DOLOMITE. 119 



brown, and sometimes black. Luster vitreous, o. a little 

 pearly. Nearly transparent to translucent. Brittle. 11= 

 3-5—4. Gr=2-8— 2-9. 



Composition. Dolomite is a compound of carbonate of 

 magnesia and carbonate of lime. The common variety con- 

 sists of 54*4 of the latter to 45*6 of the former. Infusible 

 before the blowpipe. Effervesces with acids, but more 

 slowly than calc spar. 



The principal varieties of this species are as follows : 



Dolomite. — White crystalline granular, often not distin- 

 guishable in external characters from granular limestone, 

 except that it crumbles more readily. 



Pearl spar. — This variety occurs in pearly rhombohe- 

 drons with curved faces. 



Rhomb spar, Brown spar. — In rhombohedrons, which 

 become brown on exposure, owing to their containing 5 to 

 10 percent, ofoxyd of iron or manganese. 



Miemite.—A yellowish brown fibrous variety from Miemo 

 in Tuscany. 



Gurhofite. — A compact white rock, looking like porcelain 

 and containing a few per cent, of silica. 



Dif. Distinctive characters, nearly the same as for cal- 

 careous spar. It is harder than that species, and differs in 

 the angles of its crystals, and effervesces less freely ; but 

 chemical analysis is often required to distinguish them. 



Obs. Massive dolomite is common in the Eastern States, 

 and constitutes much of the coarse white marble used for 

 building. Crystallized specimens are obtained at the Quar- 

 antine, Richmond county, N. Y. Rhomb spar occurs in talc 

 at Smithfield, R. I., Marlboro, Vt, Middlefield, Mass. ; pearl 

 spar in crystals of the above form at Lockport, Rochester, 

 Glen's Falls ; gurhofite on Hustis's farm, Phillipstown, N. Y. 



Dolomite was named in honor of the geologist and traveler, 

 Dolomieu. 



Uses. Dolomite burns to quicklime like calc spai, and af- 

 fords a stronger cement. The white massive variety is used 

 extensively as marble. The magnesian lime has been sup- 

 posed to injure soils ; but this is believed not to be the case 

 if it is air-slaked before being used. It is also employed in 

 the manufacture of Epsom salts or sulphate of magnesia. 



What is the composition of dolomite ? How does it differ from eal« 

 cite ? What are its uses ? 



