some Carbonates of the Calcite Group. 359 



attraction normal to the plane exerted by the atoms in 

 the plane upon an atom above it. That is, the difference 

 between these two factors as well as the magnitude of the 

 first one (the attraction in the plane) must be very great 

 if cleavage is to be possible. All the other crystals which 

 have been shown to have the sodium chloride arrange- 

 ment, such as other alkali halides, sodium nitrate, magne- 

 sium oxide, 43 show perfect cubic (100) cleavage. 



The other commonly developed faces of calcite, _as 

 e(01I2), /(0221), M(4041), </>(0554), v(2l31), and 2/(3251) 

 are the dense planes of the rhombohedral structure. 



Various members of this carbonate group have been 

 shown to have both the same general arrangement of 

 atoms within the unit of structure and units of about the 

 same size and shape. They are commonly said to be 

 isomorphous with one another. Calcite and sodium 

 nitrate 44 have not only the same sort of grouping of atoms 

 in the unit but the size and shape of the unit and the abso^ 

 lute distances apart of corresponding atoms are prac- 

 tically identical in the two cases ; yet they are not isomor- 

 phous (as the term is now commonly used). The crystal 

 structures of calcite and sodium nitrate are more 

 nearly alike than are those of calcite and rhodochrosite, 

 for instance. It is in strict accordance with their crystal 

 structures that sodium nitrate will grow in parallel orien- 

 tation upon a crystal of calcite but not upon a crystal of 

 any of the other members of the group. 45 The difference 

 in the crystal structure of calcite on the one hand and rho- 

 dochrosite and siderite on the other may account for the 

 fact that while a complete series of mix crystals of the 

 last two is known, 46 calcite and either of the other two are 

 not known to show complete miscibility. 



Conclusions. 



1. A probably unique solution has been obtained for 

 structures of calcite and rhodochrosite. Of the assump- 

 tions commonly made in crystal structure study, the only 

 one required in these determinations was that the atoms 



43 W. P. Davey and E. O. Hoffman have studied MgO powder (Phys. Bev., 

 (2), 15, 333, 1920). The present method of analysis has indicated the same 

 structure for large and perfect crystals. 



44 W. L. Bragg, op. cit.; E. W. G. Wyckoff, Phys. Eev. (2), 16, 149, 1920. 



45 T. V. Barker, Zs. Kryst., 45, 1, 1908. 



46 W. E. Ford, Trans. Conn. Acad, of Arts and Scl, 22, 211, 1917. 



