CARBON \TI.S 



385 



lactitic, as from the caves of Kentucky. Chalk is a soft, earthy 

 variety derived from the shells of organisms, 



Uses. Limestone is used in the building industries not only 

 as a ItuililiiiK stone, but it is heated to drive off the carbon dioxide, 

 when quicklime (CaO) remains; this is slaked with water and 

 mixed with sand for mortar; or is sintered with eight or nine per 

 cent, of clay and then ground, forming Portland cement, of which 

 nearly fifty millions of barrels are used annually in the United 

 States, and which now largely replaces the older quicklime mortars. 

 Quicklime is often air-slaked and applied to the soil as a fertilizer 

 to correct the acidity and as an aid in liberating combined potas- 

 sium, in a form in which it is available for plant growth. 



( Jranular limestones and marbles are crystalline limestones ; their 

 color varies with the impurities they contain, and some have a very 

 pleasing effect as ornamental and building stones. When very 

 pure, white, and of an even texture, as Carrara marble, it is used 

 by sculptors as statuary marble. There is a deposit of statuary 

 marble in Colorado said to equal, if not to surpass, in quality that 

 of the noted quarries of Carrara, Italy. Limestone is also used as 

 a flux in blast furnaces, where it combines with the silica of the ore 

 to form a fusible slag. Limestones are quarried in many states, 

 but the center of the industry is around Rutland, Vermont. 

 Tennessee produces colored marbles in large quantities, and Cali- 

 fornia the onyx variety. 



Artificial. Calcite forms from ordinary solutions of calcium 

 bicarbonate at low temperatures; if heated or at temperatures 

 near the boiling point, aragonite will form. Calcite will also form 

 in a fusion with alkaline carbonates, while from an alkaline solution 

 aragonite will form. 



MAGNESITE 



Magnesite. Carbonate of magnesium, MgC0 3 ; Mg = 47.6, 

 CO 2 = 52.4 ; Hexagonal ; Type, Dihexagonal Alternating ; c = 

 .8112^ 0001 A 10ll =43 7' 45"; r A r' = 72 36'; Forms r (1011), 

 m (1010), c(0001); Cleavage, rhombohedral perfect; Brittle, 

 fracture conchoidal ; H. = 3.5-4.5; G. = 3-3.12; Luster, vitre- 

 ous to dull and earthy; Transparent to opaque; Color, white, 

 gray, or brown; = 1.515, o> = 1.7171, <o- =.202, Optically ( ). 



B.B. Reacts like calcite, but dissolves in dilute HC1 more 

 slowly. Becomes flesh-colored when heated with cobalt solution, 

 2c 



