferrous carbonate, and are known as breun- 

 nerite. 



The massive magnesite is dull white or gray and very tough. 

 It is formed by the same agencies as calcite, but naturally from the 

 decomposition of minerals or rocks rich in magnesium ; it is there- 

 fore associated with serpentine, olivine, and garnets, in talcose 

 and chloritic schists. White magnesite occurs in Massachusetts, 

 Pennsylvania, and Maryland. All magnesite mined in the United 

 States is from the California deposits, especially those of Tulare 

 and Santa Clara Counties. The imported magnesite is obtained 

 from Greece and Austria. 



Uses. In addition to being the source of all magnesium salts, 

 calcined magnesite, made in the form of bricks, is used as a refrac- 

 tory lining in the basic open hearth furnaces, in cement kilns, or 

 as a lining to resist chemical corrosion at high temperatures, as 

 its fusing point is about 2250. 



Artificial. If a solution of magnesium carbonate in water is 

 heated to 300 with a porous stopper, allowing the C0 2 to escape 

 gradually, small rhombohedral crystals of magnesite will form. 

 They are also formed when sodium carbonate, magnesium chloride, 

 and water are heated in a sealed tube to a temperature of 160 to 

 200. 



DOLOMITE 



Dolomite. Pearl spar ; Calcium magnesium carbonate, 

 CaMg(C0 3 ) 2 ; Ca = 30.4, Mg = 21.7, CO 2 = 47.8 ;_ Hexagonal ; 

 Type, Hexagonal Alternating; c = .832; 0001 A 1011_= 43 51' 

 37"; r A r' = 73 45'; Forms, c (0001), m(10lO), r (1011) ; Twin- 

 ning, like calcite; Cleavage, rhombohedral perfect; Brittle, frac- 

 ture subconchoidal ; H. = 3.5-4; G. = 2.8-2.9; Color, various 

 shades of gray, light yellow, and brown ; Luster, pearly to vitre- 

 ous; Streak, white; Transparent to opaque; CD = 1.682; = 

 1.502; - = .180; Optically (-). 



B.B. Infusible, whitens by loss of C0 2 and reacts alkaline with 



