CHAPTER IX 



LIME 



Origin and Nature of Limestones and Lime. The 



generally accepted theory with regard to the origin of limestones 

 is that most of them at least are of organic origin, altho some 

 geologists still hold that the formation of the massive beds of 

 limestone deposited in the earlier geological periods was by a 

 purely chemical process. Limestone, or at least the chemical 

 compound, calcium carbonate, which constitutes the main part 

 of limestone, may occur in a great number of forms in nature. 

 Common limestone, marble, aragonite, calcite, and travertine are 

 all forms that are composed almost entirely of calcium carbonate, 

 while the rock known as dolomite is composed of calcium and 

 magnesium carbonates. 



Pure calcium carbonate is composed of 44 per cent carbon 

 dioxide and 56 per cent calcium oxide. These two chemical 

 compounds can be separated by means of heat, and this is the 

 chemical reaction utilized in the manufacture of lime. The 

 carbon dioxide, being a gas, is driven off by heat, and the calcium 

 oxide, a white solid, known as quicklime, remains in the kiln. 



Calcium, the metallic constitutent of limestone, is very 

 abundant in nature; in fact, only oxygen, silicon, aluminum, and 

 iron are more abundant. An estimate of the amount of hmestones 

 in the earth's crust by T. Mellard Reade places it as equivalent 

 to a layer around the earth with a thickness of 528 feet. Van 

 Hise estimates the amount of calcium carbonate in solution in 

 the waters of the oceans as equal to 160,000,000,000,000 metric 

 tons. Calcium cannot exist in nature as the metal or the oxide 

 on account of its great affinity for water. Water changes it 

 immediately to the form known as slaked lime or calcium 

 hydroxide. The hydroxide in turn is acted upon by carbon 

 dioxide to form calcium carbonate. The formation of quick- 

 lime and its return to the form of carbonate again takes place 

 according to the following reactions: 



■ CaCOs + heat = CaO + CO2 



This means that when calcium carbonate or limestone is heated 

 it gives up carbon dioxide and becomes calcium oxide or quick- 

 lime. Quicklime, when acted upon by water in the process 

 commonly known as slaking, gives the following reaction : 

 CaO + H2O = Ca(0H)2 



(149) 



