CARBONATI.S 



3S!) 



51"; r A r' = 730'; Common forms, r(1011), c(0001); Twin- 

 ning plane, e (01 12) ; Cleavage, rhombnhedral perfect; Brittle, 

 I'ra.-ture uneven; H. = 3.5-4 ; G. = 3.83-3.88 ; Color, various 

 shades of brown and gray; Streak, white unless oxidized, when 

 it may he brown; Luster, vitreous; Translucent; o> = 1-.873, 

 c = 1 .(>:; o>- = .240; Optically (-). 



B.B. Loses CO 2 and blackens. In R. F. becomes magnetic. 

 Fu-es at 4.5 and reacts for iron with the fluxes. Effervesces in 

 dilute HC1, especially when hot, and dissolves completely when 

 pure. 



General description. Crystals are simple rhombohedrons or 

 complex crystals, which are curved rhombohedrons as in dolomite. 

 Simple rhombohedral crystals of half a pound in weight occur 

 embedded in the cryolite of Ivigtut, Greenland. At the Buckler's 



FIG. 447. Cryolite and Siderite from Ivigtut, Greenland. 



Mine, Cornwall, England, crystals, combinations of the rhombo- 

 hedron, base, hexagonal prism, and a scalenohedron occur, but forms 

 other than the rhombohedron are rare. Siderite is also massive, 

 granular, or disseminated. 



The various colors depend upon the amount of oxide of iron, or 

 oxidation, and at times it is very dark, due to the presence of hema- 

 tite or limonite, to which it is easily transformed by oxidation. 



