28 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
cium carbonate is replaced by magnesium carbonate (MgC0 3 ), the 
general formula for magnesian limestone being therefore x CaC0 3 , 
y MgC0 3 . In this formula x may vary from 100 per cent to zero, 
while y will vary inversely from zero to 100 per cent. Where the two 
carbonates are united in equal molecular proportions, the resultant rock 
is called dolomite. It has the formula CaC0 3 , MgC0 3 and is com- 
posed of 54.35 per cent calcium carbonate and 45.65 per cent mag- 
nesium carbonate. If the calcium carbonate has been entirely replaced 
by magnesium carbonate, the resulting pure carbonate of magnesia is 
called magnesite, having the formula MgC0 3 and being composed of 
47.6 per cent magnesia (MgO) and 52.4 per cent carbon dioxide (C0 2 ). 
Rocks of the limestone 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 those rocks which have a composition between that of 
calcite and dolomite. All the more uncommon phases, carrying more 
than 45.65 per cent magnesium carbonate, are usually described simply 
as impure magnesites. 
The presence of much magnesia in finished Portland cement is con- 
sidered undesirable, 3i per cent being the maximum permissible under 
most specifications. Therefore the limestone to be used in Portland 
cement manufacture should not carry over 5 or 6 per cent of mag- 
nesium carbonate. 
Though magnesia is often described as an "impurity" in limestone, 
this word, as can be seen from the preceding statements, hardly 
expresses the facts in the case. The magnesium carbonate 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 carbonate rock. With the impur- 
ities to be discussed in later paragraphs, however, this is not the case. 
Silica, alumina, iron, sulphur, alkalies, etc., when present, are actual 
impurities, not merely chemical replacements of part of the calcium 
carbonate. 
SILICA, TRON, AND OTHER IMPURITIES IN LIMESTONE. 
A limestone consisting of pure calcium carbonate or of calcium car- 
bonate with more or less magnesium carbonate may also contain a 
greater or lesser amount of distinct impurities. From the point of 
view of the cement manufacturer, the more important of these impuri- 
ties are silica, alumina, iron, alkalies, and sulphur, all of which have 
a marked effect on the value of the limestone as a cement material. 
The silica in limestone may occur 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 different in the two cases, 
they will be taken up separately. 
