Wychoff — Crystal Structure of Alahandite. 249 



Taking its crystal structure as a criterion, one would 

 say that alabandite is in chemical nature also similar to 

 magnesium oxide in that it is composed of divalent 

 ^4ons" of manganese and of sulphur.^^ 



Summary. 



By a combination of a reflection spectrum from a known 

 crystal face with a powder reflection, and employing the 

 general method based upon the theory of space groups, 

 it is shown that the arrangement of the atoms in alaban- 

 dite is either that of the ^'sodium chloride grouping'' or 

 is a grouping approaching very close to this arrangement. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C. 

 July, 1921. 



" Ealph W. G. Wyckoff, this Journal, 1, 188, 1921. 



