THE NONMETALLIC MINERALS. 



275 



if the original sediments contained sufficient magnesium carbonate to 

 constitute a true dolomite. They are regarded rather as having resulted 

 from the alteration of limestone strata by the replacement of a part 

 of the calcium carbonate by carbonate of magnesium. 



Uses, Aside from its use as a building material, dolomite has of late 

 come into use as a source of magnesia for the manufacture of highly 

 refractory materials for the linings of converters in the basic processes 

 of steel manufacture. According to a writer in the Industrial World 1 

 the magnesia is obtained by mixing the calcined dolomite with chloride 

 of magnesia, whereby there is formed a soluble calcic chloride which 

 is readily removed by solution, leaving the insoluble magnesia behind. 

 According to another process the calcined dolomite is treated with 

 dissolved sugar, leading to the formation of sugar of lime and deposi- 

 tion of the magnesia; the solution of sugar of lime is then exposed to 

 carbonic acid gas, which separates the lime as carbonate, leaving the 

 sugar as refuse. Recently it has been proposed to use magnesia as a 

 substitute for plaster of paris for casts, etc. 



The snow-white coarsely crystalline Archean dolomite commercially 

 known as snowflake marble, and which occurs at Pleasantville, in West- 

 chester County, New York (Specimen No. 30863, U.S.N.M.), is finely 

 ground and used as a source of carbonic acid in the manufacture of 

 the so-called soda and other carbonated waters. (Specimen No. 3080-1, 

 U.S.N.M.) 



3. MAGNESITE^ 



This is a carbonate of magnesium, MgCO 3 , = carbon dioxide 52.4 

 per cent, magnesia 47.6 per cent. Usually contaminated with carbon- 

 ates of iron and free silica. 



The following analysis will serve to show the average run of the ma- 

 terial, both in the crude state and after calcining: 



The mineral occurs rarely in the form of crystals, but is commonly 

 in a compact finely granular condition of white or yellowish color some- 



Junel, 1893. 



