Part I. Relations existing between the chemical com- 

 position AND SPECIFIC gravity OF CaLCITE, RhODOCHROSITE, 



vSiderite and Magnesite 



The writer has for some time been studying the data concern- 

 ing the relations existing between the chemical composition and 

 the physical properties, particularly the specific gravity and 

 refractive indices, of the members of the Calcite Group. He has 

 collected all readily available analyses of the minerals of the 

 group and has in addition made or had made in the Mineralogical 

 Laboratory of the Sheffield Scientific School a number of new 

 analyses and physical determinations. From this study have 

 come some interesting results which will be presented in this 

 paper. Because of the few analyses that have been made of 

 smithsonite and the generally unsatis factor}^ character of the 

 material available for investigation it has been necessary to omit 

 this member of the group from the present study. 



Our knowledge of the molecular structure of minerals has been 

 largely increased in recent years by the brilliant investigations 

 made by W. H. and W. L. Bragg with the use of the X-rays. 

 The minerals of the Calcite Group were among those studied in 

 this way, with the result that we now have a theory as to their 

 structure that quite completely satisfies the observed facts.^ A 

 description of these methods of investigation cannot very well be 

 given here and the interested reader is therefore referred to the 

 original articles. It is sufficient at present to say that by means 

 of X-rays these investigators have been able to measure the 

 distances existing between the different molecular layers in direc- 

 tions perpendicular to various crystal planes. In the case of 

 calcite their measurements included the spacing parallel to the 

 following faces, c(oooi), r(iori), ^(oir2), in(ioio), and 

 a(ii2o). For the other members of the group measurements 

 are given only for the direction normal to the base, (oooi). 

 From their results they have been able to show quite conclusively 

 that the unit cell of the crystal net- work has the shape of the 



' See, W. L. Bragg. Proc. Roy. Soc, 80, Series A. 485. 1913, and W. H. 

 Bragg and W. L. Bragg, X-Rays and Crystal Structure, 112, 1915. 



