222 William E. Ford, 



MnCOg. Here, similar to the case of rhodochrosite it is appar- 

 ently the conditions under which niagnesite commonly occurs 

 that limits the amount of the MnCO, present, rather than the 

 difference in molecular structure. 



From the above it is seen that the theoretical assumptions as to 

 the miscibility of the different molecules with each other are well 

 substantiated by the study of actual analyses. Therefore, it 

 seems clear that we can say of calcite, rhodochrosite, siderite and 

 magnesite that each has an individual space lattice and the 

 entrance of the other molecules into the composition of anyone 

 of these is conditioned upon the amount that can be present and 

 still adapt itself to the structure of the mineral. The only way 

 apparently in which the space lattice of calcite could vary would 

 be in the spacing of the molecular layers. It might be possible 

 that the introduction into the mineral of other molecules with 

 different molecular spacings would proportionately modify the 

 calcite structure. Such a change would involve a change in 

 molecular volumes and would presumably make possible a com- 

 plete isomorphous series. It is of interest to examine analyses 

 of calcite to see if the introduction of other molecules does 

 appreciably modify the molecular volumes. In Table I are given 

 some fourteen analyses which, it should be stated, include all the 

 available analyses with which specific gravities have been given. 

 There are no omissions from the list except in cases where the 

 analysis clearly showed some error or impurity. 



* (i) Foote and Bradley, Am. J. Sc, 37, 342, I9i4- (2) Eisenhuth, 

 Zs. Kr., 35, 601, 584, 1901. (3) Erben, ref., Zs. Kr., 12, 664, 1887. (4) 

 Jentsch, Pogg. Ann., 96, 145, 1855. (5) Bukousky, ref., Zs. Kr., 39, 400, 

 1904. (6) Weibull, ref., Zs. Kr., 10, 512, 1885. 



