246 Wyckoff — Crystal Structure of Alahandite. 



that of (11).^ A definite choice between there three possi- 

 bilities npon the basis of a study of their diffraction 

 effects cannot be made until the scattering powers 



(§) MANGANIEISE 

 • 3UILPHUF^ 



Fig. 2. — The arrangement of the atoms in alabandite if it has exactly the 

 '^sodium chloride arrangement. ' ' 



of the different atoms for X-rays is known with 

 accuracy and until the form of f{d/n) is carefully 

 determined. If the atoms lie exactly in the ^'sodium 



* other weaker lines appear in the powder photograph. As will be seen 

 from the following table they are in equally good agreement with the 

 '^ sodium chloride arrangement," (II), or a close approach to this grouping. 















Calculated 





d/n 









Estimated 



(for II) 



Vleasured 





Calculated 



for the Plane 



n 



Intensity 



Intensity 



1.50A.U. 





1.50A.U. 



111 



2 



3 



17,700 units 



1.31 





1.30 



100 



4 



2 



10,000 



0.87 





0.87 



122 



2 



2 



17,700 



0.83 





0.83 



130 



2 



2 



16,000 



At least a portion of the lack of quantitative agreement can be ascribed to 

 the approximate character of the assumption that f{d/n) =z {d/ny. 



