COMPOUNDS OF CALCIUM. 219 



Dolomite.— Calcium-Magnesium Carbonate. Magnesian Carbonate of 



Lime. 



Rhombohedral. R A R=10Q° 15'. Cleavage perfect pa- 

 rallel to R. Faces of rhombohedrons sometimes 

 curved, as in the annexed figure. Often granular 

 and massive, constituting extensive beds. 



Color white or tinged with yellow, red, green, 

 brown, and sometimes black. Lustre vitreous or 

 pearly. Nearly transparent to translucent. Brit- 

 tle. H.=3*5-4. G. =2-8-2-9. 



Composition. JCajMg 3 C = Calcium carbonate 54 # 35, 

 magnesium carbonate 45*65 = 100. Some iron or manga- 

 nese is often present, replacing part of the magnesium or 

 calcium. Dolomite resembles calcite, but differs in that 

 unless finely pulverized it effervesces very sparingly, if at 

 all, in cold dilute hydrochloric acid. 



The princpal varieties of this species are as follows : 



Dolomite. White, crystalline granular, often not distin- 

 guishable in external characters from granular limestone. 



Pearl spar. In pearly rhombohedrons with curved faces. 



Rhomb spar, Brown spar. In rhombohedrons, which be- 

 come brown on exposure, owing to their containing 5 to 10 

 per cent, of oxide of iron or manganese. 



A cobaltiferous variety has a red tint. A white compact 

 siliceous variety has been called Gurhofite. Some hydraulic 

 limestones are dolomite. 



Biff, Distinctive characters nearly the same as for cal- 

 cite. It is harder than that species, and differs in the 

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

 chemical analysis is often required to distinguish them. 



Obs. Massive dolomite is common in Western New Eng- 

 land and Southeastern New York, and constitutes much 

 of the marble used for building. Crystallized specimens 

 are obtained at the Quarantine, 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, Niagara Falls, Rochester, Glen's Falls ; 

 gurhofite on Hustis's farm, Phillipstown, N. Y. 



Dolomite was named in honor of the geologist and trav- 

 eler, Dolomieu. 



Dolomite burns to quicklime like calcite, and affords a 

 more durable cement. The white massive variety is used 



