ALTERATIONS OF CALCITE. 239 



Or supposing that the magnesium salt is a carbonate, and that this is depos- 

 ited and an equivalent amount of calcium carbonate is taken into solution, 

 the reaction would be: 



(3) 2CaC0 3 +MgC0 3 =CaMgCA+CaC0 3 +k. . 



Either of these changes is accompanied by the decrease in volume of 12.30 

 per cent if the original calcite be compared with the produced dolomite. 

 There might be no diminution in volume, or even an increase in volume, 

 in case less than the molecular weight of calcium salt equivalent to the 

 introduced magnesium was dissolved. For instance, in an extreme case 

 the reaction might be: 



(4) CaC0 3 +MgCO s =CaMg(C0 3 ) 2 +k, 



the MgC0 3 being added through solutions, and no calcium carbonate 

 dissolved. In this case the expansion in volume over the original calcite 

 would be very great — 75.41 per cent. However, the normal case in 

 dolomitization, as noted below, appears to be the molecular replacement 

 represented by the specific equations (2) and (3). The compounds 

 concerned in these reactions are so important that the heat relations have 

 been determined as above given; so it can be asserted positively, from 

 chemical studies, that heat is liberated by them. 



The calcium of calcium carbonate may be replaced by other metals 

 besides magnesium, or calcite may be replaced by an oxide. The most 

 important of the elements which enter into such combinations, and the 

 only one which need be mentioned, is iron. At many localities, partly 

 or wholly occupying the place once held by calcite, iron carbonate is 

 found. For any definite proportion of iron replacing the calcium, equations 

 may be written paralleling those for the replacement of calcium by 

 magnesium. 



The third important alteration of calcite is to wollastonite (monoclinic ; 

 sp. gr. 2.8-2.9). This alteration is, indeed, the chief source of wollastonite. 

 The equation is: 



(5) CaC0 3 +Si0 2 =CaSiO s +C0 2 — k. 



Iii the change the volume is decreased 31.48 per cent, provided the silica 

 used is a solid and the carbon dioxide escapes. In case the silicic acid be 

 brought in solution from an outside source, the volume of the solid is 

 increased 10.81 per cent. Between these extremes there are theoretically 



