Studies in the Calcite Group. 225 



average difference between the measured and calculated values 

 is .017, which is within the limit of error of the ordinary specific 

 gravity determination. An interesting and possibly significant 

 fact is that the great majority of the differences between the 

 measured and calculated values are negative in character. This 

 was true also, of course, in the case of the molecular volumes. 

 This would be explained if the value assigned to the specific 

 gravity of calcite was too low. In fact, if the specific gravity, 

 of calcite had been taken as 2.72, the average difference, taking 

 in this case the plus and minus signs into consideration, would 

 have been — .008. 



In the case of the magnesite-siderite series, since their molec- 

 ular volumes and crystal angles are very closely the same, it 

 follows that their crystal lattices are practically identical and we 

 should expect a continuous isomorphous series between them. 

 This, for the same reason, would also be the case with the 

 siderite-rhodochrosite series. It has already been shown that 

 these two series do show such isomorphism. 



Below is given in tabular form the available analyses of these 

 two series with which the specific gravities are recorded. Num- 

 bers proportionate to the molecular volumes of each specimen 

 as derived from the percentages of the different molecules 

 present and the specific gravity are also given. It will be noted 

 that these values show a small but fairly regular progressive 

 change from the magnesite end to the siderite end of the first 

 series and from the siderite end to the rhodochrosite end of the 

 second series. It is evident, therefore, in these series, that either, 

 since the molecular structures of the chief components are so 

 closely similar, they may crystallize together and still each pre- 

 serve its individuality, or that the molecular structure of the 

 combination can adjust itself in accordance with the amounts of 

 the different constituents present. Whichever case is true, it 

 ought to be possible to calculate the molecular volume and specific 

 gravity of each specimen directly from the percentage com- 

 position. 



In the case of the CaCO^ molecule, however, it would appear 

 logical to consider that since its lattice in calcite is distinctly 

 different from the lattice of rhodochrosite, siderite and magne- 

 site, its structure when it occurs in these minerals, would have to 

 be modified to conform to their network. Since, however, the 



