90 CEMENTS, LIMES, AND PLASTERS. 



As might be expected, the limestones we have to deal with in practice 

 depart more or less widely from this theoretical composition. These 

 departures from ideal purity may take place along either of two lines: 



a. The presence of magnesia in place of part of the lime ; 



b. The presence of silica, iron, alumina, alkalies, or other im- 



purities. 



It seems advisable to discriminate between these two cases, even 

 though a given sample of limestone may fall under both heads, and 

 they will therefore be discussed separately. 



The presence of magnesia in place of part of the lime. The theo- 

 retically pure limestones are, as above noted, composed entirely of 

 calcium carbonate and correspond to the chemical formula CaCOa. 

 Setting aside for the moment the question of the presence or absence 

 of such impurities as iron, alumina, silica, etc., it may be said that 

 lime is rarely the only base in a limestone. During or after the forma- 

 tion of the limestone a certain percentage of magnesia is usually intro- 

 duced in place of part of the lime, thus giving a more or less magnesian 

 limestone. In such magnesian limestones part of the calcium car- 

 bonate is replaced by magnesium carbonate (MgCOs), the general 

 formula for a magnesian limestone being, therefore, 



zCaCOs + 2/MgCOs. 



In this formula x may vary from 100 per cent to zero, while y will 

 vary inversely from zero to 100 per cent. In the particular case of 

 this replacement where the two carbonates are united in equal molec- 

 ular proportions, the resultant rock is called dolomite. It has the 

 formula CaCOs, MgCOs, corresponding to the composition calcium 

 carbonate 54.35 per cent, magnesium carbonate 45.65 per cent. In 

 the case where the calcium carbonate has been entirely replaced by 

 magnesium carbonate, the resulting pure carbonate of magnesia is 

 called magnesite, having the formula MgCOs and the composition mag- 

 nesia (MgO) 47.6 per cent, carbon dioxide (C0 2 ) 52.4 per cent. 



Rocks of this series may therefore vary in composition from pure 

 calcite limestone at one end of the series to pure magnesite at the other. 

 The term limestone has, however, been restricted in general use to that 

 part of the series lying in composition between calcite and dolomite, 

 while all those more uncommon phases carrying more magnesium car- 

 bonate than the 45.65 per cent of dolomite are usually described simply 

 as more or less iniDure magnesites. 



Though magnesia is often described as an " impurity " in lime- 

 stone, this word, as can be seen from the preceding statements, hardly 



