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. 



Gurhqfite. — 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 ; gurhoflte 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 cal« 

 cite ? What are its uses 1 



