Part II. Relations Existing Between the Chemical Com- 

 position AND Specific Gravity of Dolomite 



Dolomite, for a long time, was considered as an isomorphous 

 mixture of the calcite and magnesite molecules in approximately 

 equal proportions. For various reasons this has been shown not 

 to be the case. For instance, the series CaCOy-MgCOs is far 

 from complete. We have calcite containing- small amounts of 

 magnesium carbonate and magnesite containing likewise small 

 amounts of calcium carbonate but large breaks in the series exist 

 on both sides of the dolomite mixtvu^e. Further, as shown by 

 Rettgers^ the specific gravity of dolomite falls distinctly below 

 the theoretical value for an isomorphous mixture of the calcite 

 and magnesite molecules. Lastly the fact, that dolomite has been 

 shown to have tetartohedral symmetry, gives another proof that 

 it is a distinct and individual compound. Yet its similarity in 

 crystalline and optical characters to the other members of the 

 Calcite Group shows that it has close structm-al relations with 

 them. 



W. L. Bragg- has shown by means of the X-ray that the 

 internal structure of dolomite is similar to that of the other 

 members of the group. Fie makes the suggestion that the 

 arrangement of the molecular planes perpendicular to the vertical 

 crystal axis is like that of calcite, see Fig. i, with however, the 

 planes that contain the metallic atoms showing alternately cal- 

 cium and magnesiuuL This arrangement would serve to explain 

 the tetartohedral symmetry of the mineral. Bragg adds, how- 

 ever, that as yet the X-ray has not given any evidence to prove 

 the truth of this assumption. 



Normal dolomite has the composition of CaMg(CO.,)o into 

 which ferrous iron commonly enters and less frequently man- 

 ganese. These last seem always to replace the magnesium in the 

 molecule and never the calcium. Lately it has been recognized 

 that CaCOg may also occur as an isomorphous constituent and 

 some analyses would suggest that under rarer conditions MgCO;; 

 might enter the molecule as well. The various isomorphous 



' Zs. Phys. Chem., 6, 227, 1890. 

 " Proc. Roy. Soc, 89, 488, 1913. 



