388 MINERALOGY 



almost any analysis of calcite; most of the magnesian lime- 

 stones are of this nature. Beds of dolomitic limestone are formed 

 by the replacement of calcium carbonate with magnesium carbonate 

 in coral reefs. This can take place in solutions of MgCl 2 and 

 MgSC>4 common in sea water, and especially in the presence of 

 CO 2 and at a temperature of 20 to 25 C. 



Mountain ranges in the West are formed by dolomite in which the 

 rivers have eroded steep walled canons, which, with the cross valleys 

 and tributary streams, cover the country with the peculiar butte 

 formations ; such a section in the Tyrol is known as the Dolomites. 



An interesting variety of nearly black dolomite, teruelite, occurs 

 at Teruel, Spain, in a matrix of gypsum. The crystals are combi- 

 nations of an acute rhombohedron and the base equally developed, 

 resembling very closely the octahedron, but the basal faces are 

 always rough and quite different from the rhombohedron. 



Uses. Massive dolomites are very desirable as building stones. 



SIDERITE 



Siderite. Chalybite ; Spathic iron ore ; Ferrous carbonate, 

 FeC0 3 ; FeO = 62.1 (Fe = 48.2),' CO 2 = 37.9 ; Hexagonal ; Type, 

 Dihexagonal Alternating; c = .8184; 0001 A 1011 == 43 22' 



FIG. 446. Fluorite and Siderite. Harz Mountains, Germany. 



