19 



pure carbonate of magnesia is called magnesite, having the 

 formula MgCO^ and the composition magnesia (MgO) 47.6 

 per cent.; carbon dioxide (CO2), 52.4 per cent. 



Rocks of this series may therefore vary in composition from 

 pure calcite-limestones 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 be- 

 tween calcite and dolomite, while all those more uncommon 

 phases carrying more magnesium carbonate than the 45.65 per 

 cent, of dolomite are usually described simply as impure magne- 

 sites. 



The presence of much magnesia in the finished cement is con- 

 sidered undesirable, 3V> per cent, being the maximum permissi- 

 ble under most specifications, and therefore the limestone to be 

 used in Portland cement manufacture should carry not over 5 

 to 6 per cent. o>f magnesium carbonate. 



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

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

 hardly expresses the facts in the case. The magnesium carbon- 

 ate present, whatever its amount, simply serves to replace an 

 equivalent amount of calcium carbonate, and the resulting rock, 

 whether little or much magnesia is present, is still a pure carbon- 

 ate rock. With the impurities to be discussed in later para- 

 graphs, however, this is not the case. Silica, alumina, iron, 

 sulphur, alkalies, etc., when presenit, are actual impurities, not 

 merely chemical replacements of part of the calcium carbonate. 



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

 purities.- \Yhether a limestone consists of pure calcium carbon- 

 ate or more or less of magnesium carbonate, it may also contain 

 a greater or lesser amount of distinct impurities. From the 

 point of view of the cement manufacturer, the more import an 

 of these impurities are silica, alumina, iron, alkalies, and sul- 

 phur, all of which have a marked effect on the value of the lime- 

 stone as a cement material. These impurities will therefore be 

 taken up in the order in which they are named above. 



The silica in a limestone may cccur either in combination with 

 alumina, as a clayey impurity, or not combined with alumina. 

 As the effect on the value of the limestone would be very di Cer- 

 ent in the two cases, they will be taken up separately. 



Silica alone. Silica, when present in a limestone containing 

 no alumina, mav occur in one of three forms, and the form in 



