390 MINERALOGY 



When mixed and crystallized with MgCO 3 , it forms mesitite, as at 

 Traversella in Piedmont, which has a rhombohedral habit and is 

 theoretically iron and magnesium carbonates in molecular propor- 

 tions. Ankerite is a mixture of calcium, iron, and magnesium car- 

 bonates, occurring with iron ores and siderite, as in Styria, Siegen, 

 Nova Scotia, and northern New York. 



Siderite like other carbonates is deposited from the bicarbonate 

 solution, but in this case it must be laid down under non-oxidizing 

 conditions or in the presence of organic matter, otherwise hematite 

 or limonite will form, as ferric carbonate is not known as a dry 

 salt. It is also formed by the replacement of calcium in calcite 



FIG. 448. Siderite, showing the Hexagonal Prism and Base. Cornwall, England. 



and limestones and is therefore an important constituent of 

 many sedimentary rocks, especially those of the coal formations. 

 Siderite as a vein filler is associated with many ore deposits. 



Uses. It is mined as an iron ore in Cornwall, England, but it is 

 of little importance as an iron ore in the United States. For- 

 merly it was mined at Roxbury, Connecticut. 



Artificial. Microscopic crystals may be obtained by precipitat- 

 ing a solution of ferrous sulphate with sodium bicarbonate and 

 heating in a sealed tube to 150 for several hours. 



RHODOCHROSITE 



Rhodochrosite. Manganese carbonate, MnCO 3 ; Mn = 61.7, 

 CO 2 = 38.3 ; Hexagonal ; Type, Dihexagonal Alternating ; 6 = 

 8184; 0001 A 101 1 = 43 22' 50"; r A r' = 73 0'; Forms, 



