358 



UNITED STATES MINERAL RESOURCES 



dant. Resources of these rocks are restricted to small 

 parts of the continents. This chapter emphasizes 

 limestone and dolomite that are used for their 

 chemical properties; uses for construction are de- 

 scribed in the chapter "Construction Stone." Mag- 

 nesite, another carbonate rock, consists of the min- 

 ei-al magnesite, MgCOa, and is used mostly as a 

 refractory material. Magnesite is discussed in the 

 chapter "Magnesian Refractories." 



Distribution, characteristics, and uses of limestone 

 and dolomite are discussed in many reports. The 

 U.S. Bureau of Mines "Minerals Yearbooks" and 

 "Mineral Facts and Problems" give information 

 about production and use. Recent data about chem- 

 istry and technology are discussed in "Mineral Facts 

 and Problems" (U.S. Bureau of Mines, 1970) and by 

 Boynton (1967). New information about depositional 

 environments and origin of limestone and dolomite — 

 the result of intensive investigations during the 

 last 20 years — aids the search for high-purity lime- 

 stone and dolomite. Among recent reports giving 

 such information are those in the symposia edited 

 by Chilingar, Bissell, and Fairbridge (1967a), Fried- 

 man (1969), and Pray and Murray (1965). Summary 

 information about the distribution of limestone and 

 dolomite in the United States was presented by Gill- 

 son (1960, p. 148-183). Ericksen and Cox (1968) 

 discussed distribution of carbonate rocks in the 

 Appalachian region. 



DEFINITIONS 



Limestone and dolomite are rocks composed, re- 

 spectively, of the minerals calcite, CaCOa, and dolo- 

 mite, CaMg(C03)2; however, both limestone and 

 dolomite commonly are called limestone by the in- 

 dustry, and intermediate varieties are not dis- 

 tinguished. Some newly deposited limestone consists 

 chiefly of the mineral aragonite, which has the same 

 composition as calcite but a different crystal struc- 

 ture. The aragonite structure is unstable, and with 

 time aragonite gradually changes to calcite. Com- 

 monly, both calcite and dolomite occur in the same 

 rock, but generally one predominates in any one 

 layer or stratigraphic unit. A complete gradation 

 between limestone and dolomite is uncommon. 



The value and use of carbonate rocks are deter- 

 mined by their composition. High-calcium limestone 

 and high-purity dolomite have a great variety of 

 uses in the chemical and metallurgical industries 

 and are by far the most valuable of these rocks. 

 High-calcium limestone is defined here as consisting 

 of at least 97 percent CaCOs by weight, but many 

 writers use 95 percent as a defining limit (Ericksen 

 and Cox, 1968, p. 227) . High-purity dolomite has at 



least 87.5 percent CaMg(C03)2 (40 percent MgCOa). 

 Pure dolomite, CaMg(C03)2, consists of 45.7 percent 

 MgCOa by weight. Cement limestone may contain a 

 considerable amount of clastic impurities if they are 

 relatively uniformly distributed throughout the rock 

 and provides the alumina and silica needed for port- 

 land cement. Cement limestone must contain less 

 than 6.5 percent dolomite because raw materials for 

 Portland cement cannot contain more than 3 percent 

 MgCOs; the specifications for portland cement per- 

 mit no more than 5 percent MgO. During the manu- 

 facture of cement, CO2 is lost from CaMg(C03)2, 

 causing relative enrichment of the magnesium in 

 the finished product (Boynton, 1957, p. 98) . 



Carbonate rocks containing more than 25 percent 

 fine clastic material are unsuitable for any use ex- 

 cept as fill for construction. Chert in crushed stone 

 to be used as concrete aggregate is undesirable 

 because the chert may react with the cement, and 

 the resulting chemical compounds might weaken the 

 concrete and cause spalling. 



PRODUCTION, USES, AND DESIRABLE 

 PROPERTIES 



The present consumption of limestone is immense ; 

 more than 628 million tons of limestone and dolo- 

 mite was used in the United States in 1969 (fig. 43) . 

 Although essential to industry, limestone and dolo- 

 mite are among the cheapest of the mineral com- 

 modities — the average price of crushed limestone 

 and dolomite at domestic quarries was $1.46 per ton 

 in 1969. Commonly, a user pays more for shipping 

 the limestone than for the rock itself. For example, 

 the M. W. Kellogg Co. (1972, p. 13) reported the 

 following freight rates for crushed stone in the 

 United States, expressed in cents per ton-mile: 

 truck, 5-10 (at 10 miles) to 2-5 (at 100 miles) ; 

 rail, 4.5-6 (at 10 miles) to 0.75-1.5 (at 250 miles) ; 

 and water (barge or ship), 0.9-1.5 (at 20 miles) to 

 0.25-0.50 (at 500 miles) . Because of shipping costs, 

 limestone of relatively poor quality may be used in 

 preference to better quality limestone that must be 

 shipped in from a greater distance. For example, the 

 Vanport Limestone is mined in western Pennsyl- 

 vania, eastern Ohio, and northern West Virginia 

 for use in nearby steel plants, even though it con- 

 tains only 90-95 percent CaC03 instead of the 95 

 percent or more CaC03 usually required for flux. 



Most limestone and dolomite is produced from 

 open pits or quarries ranging from tens to a few 

 hundreds of feet in depth. Relatively small amounts 

 of stone, chiefly high-purity material, are produced 

 from room-and-pillar underground mines, most of 

 which are less than 500 feet deep. Open-pit mining 



