2i6 WiUiam E. Ford, 



The interesting' feature in this comparison of molecular 

 volumes is the fact that calcite stands at the head of the list with 

 quite a large difference between its volume and that of dolomite 

 which comes next in order. On the other hand the volumes of 

 the other four are fairly close together. As shown in the column 

 of the above table headed "Difference" the differences in the 

 volumes of dolomite and rhodochrosite, of rhodochrosite and 

 siderite and of siderite and magnesite are practically identical. 



As the space lattice of a crystal depends not only on the volume 

 of its individual cell but also upon the angles that the bounding 

 planes of this cell make with each other, the following comparison 

 is of interest : — 



Again we see emphasized that the space lattice of calcite is 

 distinctly different from that of the other four members of the 

 group. 



When another molecule enters a compound in isomorphous 

 replacement it seems certain that either one of two things must 

 happen. Either the molecule that enters must conform to the 

 crystal lattice of the mineral or the lattice must progressively 

 change with the change in composition. In the first case a vari- 

 ation in composition would produce no corresponding change in 

 crystal angles or molecular volume while in the second case some 

 such change would take place. A study of the cleavage angles of 

 the different minerals of the Calcite Group shows no evidence of 

 any variation accompanying changes in their compositions. The 

 largest amount of data is available with dolomite. The literature 



