152 Wyckoff — Crystal Structure of Magnesium Oxide. 



of magnesium so that the compound is an aggregate of 

 doubly charged oxygen and magnesium ^'ions." 



The writer wishes to express his thanks to C. J. Ksanda 

 for assistance in carrying out parts of this determination. 



Summary. 



An attempt has been made using Laue photographs and 

 X-ray spectrum measurements to get a unique solution 

 for the crystal structure of magnesium oxide. If it 

 possesses holohedral symmetry then the only simple 

 structure which is possible is the ^^ sodium chloride 

 arrangement,'^ (c). Certain cases of grouping, showing 

 tetartohedral symmetry and of the more complicated 

 holohedral arrangements, {j) and (/b), each with thirty- 

 two molecules associated with the unit, are in agreement 

 with the existing experiments. These other possibilities, 

 however, differ but slightly from the '^sodium chloride 

 arrangement, '^ and can not be positively treated by the 

 experimental facilities now available. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C. 

 October, 1920. 



