354 R. W. G. Wyckoff — Crystal Structures of 



The Nature of these Compounds in the Light of their 

 Crystal Structures. 



These substances are conveniently considered as an as- 

 semblage of an equal number of metal atoms and carbon- 

 ate groups. The rhombohedron of calcite is then simply 

 a deformed sodium chloride arrangement : the carbonate 

 groups holding positions analogous to those of the nega- 

 tive chlorine atoms of NaCl and the calcium atoms cor- 

 responding to the sodium atoms. It is of interest in this 

 connection to see if there is any evidence for the actual 

 existence of a carbonate group in the crystals of the cal- 

 cite group. 



On the basis of the structure just determined and tak- 

 ing u = 0.25, the distance between the carbon atom in cal- 

 cite and each of its three nearest oxygen atoms is found 

 to be 1.21 X 10 s cm. The distance of a calcium atom 

 from the nearest oxygen atoms is 2.30 X 10 s cm. ; and the 

 shortest distance of calcium atom to carbon atom is 

 3.04 X 10 s . The corresponding distances for rhodochro- 

 site are : carbon to oxygen 1.22 s X 10 8 cm. ; oxygen to man- 

 ganese 1.96 X 10 8 cm., carbon to manganese 2.83 X 10~ 8 

 cm. These values are the same in siderite as in rho- 

 dochrosite. The distance between carbon and the three 

 oxygen atoms arranged closely about it is thus seen 

 to be the same in the two cases within the limit of 

 the experimental determination of the value of u, 

 though the other distances between atoms vary quite 

 widely. This is what would be expected if the combining 

 forces of the oxygen atoms were directed in large measure 

 toward the carbon atoms with the formation of a C0 3 

 group, and if the crystal were then built up as a result of 

 the attraction between these groups and metal atoms. 

 There is thus distinct evidence for the existence of definite 

 groups of associated atoms in the crystals of the calcite 

 group. 



There is good reason to believe that the constituent 

 atoms, or groups of atoms, in a large class of crystals are 

 electrically charged. 36 These carbonates have the struc- 

 ture and in general the behavior that they would have if 

 their crystals were an assemblage of metal " ions" bear- 

 ing two positive charges and of carbonate "ions" with 



36 Discussed by B. W. G. Wyckoff, Journ. Wash. Acad. Sci., 9, 565, 1919. 



