Studies in the Coleitc Group. 



231 



The variation in composition shown by these analyses of dolo- 

 mite is represented graphically in Fig. 3. This is a portion of 

 the triangular projection of which CaCO^, MgCOo and 

 (Fe,Mn)C03 are the three points (see Fig. 5). Since man- 

 ganese carbonate in molecular weight, etc., is practically equiva- 

 lent to ferrous carbonate these two radicals have been combined 



Fig. 3. 



and treated as one. The heavy solid line running diagonally 

 across the figure gives the locus of the points representing the 

 simple mixtures of CaMg(C03)2 and Ca(Fe,Mn) (COg),. The 

 heavy broken line that lies above and parallel to this line, repre- 

 sents the maximum limit of the amount of CaCO... that may 

 enter the mineral as established by Foote and Bradley. It is to 

 be noted that while a considerable number of the analyses lie 

 on or very close to the line, CaMg-(C03)2-Ca(Fe,Mn) (CO;,).,, 

 a large number also fall above it and therefore show the presence 

 of isomorphous CaCO,,. There are a few analyses whose points 

 fall below this line. These are the ones that show an excess of 

 MgCOa, which, as shown in Table III, in most cases is limited 

 in amount. For the present and until more evidence is available 

 it would seem reasonable to reject the analysis showing over 16 

 per cent of extra MgCO... and place the limit of this possible 

 excess at about 7 per cent. It is possible, of course, that MgCO. 

 does not occur in dolomite in this way and that its presence 



