2 24 IVilliam E. Ford, 



entrance would proportionally affect the structure of the whole 

 mineral, we should have the molecular volumes that are given 

 in the column headed "Calc." It will be seen that the values in 

 this second column show a steady decline as the percentages of 

 CaCOs decrease and in general increasing differences from the 

 values derived from the measured specific gravities. Therefore 

 we can safely say that the molecular structure of calcite shows 

 little or no change with variation in composition and that for 

 practical purposes its space lattice can be considered as constant 

 in all particulars. 



This conclusion has an important bearing on the study of the 

 connection existing between the variation in specific gravity and 

 variation in composition. We must recognize that the different 

 molecules as they enter calcite must have specific gravities that 

 correspond to its particular space lattice and which are different 

 from the specific gravities they would have as separate species. 

 For instance any FeCOg entering into the calcite structure will 

 occupy a larger volume and therefore have a lower specific 

 gravity than when it occurs in the more constricted lattice of 

 siderite. Having given, therefore, the specific gravity of pure 

 CaCOy, it is possible to calculate what the specific gravities of 

 the isomorphous molecules must be when they occupy its partic- 

 ular crystal lattice. The following equation expresses these 



, . Molecular Weight t- i t- • . . 



relations, —^ 7^ — -^ .^ — = K, where K is a constant corre- 



Specmc Gravity 



spending to the molecular volume. In the case of calcite this 



becomes '— = K. or K ^ ^6.86. To find, therefore, the spe- 



2.715 



cific gravity of FeCOg when occupying the calcite lattice 



we use the following expression, ->^x = Sp. G., from which 



the value of 3.148 is obtained. In this way the specific gravities 

 for the different molecules as they enter calcite have been 

 calculated and are given below : 



CaCOs MgCO. MnCO. FeCOa 



In calcite 2.715 2.288 3.123 3.148 



Using these figures the theoretical specific gravities have been 

 calculated for the calcite analyses and are given together with 

 the measured values in the second half of the above table. The 



